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Intermediate Electrician Certification Exam Questions 1. Kirchhoff’s first law states that the algebraic sum of the currents flowing through any node is (zero). 2. When the transformer is operating under load, the secondary side voltage (rises) as the load current increases. 3. The starting performance of an asynchronous motor refers primarily to the relationship between the secondary side voltage and time, given a constant power factor of the load. 4. When executing a program, the PLC periodically scans from the first step in sequence until it reaches the (END) instruction. 5. (Synchronous motor) It can operate as a generator as well as as a motor. 6. Whether it is a single-phase fully controlled or semi-controlled bridge rectifier circuit, the formula for calculating the output voltage with a resistive load is the same. 7. In ladder diagrams, the number of times the normally open and normally closed contacts of each component can be used is unlimited. 8. In a three-phase four-wire system with a grounded neutral, the line voltage refers to the (voltage) between the phase wires. 9. When three-phase transformers are operated in parallel, it is required that their connection groups be (the same); otherwise, they cannot operate in parallel. 10. At the instant when the stator voltage of a three-phase asynchronous motor suddenly drops to 80% of its original value, the slip rate remains unchanged, but its electromagnetic torque decreases to 64% of its original value. 11. After maintenance of the contactor, it will become unusable due to damage to the (arc extinguishing) device. 12. The electromagnetic trip in the low-voltage circuit breaker serves the purpose of (overcurrent) protection. 13. A programmable controller generally consists of five components: (CPU), (memory), (input/output interfaces), (power supply), and (programmer). 14. When selecting a time relay, the following points should be considered: (Choose the appropriate series and type based on the system’s required delay range). (Select the suitable delay mechanism according to the functional characteristics of the control circuit). (Choose the voltage level of the coil based on the control voltage). 15. The external characteristic of a transformer when operating under load refers to the relationship between the secondary side voltage and the load current, given a constant power factor of the load. 16. Multi-speed motors typically change the (number of pole pairs) by altering the wiring of the stator windings, thereby changing the motor’s (speed). 17. During maintenance, it was found that the short-circuit ring of the AC contactor was damaged; this contactor (cannot) be used anymore. 18. The electrical testing cycle for insulating tools used in live working is once every (half year). 19. The meanings of neutral point, zero point, neutral wire, and zero wire are explained as follows: (1) The neutral point refers to the common point of the three-phase windings when the three-phase power windings of a generator or transformer are connected in a star configuration. (2) Zero point refers to the grounded neutral point. (3) The neutral wire refers to the conductor that leads from the neutral point. (4) The neutral wire refers to the conductor that leads from the zero point. 20. Proper use of a high-voltage voltage tester: The correct way to use it is as follows: (1) The function of a voltage tester is to determine whether electrical equipment or circuits have voltage. (2) The rated voltage of the voltage tester must be compatible with the voltage level of the equipment being tested. (3) The electric tester must be tested on live equipment before use to check whether it is in good condition. (4) The indicator voltage tester that must be grounded is grounded at its end. (5) Insulating gloves must be worn when performing voltage testing, and a supervisor must be assigned. 21. The function of the arc extinguishing chamber in a high-voltage circuit breaker is to extinguish arcs. Types of arc extinguishing methods: 1. (Longitudinal blowing, transverse blowing). II. (Vertical and horizontal blowing). 23. A low (power factor) in the circuit can lead to insufficient utilization of the capacity of power supply equipment and increased power losses in the transmission lines. 24. In the case of symmetric loading, the (total power) is the same regardless of the connection method used. 25. During the normal operation of the motor, if the supply voltage drops excessively for some reason – falling below 85% of the rated voltage – it will cause the motor’s speed to decrease significantly, or even force the motor to stop running ； This can cause some electrical appliances to release energy, preventing the control circuit from functioning properly and potentially leading to accidents. To this end, it is necessary to take measures to quickly cut off the power supply when the voltage drops below a certain allowable level; this type of protection is known as (under-voltage protection). 26. When the equipment is operating normally, a power outage may occur suddenly for some accidental reason, causing the equipment to stop working as well. The protective measure adopted to prevent accidents caused by the motor starting on its own when the power supply voltage is restored is called (voltage loss or zero-voltage protection). 27. In a low-voltage power supply with a three-phase three-wire system and an ungrounded neutral point, reliably connecting the metal casing or frame of electrical equipment to the ground is called (protective grounding). 28. When protective neutral grounding is employed, in addition to grounding the neutral point, it is also necessary to ground the neutral wire at one or several points; this is called repeated grounding. 29. The three elements of sinusoidal alternating current are (frequency), (amplitude), and (initial phase). 30. The tools that electricians carry with them mainly include (wire cutters), (screwdrivers), (electrician’s knives), (voltage testers) and (rubber gloves), among others. 31. Common motor bearing greases include (calcium-based grease), (sodium-based grease), calcium-sodium mixed-based grease, aluminum-based grease, and molybdenum disulfide grease. 32. The technical indicators for evaluating speed control performance are four: speed control range, smoothness of speed control, slip rate, and economic efficiency of speed control. 33. The insulation resistance of three-phase AC asynchronous motors for low-voltage applications shall not be less than (0.5) megohms. 34. A four-pole three-phase AC asynchronous motor with a supply frequency of 50 Hz and a slip rate of 6% has a synchronous speed of 1500 revolutions per minute; its rotor speed is 1410 revolutions per minute. 35. The Y‑△ reduced-voltage starting method is used for three-phase squirrel-cage asynchronous motors. At startup, the stator windings should be connected in a Y configuration, with each phase winding carrying a voltage of 220 volts. During normal operation, the stator windings should be connected in a Δ configuration, with each phase winding carrying a voltage of 380 volts. 36. The common starting methods for three-phase wound non-standard motors are starting by connecting a resistor in series with the rotor winding, and starting by connecting a frequency-sensitive rheostat in series with the rotor winding. Its main advantages are (reducing the starting current and increasing the starting torque, thereby improving starting performance). 37 The speed control methods for three-phase AC asynchronous motors include: (pole-changing speed control), (frequency conversion speed control), and (slip variation speed control). 38. The grounding resistance of the protective grounding wire for the enclosure of a three-phase AC asynchronous motor should be less than (4) Ω. 39. Servo motors come in two types: (AC) servo motors and (DC) servo motors. Its function is to convert electrical signals into mechanical displacement. The protective characteristic of a thermal relay is that, at the set current, the thermal element does not activate over an extended period of time ； When (in the hot state) it starts, act within 20 minutes ； When the current increases to 1.5 times the set current, it operates within 2 minutes starting from (hot state) ； Starting from the (cold) state, when a 6-fold setting current is passed through, it takes more than 5 seconds for it to operate. 41. A pressure relay is mainly composed of four parts: a diaphragm, a strut, a microswitch, and an adjustment nut. Its main purpose is (to control the pressure of water, oil, and gases). 42. The methods of reduced-voltage starting for three-phase squirrel-cage asynchronous motors include starting with stator resistance in series for voltage reduction, (Y–△) reduced-voltage starting, starting using an autotransformer for voltage reduction, and stepped-down delta starting. 43. The severity of electric shock depends on the magnitude of the current flowing through the body, the duration of exposure to the current, the path taken by the current, the frequency of the current, and the resistance of the person who has been shocked. 44. In addition to the function of connecting or disconnecting the main circuit and control circuit, AC contactors can also provide (under-voltage or loss-of-voltage protection). 45. When a rectifier circuit is equipped with capacitor filtering, its load-carrying capacity is poor, and the output voltage changes as the load varies. 46. When measuring the current in a circuit with an ammeter, it should be connected in series within the circuit. What consequences will occur if, while measuring the current in a load, one accidentally connects the ammeter in parallel with the load? Assume the load resistance is very large. Answer: The internal resistance of an ammeter is very low. If the ammeter is mistakenly connected in parallel with a load, the equivalent load resistance of the circuit will be the parallel combination of the ammeter’s internal resistance and the load resistance. Due to the low internal resistance of the ammeter, the equivalent resistance after parallel connection is very small, which increases the current in the circuit to a level far exceeding the normal current required by the load. Almost all of this current flows through the ammeter, and it is entirely possible for this current to exceed the ammeter’s maximum rating by many times, resulting in damage to the ammeter. Furthermore, due to the high current in the circuit, the power supply equipment may also fail due to overload. 47. Why is it necessary to have a neutral wire when asymmetric loads are connected in star configuration? Answer: The function of the neutral line is to ensure symmetrical phase voltages for asymmetric loads. If the load is connected in star configuration without a neutral wire, and the load is asymmetric, the magnitude of the phase voltage across each phase of the load is determined by the value of the load impedance. Some phase voltages may be higher than the rated voltage of the load, while others may be lower than that rating; this can prevent the load from functioning properly, and in severe cases it can even cause damage to the load. Therefore, when the load is asymmetric, it is necessary to ensure a reliable connection of the neutral line. 48. What precautions should be taken when using a drill? Answer: The following precautions should be observed when using a drill: 1. The drill must not be used in flammable or explosive environments ； 2. A drill of the appropriate size should be selected based on the diameter of the hole, to prevent damage caused by motor overload ； 3 The metal casing of the drill should be reliably grounded. After connecting to the power supply, first use a voltage tester to check whether the casing is charged; only after confirming this is the case can it be used ； 4 Before drilling, let the drill spin idly a few times to check if its rotation is normal ； 5. Keep the drill clean regularly, replace the brushes in a timely manner to extend its service life. 49. Briefly describe the working principle of a three-phase AC asynchronous motor. Answer: Its working principle is as follows: When three-phase symmetric alternating current is applied to the motor’s symmetric three-phase stator windings, a rotating magnetic field is generated in the space within the stator. As the magnetic field in the facility rotates in a clockwise direction, relative motion occurs between the rotor and the rotating magnetic field; the rotor wires cut through the magnetic field lines, thereby generating an induced electromotive force. Since the magnetic field rotates in a clockwise direction, it is as if the magnetic field remains stationary, while the rotor conductors move in a counterclockwise direction to cut through the magnetic field lines. The direction of the induced electromotive force can be determined using the right-hand rule. Since the rotor conductors are closed, an induced current flows through them under the action of the induced electromotive force; this is known as the rotor current. The rotor current interacts with the magnetic field in a rotating magnetic field to generate an electromagnetic force, whose direction is determined by the left-hand rule. The electromagnetic force exerts a torque on the rotating shaft, known as the electromagnetic torque, whose direction is consistent with that of the rotating magnetic field. It drives the rotor to rotate in the direction of the rotating magnetic field. If a mechanical load is applied to the motor shaft, the motor will output mechanical energy. Electric energy is supplied to the stator windings of the motor, and through electromagnetic induction this energy is transferred to the rotor, where it is converted into mechanical energy for output. This is the working principle of a three-phase AC asynchronous motor. 50. What issues should be considered when using brushes? Answer: The following points should be taken into account when using brushes: 1. The same model of brush should be used for the same motor. 2. Generally, new brushes should be replaced when the brush wear reaches 1/3 of its original height. Each time the brushes are replaced, all brushes on the motor should be replaced at the same time. 3. When replacing the brushes, use 0 or 00 grade sandpaper to grind the brushes in the direction of the motor’s rotation, so that the brushes fit well with the commutator or slip rings; the contact area should be 80% to 90%. 4. The brush and the brush holder frame fit properly, allowing free movement without being too tight or too loose. 5. After replacing the brushes, it is necessary to adjust the pressure of the brush springs promptly so that the pressure on each brush is roughly uniform and there is good contact with the working surface of the commutator or slip rings. 51. What are the hazards of excessive starting current in three-phase AC asynchronous motors? Answer: When a three-phase AC asynchronous motor is started directly, the starting current can reach 4 to 7 times the rated current. Such a high starting current poses significant hazards to both the motor and the wiring: 1. The high starting current can cause the motor to overheat, accelerating the aging of its insulation and reducing its service life ； 2. The high starting current causes a large voltage drop in the power supply lines. Especially in large-capacity motors, the voltage loss is greater. Since the torque of a motor is proportional to the square of the voltage, this not only reduces the torque of the motor and makes starting difficult, but it also affects the proper operation of other electrical devices on the same circuit, sometimes even forcing them to shut down ； 3. The high starting current, at the moment the motor starts up, exerts stress on the motor itself as well as on the mechanical equipment; intense vibrations accelerate the wear and damage of these devices. 52. What are the main reasons for abnormal noises in a motor during operation? Answer: The main reasons for abnormal noises in a motor while it is running include the following: 1. Abnormal three-phase supply voltage or operation with one phase missing ； 2. Unbalanced three-phase current in the motor ； 3. Loose rotor blades or broken casing ； 4. Lack of oil in the bearing, dirt inside the bearing, or damaged bearing ； 5. The coupling is loose. 53. What are the main causes of motor overheating? Answer: The main reasons for motor overheating include the following: 1. The motor operates under overload conditions ； 2. The power supply voltage is too low or too high ； 3. Single-phase operation of the motor ； 4. Blockage in the motor air duct ； 5. The stator winding is grounded or short-circuited. 54. What are the main reasons for a decrease in the insulation resistance of a motor? Answer: The main reasons for a decrease in the insulation resistance of a motor include the following: 1. The motor is exposed to rain or water droplets, causing the windings to become damp ； 2. Insulation aging caused by winding overheating ； 3. Excessive dust on the windings ； 4. Poor insulation between the motor leads and the junction box. 55. What does daily maintenance of a three-phase AC asynchronous motor include? Answer: Daily maintenance of a three-phase AC asynchronous motor generally includes the following: 1. The motor should be kept clean regularly to prevent water droplets, oil stains, and dust from entering its interior ； 2. Regularly inspect the bearings, and apply oil to them on a periodic basis, as well as change the oil and clean the bearings ； 3. The operating current of the motor should not exceed the rated current ； 4. Regularly check whether the temperatures of various parts of the motor meet the technical specifications ； 5. Regularly check whether the noise and vibration of the motor are normal ； 6. Always keep the slip rings or brushes of wound-rotor asynchronous motors smooth, clean, and in good contact ； 7. Regularly check that the insulation resistance of the motor stator windings is not below the specified standard ； 8. The motor housing grounding must be firm and intact ； 9. Ensure unobstructed ventilation of the motor ； 10. Regularly tighten the motor leads to keep the wiring neat and tidy. 56 How to select an AC contactor? Answer ； When selecting an AC contactor, the following parameters should be considered: 1. The rated voltage of the main contacts of the AC contactor should be greater than or equal to the rated voltage of the circuit ； 2. The rated current of the main contacts of the AC contactor should be greater than or equal to the rated current of the device being controlled ； 3. The types and quantity of auxiliary contacts of the AC contactor should meet the requirements of the circuit ； 4. The voltage level of the electromagnetic coil of the AC contactor should be consistent with the power supply voltage of the control circuit. 57. What is interlock protection? What is its main function? Answer: In control circuits where two opposite actions are not allowed to occur simultaneously, the normally closed contacts of two devices are often connected in series within each other’s control circuits; when one of these devices operates, it prevents the other from being powered. This interdependent relationship is known as interlock control, or simply interlock. Its main function is to prevent failures that can occur when two actions take place simultaneously. 58 What are the main measures to prevent electric shock? Answer: The main measures to prevent electric shock are: 1. The installation of various electrical equipment must comply with technical specifications, and it should be equipped with protective devices ； 2. The metal casing of electrical equipment should be grounded or connected to the neutral wire ； 3. Select wires and cables appropriately ； 4. Use various safety equipment, tools, and instruments properly ； 5. Strengthen the management of temporary lines, and use safe voltage as much as possible ； 6. Establish and improve various safety regulations and systems, strengthen safety education, and enhance organizational measures as well as various technical measures. 59. (Relative error) is the ratio of the absolute error of the indicated value to the true value. 60. Measurement uncertainty: A parameter that characterizes the variability associated with the value reasonably assigned to the measured quantity. In the complete presentation of measurement results, the measurement uncertainty should be included. 61. In a stable state of the instrument, the ratio of the change in output to the corresponding change in input that is measured is defined as (sensitivity). 62. Temperature: Temperature is a physical quantity that characterizes an object’s degree of hotness or coldness. 63. Absolute pressure: refers to the total pressure in the space surrounding an object; it is equal to the sum of gauge pressure and atmospheric pressure. 64. Measurement accuracy: The degree of consistency between the measurement result and the true value of the quantity being measured; it is a qualitative concept. 65. Automatic control: It involves the use of automatic control devices to enable the (controlled object) to operate automatically in accordance with predetermined patterns, so that the quantity being controlled can change according to those predetermined patterns within a certain range of accuracy, or remain stable within a specific range. 66. Under constant external conditions, when using the same instrument to measure a certain parameter in both forward and reverse directions, the instrument readings for the same value of that parameter are not identical; the difference between these two readings is known as the (variation). 67. Metrological standard: (A measuring instrument or substance) that uses the **specified accuracy class as the basis for calibration. 68. Pressure: The force exerted uniformly and vertically on a (unit area). 69. Flow rate: The amount of fluid that passes through the cross-section of a pipe or channel per unit of time. 70. Perturbation: refers to an external factor that disrupts the system’s equilibrium state and causes changes in the controlled quantity. Apart from control effects, all external factors that cause fluctuations in the controlled quantity are collectively referred to as disturbances. 71. Atmospheric pressure: The pressure exerted on the ground by the gravity acting on the column of air at the Earth’s surface. 72. Stability: Refers to the performance of the instrument’s indication value (which does not change over time or under varying operating conditions). 73. Measurement: It is the process of comparing a known standard physical quantity with an unknown (the physical quantity being measured). 74. Under exactly the same conditions, when the same quantity is measured multiple times, an error whose absolute value and sign remain constant, or that changes according to a certain pattern when the conditions change, is called a (systematic error). 75. Pressure expressed with atmospheric pressure as the zero reference is called gauge pressure; when gauge pressure is negative, it is referred to as vacuum. 76. (Random error) is the error in which, under identical actual conditions and when measuring the same quantity multiple times, both the absolute value and the sign vary in an unpredictable manner. 77. (Large error) is an error that significantly distorts the measurement results. 78. (Accuracy class) is the maximum permissible error, with the plus/minus sign and percent sign removed. It is one of the important indicators for measuring the quality of instruments.79. (Electric heating) uses thermal energy to compensate for the heat lost by the object being heated in the instrument pipelines. 80. Thermal resistance thermometer: A thermal resistance thermometer is a temperature measuring instrument that utilizes the property whereby the resistance value of a conductor or semiconductor changes with temperature; it displays the measured temperature based on these changes in resistance value using a quadratic meter. 81. Distributed control system: It is a centralized-distributed control system based on microprocessors, and its main feature is (centralized management and decentralized control). 82. Throttling device: A throttling device is a measuring element installed directly on the pipeline; its function is to convert fluid (flow rate) signals into corresponding (pressure difference) signals. 83. When the same pressure is applied to both the positive and negative chambers of a differential pressure transmitter, the drift in the transmitter’s output zero point is referred to as (static pressure error). 84. Controlling digital signals in a systematic manner, in accordance with the requirements of the production process, is called (sequential control). 85. Closed-loop control system: A control system is referred to as a closed-loop control system if there are both (forward) and (reverse) actions between the controller and the controlled object. Also known as a feedback control system. 86. Electromagnetic flowmeter: It is a type of flow measurement instrument that operates on the principle that the movement of a conductor in a magnetic field generates an induced electromotive force, and this induced electromotive force is proportional to the flow rate; by measuring this electromotive force, the flow rate in the pipeline can be determined. 87. Secondary instruments: refer to the general term for various instruments whose indication signals do not come directly from the process medium. The signal from secondary instruments is usually a standard signal converted by a transmitter. 88. Source component: Usually refers to the processed parts installed at (the primary measurement point). Such as the pressure tapping fitting in pressure detection systems, and the thermometer boss in temperature measurement systems. 89. Reference error: (the ratio of the absolute error to the range, expressed as a percentage), that is, Reference error = Absolute error / Range × 100%. The accuracy class of an instrument is determined based on this reference error. 90. What are the hysteresis, dead zone, backlash, and sensitivity limit of the instrument? Answer: Hysteresis is a phenomenon characterized by a characteristic curve that consists of an ascending segment as the input increases and a descending segment as it decreases. The dead zone is a limited region in which changes in the input do not cause any noticeable change in the output. Hysteresis is the maximum difference between the corresponding output values for the same input when the input value increases and decreases. Sensitivity limit: refers to the smallest change in the input signal that can cause a change in the instrument’s output signal. The sensitivity limit of a general instrument should not exceed half of the absolute allowable error of the instrument. 91. Transmitter range ratio: the ratio of (maximum measurement range) to (minimum measurement range). The range ratio is large, providing ample adjustment flexibility, which facilitates changing the range of the transmitter without the need to replace the instrument. 92. (Standard throttling devices) The relevant calculation data have been obtained through systematic testing, and there are standardized charts and calculation formulas; throttling devices that are designed, manufactured, and installed in accordance with these standards. 93. Transmitter available range ratio: The range ratio is the value at which the transmitter’s accuracy decreases to 2.5 times that of its maximum range when that range is in use. 94. Briefly describe the advantages and disadvantages of two-wire and three-wire systems when using a thermistor as the sensing element? Answer: The advantage of the two-wire system is that it eliminates one wire; the disadvantage is that since both wires are on the same bridge arm, the resistance of these wires changes with temperature, which directly affects the balance of the bridge circuit and results in large errors in the instrument readings. The three-wire system involves connecting three wires to the ends of the bridge arms; although the resistance of these wires changes with temperature, it does not affect the balance of the bridge circuit. The downside is that it requires one additional wire. 95. What are the components of a thermal resistor? Why is a three-wire system used for the leads? Answer: It consists of the thermal resistor element, an insulating sleeve, a protective sleeve, and a terminal box. The purpose of using a three-wire system is to reduce the additional errors caused by variations in lead resistance. 96. What are the three main parts of low-voltage electrical equipment? (Electromagnetic system), (Contact system), (Arc extinguishing system). 97. What is the working principle of a bimetallic thermometer? It consists of two pieces of metal with different coefficients of expansion firmly bonded together; one end is fixed, while the other end is connected to a pointer through a transmission mechanism. When the temperature of the pointer changes, the difference in expansion coefficients causes the bimetallic strip to undergo angular displacement, which in turn drives the pointer to indicate the corresponding temperature; this is the working principle of a bimetallic thermometer. 98. What are the effects of reversing the inlet and outlet of a control valve? Answer: Generally, the structure of a butterfly valve is symmetrical; it has no distinction between an inlet and an outlet, so there is no issue of it being installed in the wrong direction. The inlets and outlets of control valves in other types of structures can affect the flow characteristics of the valve, causing changes in flow density; this can lead to leaks at the gasket area. In the case of a single-seat valve, installing it in the reverse orientation can change the direction of the balancing forces, thereby affecting the stability of the valve. 99. From which aspects should the selection of an actuator be considered? (1) The structural type of the control valve, (2) The characteristics of the process variable being controlled. (3) Diameter of the control valve. (4) Installation location of the control valve. (5) Operating environment of the control valve. (6) The power supply conditions for driving the actuator. 100. What is the working principle of an electrical valve positioner? Answer: An electrical valve positioner converts the output current signal from the controller into a pressure signal, which is used to drive the control valve. It also feeds back the displacement of the valve stem; through mechanical transmission, a dynamic balance is established between the feedback force and the electromagnetic force, thereby enabling the valve to be positioned quickly. 101. Why is cold-junction temperature compensation necessary in the thermocouple input circuit? Answer: This is because when using thermocouples, it is always desirable for the cold junction temperature compensation to remain constant, but this is difficult to achieve in industrial production. Therefore, cold junction temperature compensation is used; as the output of the thermocouple changes due to variations in the cold junction temperature, the output of the bridge circuit also changes, with the magnitudes of these changes being equal but in opposite directions. In this way, the measurement errors caused by changes in the cold junction temperature of the thermocouple can be compensated for. 102. Briefly describe the working principle of a thermal resistor? Answer: The electrical conductivity of conductive and semiconductor materials is highly dependent on their temperature; as the temperature around them changes, their resistance values also change. Thermistors are made use of these properties of conductors and semiconductors, and are designed specifically for measuring temperature. 103. Briefly describe the reasons for measurement errors in the use of differential pressure flow meters? Answer: 1. Changes in the condition of the medium being measured; 2. Wear at the inlet edge of the orifice plate; 3. Incorrect installation of the throttling device; 4. Incorrect installation of the differential pressure gauge and its connecting pipelines; 5. Changes in the structure of the inner surface of the throttling device or in the cross-section of the orifice. 104. During automatic control, if the output of the regulator suddenly opens circuit, what will happen to the operation of the actuator? Why? Answer: It operates in one direction; when the regulator output is open-circuited, the servo amplifier receives only one feedback signal, which causes one thyristor to be triggered to turn on and enable the actuator to operate in one direction. What are the requirements for selecting a pressure gauge? Answer: 1. Requirements of the process for pressure measurement, 2. The medium to be measured, 3. The measurement environment, 4. Measurement range and installation method. 106. What are the sensitivity, hysteresis, and resolution of a meter? Answer: The sensitivity of a meter refers to the ratio of the change in its output signal to the corresponding change in the input signal. The difference between two input values for which the measured quantity changes slowly in both increasing and decreasing directions, resulting in the instrument indicating the same value, is called the hysteresis of that instrument at that point. Resolution reflects the instrument’s ability to respond to slight changes in the input value. 107. What issues should be considered when using compensation wires with thermocouples? Answer: 1. The compensation wire must be matched with the thermocouple; different models of thermocouples require different compensation wires. 2. When connecting the compensation wire to the thermocouple, the positive pole should be connected to the positive pole, and the negative pole to the negative pole. 3. The two junctions where the compensation wire is connected to the thermocouple must be at the same temperature. 108. There are several types of compensation wires that we commonly use. How can we determine the type of compensation wire when it’s difficult to identify it? Answer: Copper-nickel copper, copper-copper alloy, nickel-chromium-copper alloy ； 1. Poor contact between the thermocouple terminals and the thermal electrodes; 2. Damage to the insulation in the thermocouple’s measurement circuit, leading to continued short circuits or grounding; 3. Improper installation of the thermocouple or external vibrations; 4. The thermal electrodes are partially broken; 5. External interference such as alternating current leakage and electromagnetic field induction. 109. When using a thermocouple to measure temperature, it is found that the thermoelectric potential output is unstable. What causes this? Answer: 1. Poor contact between the thermocouple terminals and the thermal electrodes; 2. Damage to the insulation in the thermocouple’s measurement circuit, resulting in continued short circuits or grounding; 3. The thermocouple being not properly installed or subjected to external vibrations; 4. The thermal electrodes being partially disconnected; 5. External interference such as alternating current leakage and electromagnetic field induction. 110. You have a temperature-sensing resistor whose calibration mark is no longer visible. How can you use simple methods to determine its calibration mark? Answer: Use the R×1 or R×10 setting on a multimeter to measure the resistance value of the resistor; let this value be Rt. Then estimate the ambient temperature as t. Finally, by checking the resistance-temperature table, it is possible to quickly identify the rating of the resistor element. 111. What is the working principle of a bimetallic thermometer? Answer: It consists of two pieces of metal with different coefficients of expansion that are firmly bonded together; one end is fixed, while the other end is connected to the pointer through a transmission mechanism. When the temperature of the pointer changes, the difference in expansion coefficients causes the bimetallic strip to undergo angular displacement, which in turn drives the pointer to indicate the corresponding temperature; this is the working principle of a bimetallic thermometer. 112. Describe the steps for putting an electromagnetic flowmeter into operation Answer: 1. Open the valve to fill the system with fluid. .2. Eliminate leakage points. 3. Remove any residual gas from the system. 4. Power on the transmitter and converter and preheat them. 5. Close the valve to fill the transmitter with static liquid. 6. Re-adjust the converter zero point. 7. Reopen the valve to achieve 100% flow rate, and check whether the output is correct and stable. 113. When an electromagnetic flowmeter is in operation, if the signal becomes weaker or drops suddenly, what could be the possible reasons? Answer: When there is no medium in the measurement conduit, the electrodes are actually insulated from each other. The main cause of the aforementioned situation is a deterioration in the insulation between the electrodes or a short circuit between them. To address this issue, several factors should be considered: 1. Dirt may have accumulated on the inner walls of the measurement conduit; it is necessary to clean these walls and wipe the electrodes ； 2. The lining of the measurement conduit may be damaged and should be replaced ； 3. The signal socket may be corroded and should be cleaned or replaced. 114. What is two-wire system? Answer: It refers to the situation where the field transmitter is connected to the instruments in the control room using only two wires, which serve both as power lines and signal lines. 115. What is a safe spark? What is a safe spark-type explosion-proof instrument? Answer: A safe spark is one whose energy is not sufficient to ignite the surrounding flammable materials. Safety spark-type explosion-proof instruments are those in which the sparks generated under normal conditions as well as in emergency situations are all safe sparks. 116. When using a thermal resistor to measure temperature, the display instrument exhibits the following faults: 1. The reading shown on the instrument is lower than the actual value, or the reading is unstable. 2. The gauge indicates infinity. 3. If the gauge indicates a negative value, try to analyze the cause based on different fault symptoms. Answer: 1. There are metal shavings, dust inside the protective tube, and dust accumulated between the terminals ； Thermal resistor short circuit 2. Thermal resistor or lead wire open circuit ； The terminal screws are loose, resulting in poor contact. 3. There is an error in the wiring between the display instrument and the thermal resistor, or the thermal resistor is short-circuited. 117. What are the two main methods for ultrasonic level detection? Answer: One type is the sound wave blocking method, which utilizes the different degrees of absorption and attenuation of sound waves in gas, liquid, and solid media to detect whether there is liquid or solid material in front of the ultrasonic probe ； One type is an ultrasonic level gauge that can continuously measure level changes; it utilizes the property that ultrasonic waves are reflected at the interface between media to detect the level. 118. What is a pressure switch? How does it work? Answer: A pressure switch is a simple pressure control device. When the measured pressure reaches the rated value, the pressure switch can emit an alarm or a control signal. The working principle of a pressure switch is as follows: when the measured pressure exceeds the rated value, the free end of the elastic element moves, which directly or after comparison drives the switching element, thereby changing its on/off state and achieving control over the measured pressure. The switch types include normally open and normally closed. There are two-digit and three-digit adjustment modes. 119. When using a differential pressure transmitter to measure steam flow, why is it necessary to wait for a certain period of time after draining before activating the differential pressure transmitter? Answer: For differential pressure transmitters used to measure steam, draining removes the condensate from within the pressure guiding tube. Before putting the transmitter into operation, it is necessary to ensure that the pressure guiding tube is filled with condensate, so that the levels of condensate in the positive and negative pressure tubes are equal and remain constant. Measurement errors do not occur only when the differential pressure changes sharply. 120. How to determine whether a differential pressure transmitter is operating properly in the field? Answer: Since the common faults of differential pressure transmitters are zero drift and clogging of the pressure guiding tubes, it is rare to calibrate the scale point by point on-site; instead, the zero point and the trend of changes are checked. The specific methods are as follows: 1. Zero point check: Close the positive and negative pressure shut-off valves. Open the balance valve; at this point, the current of the electric differential pressure transmitter should be 4 mA. 2. Trend inspection: After reaching zero, each valve returns to its original open position, and the drain valve of the negative pressure chamber is opened. At this point, the output of the transmitter should be at its maximum; for electric differential pressure transmitters, this is above 20mA. If only the drain valve of the positive pressure chamber is opened, the output is at its minimum, that is, the electric differential pressure transmitter reads 4mA. When the drain valve is opened, very little or no fluid under test is discharged. This indicates that the pressure guide tube is blocked and needs to be cleared. 121. How to operate the instrument three-valve manifold? Answer: The sequence of opening is: open the positive pressure valve, close the balance valve, then open the negative pressure valve. The sequence for shutting down is: close the negative pressure valve, open the balance valve, and close the positive pressure valve. 122. What are the common anti-interference measures we use on site? Answer: Floating, shielding, grounding, filtering. 123. What are the common insulation and heat tracing measures used on site? Answer: Insulation box for heat retention, steam tracing, and electric tracing. 124. When using thermocouples with instruments, if the thermoelectromotive force shown is higher than the actual value and thus the instrument reads too high, what causes this? Answer: The thermocouple is not compatible with the display instrument ； The compensation wire is not compatible with the thermocouple ； There is a DC interference signal entering. 125. What are the main components of an electronic belt scale? Answer: An electronic belt scale consists of a weight display controller, load cells, a load-bearing mechanism, and a speed sensor. 126. What are the principles for the location of pressure tapping ports? Answer: 1. The pore size should not be too large; however, when there are frequent and significant pressure fluctuations and high requirements for dynamic response, the pore size should be increased appropriately ； 2. The axis of the pressure tap should be as perpendicular as possible to the flow lines, with a deviation not exceeding 5. --10. ； 3. The surface of the pressure tapping should be **pitted and uneven, with no obvious chamfers or other types of flaring. 127. What is the measurement principle of a orifice plate flow meter? Answer: When the fluid under test (gas or liquid) fills the pipeline and flows through a throttling device, the flow stream narrows locally at the throttling point, resulting in the conversion between the kinetic energy and potential energy of the fluid. This leads to an increase in flow velocity and a decrease in static pressure, thereby creating a pressure difference before and after the throttling element. Assuming that the fluid in the pipeline is continuous and incompressible, the volumetric flow rate and mass flow rate flowing through the pipeline can be determined by measuring the pressure difference ΔP before and after the throttling device. 128. To ensure the accuracy of measurements, under what requirements should the temperature sensing element be installed? Answer: 1) When installing the temperature sensing element in the pipeline, it is necessary to ensure full contact between the element and the fluid; the element should face the flow direction of the medium being measured, and at least be in flow alignment with that medium ； 2) The working end of the temperature sensing element should be located at the point in the pipeline where the flow velocity is highest ； 3) There must be sufficient insertion depth ； 4) The junction box cover should face upward. 129. What are negative pressure and vacuum level? What type of elastic element is commonly used to measure the negative pressure of a medium? Answer: 1) When the absolute pressure of the medium under test is lower than standard atmospheric pressure, it is referred to as negative pressure. (2) Relative vacuum degree indicates the level of negative pressure; it is expressed as a percentage, which is the ratio of the difference between atmospheric pressure Patm and absolute pressure Pabx to atmospheric pressure. In other words, relative vacuum degree = … (Absolute vacuum degree = …) (3) The elastic element commonly used to measure negative pressure in fluids is the diaphragm micromanometer 130. For fluids under high temperature and high pressure, what methods are usually employed to measure their flow rate? Try to briefly describe its working principle. Throttle flow meter. Answer: Principle: A throttle element is placed inside the pipeline; as the fluid flows through this element, its flow rate is reduced, which creates a pressure difference before and after the throttle element. The flow rate can be determined by measuring this pressure difference. 131. What precautions should be taken when installing, putting pressure instruments into operation, and using them? Answer: (1) Before putting the gauge into use, the pressure-taking pipeline should first be flushed under pressure; once it has cooled down, the pressure gauge can be installed and put into use. (1 point) (2) Before putting the differential pressure gauge into use, open the balance valve first, then open the secondary needle valve on the high-pressure side; after that, close the balance valve and open the secondary needle valve on the low-pressure side. This is done to prevent the gauge from being overloaded by static pressure or experiencing loss of condensate water. (1 point) (3) After the pressure gauge is installed, its sensitivity in transmitting pressure should be checked; otherwise, the pipeline should be flushed again before it can be put into use. (1 point) (4) For pressure gauges whose mechanical zero point has not been adjusted in advance, the readings taken during use should take into account the correction due to the height of the liquid column. (1 point) (5) For pressure gauges that represent important parameters, in order to facilitate reading during operation, the height correction value is pre-adjusted using the gauge’s mechanical zero adjustment method; thus, no further correction is needed during operation. (1 point) (6) For precise pressure measurement, it is necessary to take into account factors such as atmospheric pressure readings and instrument correction values in the calculations. (1 point) 132. What are the main factors that affect the accuracy of water level measurement using differential pressure water level gauges? What are the main measures taken to reduce errors? What are the two main categories into which control systems are classified based on their system structure? Explain them separately. Answer: There are two factors that affect measurement error: changes in the ambient temperature of the balance vessel. (2 points) The main measures taken are to insulate the balance vessel and the steam-water connection pipes, as well as to compensate for the pressure in the drum by using appropriately designed structural dimensions. More ideal compensation can be achieved by measuring the drum pressure and ambient temperature, as well as using calculation formulas for their effects on the density of saturated water, saturated steam, and condensed water. (i.e., full compensation) 133. What are absolute pressure, atmospheric pressure, gauge pressure, and vacuum? What is their relationship to each other? Answer: Absolute pressure refers to the total pressure exerted by the medium under measurement on a unit area of the container ； The average pressure generated by the column of air at the ground surface is atmospheric pressure ； The difference between absolute pressure and atmospheric pressure is called gauge pressure ； If the absolute pressure of the fluid being measured is lower than atmospheric pressure, the pressure indicated by the gauge is negative, and this value is referred to as vacuum level. 134. Function of the belt scale speed sensor: The speed sensor converts the rotation speed of the roller (which is proportional to the belt transport speed) into a frequency signal f. ? 135. What precautions should be taken when using gravity sensors? Answer: Be aware of zero drift and temperature drift. 136. How many fixing methods are there for installing temperature sensing elements on pipes and equipment? Answer: Generally, there are two methods: 1) fixation using threaded connectors, 2) fixation using flanges. 137. Why does the rotor of a remote-transmission rotameter tend to get stuck? How to improve? Answer: The remote transmission part of the remote-transmission rotor flowmeter relies on magnetism to couple with the rotor. Although the medium under test is mechanically filtered, some of the magnetic particles present in it inevitably end up inside the cone tube and get attracted by the magnets at the upper end of the rotor. Over time, the gap between the moving parts and the cone tube gradually decreases, resulting in friction and causing stagnation. Improvement methods: First, regularly disassemble and clean the rotor to remove any ferromagnetic materials that have stuck to it; second, install an additional magnetic filter in the pipeline in front of the instrument. 138. Differential pressure flowmeters are generally not suitable for use at levels below 30% of their full scale. Why? What should be done if this situation occurs? Answer: In flow measurement, **the standard specifies that the range ratio applicable to throttle devices is 30%. This is because the differential pressure is proportional to the square of the flow rate, and accuracy cannot be guaranteed when the flow rate is below 30%. Furthermore, when the flow rate is less than 30%, the Reynolds number is often below the critical Reynolds number, and the flow coefficient is not constant, resulting in inaccurate flow measurement. The following approach can be taken: negotiate with the process team to reduce the maximum flow rate; if the Reynolds number is high enough, an orifice plate or differential pressure device can be used as a solution. Switch to another type of flowmeter 139. Why can’t electromagnetic flow transmitters be used under negative pressure? Answer: The electromagnetic flowmeter transmitter has a lining provided by the manufacturer, which ensures pressure resistance but not tensile strength. If used under negative pressure, the lining is prone to peeling off, especially with polytetrafluoroethylene linings. 140. What is the task of tuning regulator parameters? What are the several methods commonly used in engineering for tuning regulator parameters? Answer: The task of tuning regulator parameters is to determine the optimal values for these parameters—namely the proportional band δ, integral time TI, and derivative time TD—based on the established control strategy, so that the system can achieve good control quality. The engineering tuning methods for regulator parameters mainly include the critical proportionality method, the attenuation curve method, and the empirical trial-and-error method. 141. In what situations should proportional (P), proportional-integral (PI), and proportional-integral-derivative (PID) control laws be used? Answer: The proportional control law is suitable for control systems where the load changes are small, the pure lag is not significant, the process requirements are modest, and a margin of error is permissible. The proportional-integral control law is suitable for control systems in which the time constant of the process control channel is small, system load variations are significant and it is necessary to eliminate the residual errors caused by such variations, the pure lag is not large and the time constant is not too high, and the controlled parameter is not allowed to deviate from the set value. The proportional-integral-derivative control law is suitable for systems with large capacity lag, relatively small pure lag, and no tolerance for error. 142. What is the significance of using a reclosing device? Answer: There are various reasons for failures in the protected circuits or equipment. Especially when a failure occurs in a protected overhead line, it may be a temporary one; once the fault is resolved, simply closing the circuit breaker again will restore normal operation, thereby reducing the losses caused by power outages. 143. What are the main technical parameters of a current relay? Answer: The main technical parameters of a current relay are: operating current, return current, and return coefficient. 144. What could be the causes of abnormal noises during transformer operation? Answer: The possible reasons for abnormal noises during transformer operation are as follows: 1. Caused by overload ； 2 Internal poor contact causing discharge and arcing ； 3 Individual parts are loose ； 4 The system has a ground connection or short circuit ； 5 High-power starting, with large load variations such as in electric arc furnaces ； 6 Ferromagnetic resonance. 145. Under what circumstances should a power capacitor be taken out of service? Answer: Power capacitors should be taken out of service immediately and the dispatch center should be informed in the following situations: 1. The power capacitor explodes ； 2 The joint is severely overheated or melted ； 3 The bushings experienced severe discharge flashovers ； 4 The capacitor is leaking oil severely or on fire ； 5 The ambient temperature is above 40 ℃. 146. Why does a signal action need to have self-holding? Answer: Since the operation of the impact relay is initiated by an impact current, which causes the signal warning or accident signal circuit to activate, in order for the impact relay XMJ to be able to operate repeatedly, it is necessary for it to return immediately after the first pulse signal triggers, so as to be ready for another operation. If a self-holding circuit is not installed at this time, once the pulse signal disappears, the audio buzzer or alarm will stop functioning immediately. To ensure the reliable transmission of the audio signal and prevent it from stopping due to the return of relay XMJ, self-holding is incorporated into the circuit of the intermediate relay that activates the audio. 147. What is internal overvoltage? Answer: Internal overvoltage occurs due to reasons such as switching operations, accidents involving grounding or disconnection, or other factors that cause a sudden change in the state of the power system. This leads to a transition process from one steady state to another, and during this process dangerous overvoltages may arise. Such overvoltages are caused by the oscillation and accumulation of electromagnetic energy within the system, which is why they are called internal overvoltages. This can be divided into operational overvoltage and resonant overvoltage; the former arises from system operations or faults, while the latter results from resonance caused by an unfavorable combination of parameters in the capacitive elements and inductive elements in the power grid, especially ferromagnetic inductors with cores. 148. Why can transformer taps serve a regulating function? Answer: The voltage in a power system changes depending on the operating mode and the level of load. Either too high or too low voltage can affect the service life of equipment. Therefore, to ensure power supply quality, adjustments must be made based on changes in system voltage. Changing the tap changes the coil turn rest. That is, changing the transformer alters the voltage, thus serving a voltage regulation function. ∵ U 1 /U 2 =W 1 /W 2; U 1 × W 2 =U 2 × W 1. Therefore, U 2 =U 1 × W 2 /W 1. 149. What are the general regulations for the installation of explosion-proof electrical equipment? Answer: 1. Electrical equipment and circuits, especially those that generate sparks during operation, should be installed as far away as possible from areas with explosion hazards. If installation in such areas is necessary, they should be placed in locations with lower levels of risk, along with effective safety measures in place. 2. Electrical equipment and wiring in areas prone to explosions must be protected from mechanical damage, and they should meet the requirements regarding protection against corrosion, moisture, sunlight, wind and rain, as well as sandstorms. 3. Electrical equipment in areas with explosion hazards should be equipped with reliable protection against overload and short circuits. 4. In areas with explosion hazards, sockets that must be installed should be placed in locations where dust and debris cannot accumulate easily ； Local lighting fixtures should be installed in locations where gases are less likely to cause impact in the event of an accident. 150. What is the difference between an AC welding transformer and a regular transformer? Answer: 1. A regular transformer operates under normal conditions, whereas a welding transformer operates under short-circuit conditions. 2. When the load on a regular transformer changes, the voltage across its secondary winding changes very little. In contrast, a welding transformer requires a certain arc-starting voltage of 60–75 volts during welding; as the welding current increases, the output voltage drops rapidly. Even when the voltage drops to zero, the current flowing through the secondary winding does not become excessive. 3. In a regular transformer, the primary and secondary coils are concentrically wrapped around a single core rod, whereas in a welding transformer, the primary and secondary coils are mounted on separate core rods. 151. What are magnetic field and magnetic flux density? Write down the formula for magnetic flux density. Answer: There is a special substance present in the space around a magnet, one that can exert a force; this special substance is called a magnetic field. The physical quantity that reflects the strength of a magnetic field is called magnetic flux density, denoted by B. In a magnetic field, when a straight conductor of length 1 with a current of I is placed at a point perpendicular to the direction of the magnetic field, and a force F acts on it, then the magnetic flux density is given by B = F/IL. Magnetic flux density is a vector whose direction corresponds to the direction of the magnetic field at that point. Its unit is the tesla, abbreviated as “T”, and it is denoted by the symbol T; in engineering, the gauss GS is commonly used as a unit. 152. How to select a motor correctly? Answer: 1. Select the motor based on the power supply voltage and operating conditions, as well as the object to be driven. The rated voltage of the motor selected must match the supply voltage. 2. Select the type of motor based on the installation location and operating environment. 3. Select the motor based on capacity, efficiency, speed, and power factor. The capacity of the motor should neither be too large nor too small. Excessive capacity will cause the cart to strain, which is uneconomical and results in wasted electrical energy ； If it is too small, it will affect the motor’s service life; in severe cases, the motor may not be able to handle the load and burn out. The speed of the motor must match the speed required by the driven machinery. 153. What is the efficiency of a motor? What factors is it related to? Answer: The percentage of the ratio of the motor’s output power P2 to its input power P1 is called the efficiency of the motor. It is denoted by the letter “η”. That is: η = P2/P1 × 100%. The efficiency of a motor is related to the load it drives, the motor’s speed, the type of motor, and the voltage of the power supply. The efficiency of a typical asynchronous motor ranges from 75% to 92%; it is lower when the load is light and higher when the load is heavy ； When the speed of the motor decreases, its efficiency generally drops as well ； When the supply voltage is higher or lower than the motor’s rated voltage, its iron loss and copper loss increase, which reduces efficiency under full load ； Wound motors and deep-groove motors of large to medium capacity have low efficiency. 154. What are the operating states of a circuit? What are the relationships between voltage, current, and power in various operating states? Answer: A circuit has three operating states: loaded state, unloaded state, and short-circuit state. 1. Loaded state: The load resistor is connected to the power supply; I = U = E/IR. P = PE – ΔP, where PE = EI is the power generated by the power supply ； △ P=I²r. ——Power lost in the internal resistance of the power supply ； P=UI —— the power delivered by the power source to the external circuit load. 2. No-load condition: The resistance of the external circuit is infinite for the power supply; R → ∞, I = 0, U = E, P = PE = ΔP = 0. 3. Short-circuit condition: When the resistance of the external circuit is R = 0, current does not flow through the load; I = IS, U = 0, P = 0, PE = ΔP = I²/R. 155. What is the value of the ground resistance? Answer: The value of ground resistance is primarily determined by the resistance associated with the grounding electrodes in the grounding network, plus the resistance of the wires used for grounding; the resistance of these wires can generally be ignored. As a reference value based on experience, the typical soil resistivity is such that one grounding electrode has a resistance of 10–15 ohms. The grounding electrodes in a grounding grid are connected in parallel; the total resistance of the grounding grid can be estimated by dividing the resistance of a single electrode by the number of electrodes, and then multiplying the result by a coefficient of 0.8–0.9. The grounding resistance of all electrical devices connected to this device is this value. 156. What should be noted when connecting copper and aluminum wires? Answer: Due to the different chemical properties of copper and aluminum, electrochemical corrosion occurs easily at their contact points; over time this can lead to poor contact, poor conductivity, or even breakage of the joint. Therefore, when connecting copper and aluminum wires, copper-aluminum connectors or copper-aluminum compression fittings should be used. When connecting copper and aluminum busbars, the method of tin-plating the copper busbar before connecting it to the aluminum busbar can be used. 157. What is the arc extinguishing principle of the grid in an AC contactor? Answer: The arc-extinguishing principle of the grid in an AC contactor is that the magnetic resistance of the iron grid above the contacts is very low; most of the magnetic flux above the arc enters the grid, resulting in a magnetic field distribution in the air surrounding the arc that is denser at the bottom and less dense at the top, thereby pulling the arc into the arc-extinguishing grid. The arc is divided into several short arcs by the grid plates, which raises the ignition voltage above the supply voltage and induces a cathode effect; moreover, the grid plates absorb a large amount of heat from the arc, causing it to be extinguished. 158. What is the function of a reactor? Answer: A reactor is essentially a coil without magnetic conductivity, and it can be arranged in three configurations: horizontal, vertical, or in a triangular shape, depending on the requirements. When a short-circuit fault occurs in an electrical power system, a large amount of short-circuit current is generated. To maintain the dynamic and thermal stability of electrical equipment, reactors are often connected in series at the outgoing circuit breakers to increase the short-circuit impedance and limit the short-circuit current. Moreover, the voltage drop across the reactor is significant, which helps maintain the bus voltage level and reduces fluctuations in it. The main function of a series reactor is to suppress higher harmonics and limit inrush currents, thereby preventing harmonics from causing damage to capacitors and avoiding excessive amplification of grid harmonics as well as resonance resulting from the connection of capacitor banks. 159. What are the requirements for running wires through tubes? Answer: 1. The total area of the insulating layers covering the wires inside the tube should not exceed 40% of the inner diameter cross-section ； 2. There must be no joints or twists in the wires inside the tube ； 3. The phase wires of the same circuit, regardless of their number, should be placed inside a single conduit; wires from different circuits or with different voltages must not be placed in the same conduit. AC and DC wires shall not be placed in the same conduit either, and a single phase wire must not be inserted into a steel conduit alone. 160. Why does the motor stop immediately when direct current is applied to the stator after the asynchronous motor is disconnected from the power supply? Answer: After the stator windings of the motor are disconnected from the AC power supply, direct current is immediately applied to the two-phase stator windings, creating a stationary magnetic field within them. Due to the inertia of the rotor, it continues to rotate in its original direction, cutting through these magnetic field lines; as a result, an induced electromotive force and induced current are generated in the rotor circuit. The interaction between the rotor current and the stationary magnetic field generates a braking torque in the opposite direction of rotation, causing the motor to stop quickly. 161. What is a closed-loop speed control system? Answer: A system that can take into account the changes in motor speed caused by load variations, reflect them at the input side, and make appropriate adjustments to achieve this goal is called a closed-loop speed control system. 162. Why is the low-voltage winding of a transformer inside, while the high-voltage winding is outside? Answer: The arrangement of the high-voltage and low-voltage windings in a transformer is determined by various factors. However, in most transformers, the low-voltage winding is arranged inside the high-voltage winding. This is mainly for insulation reasons. Theoretically, a transformer can perform its function regardless of how the high-voltage winding or the low-voltage winding is arranged. But since the core of the transformer is grounded, and the low-voltage winding is close to the core, it is easy to achieve from an insulation perspective. If the high-voltage winding is placed close to the core, due to the very high voltage of this winding, a large amount of insulating material and a greater insulation distance are required to meet the insulation requirements. This not only increases the volume of the winding but also wastes insulation material. Furthermore, since the voltage regulation of the transformer is achieved by changing the taps of the high-voltage winding, that is, by altering its number of turns, it is easier to arrange the high-voltage winding outside the low-voltage winding, as well as to handle the leads. 163. Why can’t transformers step up or down direct current? Answer: A transformer can change voltage when an alternating electric potential is applied to the primary side, thereby generating an alternating magnetic flux; this alternating magnetic flux induces an electric potential in the secondary side, and the magnitude of this induced potential is proportional to the rate of change of the magnetic flux. When a DC current is applied to the transformer, since both the magnitude and direction of the current remain constant, there is no alternating magnetic flux in the core; that is, the magnetic flux remains constant, and its rate of change is zero. Therefore, the induced electromotive force is also zero. At this point, the entire DC voltage is applied to the winding with very low resistance, resulting in a very high current and an effect similar to a short circuit. Alternating current, on the other hand, changes periodically; when alternating current is applied to the primary winding, the magnetic flux generated within the core also changes. Therefore, when the number of turns in the secondary winding is greater than that in the primary winding, the voltage can be increased ； Conversely, the voltage can be reduced when the number of secondary turns is less than that of the primary. Since the magnitude and direction of direct current do not change over time, when a constant direct current is applied to the primary winding, the magnetic flux generated within its core remains constant as well; consequently, no voltage is induced in the secondary winding, and thus no voltage transformation occurs. 164. How to conduct an no-load test on a motor? Answer: Before the test, the motor is inspected; once no issues are found, a three-phase power supply is connected to make the motor spin idly without any load. Next, it is necessary to check the operating condition of the audio equipment, the operation of the bearings, and the three-phase currents. Generally, the no-load current of large-capacity, high-speed motors is 20–35% of their rated current, while that of small-capacity, low-speed motors is 35–50% of their rated current. The no-load current should not be too high or too low, and the three phases must be balanced. The no-load test should last at least 1 hour; meanwhile, the temperature rise of the motor also needs to be measured, with this rise not exceeding the allowable limits based on the insulation class. 165. How to conduct a motor short-circuit test? Answer: The short-circuit test involves using a braking device to prevent the motor rotor from rotating, while gradually increasing the output voltage of the three-phase voltage regulator from zero. The voltage rise stops when the current reaches the motor’s rated current; this voltage is known as the short-circuit voltage. For motors with a rated voltage of 380 volts, their short-circuit voltage is generally between 75 and 90 volts. A high short-circuit voltage indicates too large leakage reactance. A too low line voltage indicates that the leakage reactance is too small. Both of these are detrimental to the proper operation of the motor. 166. What protections are generally available for small and medium-sized asynchronous motors? Answer: ① Short-circuit protection: Generally, fuses serve as short-circuit protection devices ； ②Voltage loss protection: The electromagnetic coil of the magnetic starter serves to provide voltage loss protection in the motor starting control circuit. Automatic air switches and autotransformer voltage reduction compensators are generally equipped with voltage loss trip devices, so as to provide overload protection for motors in the aforementioned two situations ； ③Overload protection: A thermal relay is the overload protection device for motors. 167. What are the requirements for grounding in 10 kV substations? Answer: The metal enclosures of transformers, switching equipment, and current transformers PT and CP; distribution cabinets, control and protection panels, metal frames, lightning protection equipment, cable terminals, and metal barriers, etc. The grounding device must meet the following requirements: ① The indoor angle steel foundations and supports should be connected using flat steel with a cross-sectional area of at least 25 × 4 mm2 to form the grounding main line, which is then led outside to connect with the outdoor grounding device ； ②The grounding electrode should be placed at a distance of three meters from the walls of the substation; its length should be 2.5 meters, with a distance of 5 meters between the two grounding electrodes being appropriate ； ③It is preferable for the grounding grid to be in the form of a closed loop; if the grounding resistance does not meet the requirements, external grounding electrodes can be added ； ④The grounding resistance of the entire grounding grid should not exceed 4 ohms. 168. What is back-emf braking? Answer: When the motor drives production machinery, in order to stop it quickly or reverse its direction, the power supply can be reversed by swapping any two phases; this causes the rotating magnetic field of the motor to change direction immediately, and as a result, the direction of the torque also changes. Since the direction of the torque is opposite to the direction in which the rotor rotates due to inertia, it exerts a braking effect. This braking method is fast, but it consumes a large amount of electrical energy. Additionally, when the rotation speed drops to zero, the power supply must be cut off promptly; otherwise, reverse power generation will occur. 169. What is lightning protection grounding? What components does a lightning protection grounding system include? Answer: Grounding designed to rapidly conduct lightning current into the ground in order to prevent damage caused by lightning is called lightning protection grounding. The lightning protection grounding system includes the following components: 1. Lightning receptor ； 2. Ground lead upper and lower wires ； 3. Grounding device. 170. What is the common method used to start a wound-rotor three-phase asynchronous motor? What are the advantages and disadvantages of various methods? Answer: There are usually two methods for starting a wound-rotor asynchronous motor: 1. Starting by connecting a three-phase symmetric variable resistor in series with the rotor circuit. This method can both limit the starting current and increase the starting torque. By selecting an appropriate value for the resistance connected in series, it is possible to make the starting torque close to the maximum torque. By appropriately increasing the power of the resistance connected in series, this resistor can also serve as a speed control resistor, thus serving two purposes. It is suitable for loads that require a high starting torque as well as the ability to control speed. Disadvantages: The multi-stage regulation control circuit is complex, and it results in high power consumption due to resistors. 2. The rotor circuit is started by connecting a frequency-sensitive rheostat in series. At the start of startup, the frequency of the rotor circuit is high; as a result, the equivalent resistance and inductive reactance of the frequency-sensitive resistor increase, which in turn increases the starting current as well as the starting torque. As the speed rises, the frequency of the rotor circuit decreases, and the equivalent impedance also decreases automatically. Once startup is complete, the frequency-sensitive resistor is disconnected. Advantages: simple structure, low cost, no need for manual adjustment during startup, easy to manage, and capable of starting under heavy loads. Disadvantages: the inductor inside the rheostat results in a lower starting torque compared to that of a series resistor, so it cannot be used for speed control. 171. What are the common voltage-reduction starting methods for cage-type three-phase asynchronous motors? Answer: The common voltage-reduction starting methods for cage-type three-phase asynchronous motors are as follows: 1. Y-Δ starting. For a cage-type three-phase asynchronous motor that operates in Δ configuration under normal conditions, it is connected in star configuration during starting, which reduces the armature voltage to 1/√3 of the rated voltage. Once the speed approaches the rated value, it is switched back to Δ configuration, allowing the motor to operate at full voltage. With Y-△ switching, the actual starting current and starting torque are reduced to 1/3 of those in direct starting, allowing only light-load starting. Advantages: The starting device has a simple structure and is cost-effective; it should be given priority ； Disadvantage: The starting torque is low, making it suitable only for motors connected in delta configuration for normal operation. 2. Autotransformer step-down starting, also known as compensated starting, involves using an autotransformer to reduce the voltage applied to the motor’s stator windings during startup, thereby reducing the starting current. Once the speed approaches the rated value, the autotransformer is disconnected and the motor operates at full voltage. Under autotransformer step-down starting, the actual starting current and starting torque are 2×W2/W1 those of starting at full voltage. W2/W1 — refers to the voltage reduction ratio; W2, W1 — are the number of turns in the primary and secondary windings of the autotransformer. Advantages: Not limited by the winding connection method of the motor; can achieve a higher starting torque than with the Y-△ transformation ； The secondary side of the autotransformer is equipped with 2–3 sets of plugs for the user to choose from, making it suitable for motors with large capacity and high starting torque requirements. 172. When transformers are operated in parallel, what will happen if the short-circuit voltages of the two transformers are not equal? What are the regulations in our country regarding the difference in short-circuit voltages of transformers? Answer ； If the short-circuit voltages of two transformers operating in parallel are not equal, it will lead to uneven load distribution; the transformer with a lower short-circuit voltage bears a heavier load while the one with a higher short-circuit voltage carries a lighter load. The greater the difference between these values, the more uneven the distribution becomes. In our country, it is stipulated that the difference in short-circuit voltages between two transformers operating in parallel shall not exceed 10%. 173. What consequences will occur if a three-phase load that should be connected in star is mistakenly connected in delta? What consequences will arise in turn? Answer: If a three-phase load that should be connected in star configuration is mistakenly connected in delta configuration, it will result in the load being subjected to an excessive voltage, which can cause the load to burn out; this applies to three-phase loads that should be connected in delta configuration. When connected in star configuration by mistake, the load voltage will be too low, resulting in insufficient output power. 174. Briefly describe the working principle of a thermal relay? Answer: The thermal relay does not operate when the main circuit is carrying normal current. When an overload current flows through, the heat generated by the thermal element causes the bimetallic strip to expand and deform, thereby pushing the operating mechanism and causing the open contacts of the thermal relay connected to the control circuit to disconnect, thus breaking the control circuit. In this way, the main contacts in the main circuit are also disconnected. 175. Why is a three-phase four-wire system used when the three-phase load is asymmetric? Why is it not allowed to install a fuse on the neutral wire in a three-phase four-wire power supply? Answer: The three-phase four-wire system is used to ensure that the phase voltages of unbalanced three-phase loads are equal. If the fuse on the neutral line blows, it will cause asymmetry in the voltage of the three-phase load, resulting in some phase voltages being too high and exceeding the rated voltage of the load, while other phase voltages are too low and falling below the rated voltage of the load; this is not acceptable. 176. Why is only one point allowed to be grounded on the primary side of a current transformer? Answer: In the secondary circuit of a current transformer, grounding is permitted only at one point; multiple points of grounding are not allowed. This is because if there are more than two points of grounding, it may lead to current splitting, thereby increasing errors in electrical measurements or affecting the proper operation of relay protection devices. 177. What is the safe current-carrying capacity of a wire? Answer: The maximum current that a wire can carry over an extended period of time, without exceeding its maximum allowable temperature, is known as the safe current-carrying capacity of the wire, also referred to as its safe current. 178. What are the various insulation grades for motor windings? What are their respective allowable temperature rises? Answer: The commonly used insulation grades include: Class A insulation: the maximum allowable temperature rise is 60℃ ； Class E insulation: The maximum allowable temperature rise is 75℃ ； Class B insulation: The maximum allowable temperature rise is 80℃ ； Class F insulation: The maximum allowable temperature rise is 100℃ ； Class H insulation: The maximum allowable temperature rise is 125°C. 179. What are the starting methods for wound-rotor asynchronous motors? What are their main advantages? Answer: There are two starting methods for wound-rotor asynchronous motors: starting by connecting a resistor in series with the rotor windings, and starting by connecting a frequency-sensitive rheostat in series with the rotor windings. Its main advantages are: it limits the starting current while simultaneously increasing the starting torque, thereby enabling the motor to achieve good starting performance. 180. In a wound-rotor asynchronous motor, can it still start if the rotor is open-circuited? Why? Answer: If the rotor of a wound-rotor asynchronous motor is open-circuited, the motor cannot start. This is because the rotor is open-circuited; no current flows through the rotor windings, so it cannot interact with the magnetic field to generate electromagnetic force. As a result, no electromagnetic torque is produced, and the rotor cannot start rotating. 181. Briefly describe the method of starting a wound-rotor asynchronous motor by connecting resistors in series with the rotor windings. Answer: The method of starting a wound-rotor asynchronous motor by using resistance in series with the rotor windings is as follows: before starting the motor, the resistance value is set to its maximum level first. After power is applied, the motor starts with reduced voltage. As the motor speed continues to increase, the resistance value gradually decreases until it is completely eliminated, at which point the motor enters stable operation. 182. Stars—Delta voltage reduction starting method: What are its main advantages and disadvantages? Answer ； The main advantages of star-delta reduced-voltage starting are ； 1. When the motor starts in star connection, the voltage across each phase winding is only 1/√3 of the voltage during operation. 2. The starting current is reduced to only 1/3 of that in direct start mode, thus being significantly lower. 3. The equipment is simple and operates reliably. The main drawback is ； Since the electromagnetic torque is proportional to the square of the voltage applied to the stator windings, the torque during startup decreases, becoming only 1/3 of that during direct startup; therefore, it is suitable only for light-load or no-load startups where low starting torque requirements apply. 183. Three-phase squirrel-cage asynchronous motors use star-delta reduced-voltage starting. What is the principle behind this? Answer: The principle of star-delta reduced-voltage starting is as follows: during startup, the stator windings are connected in a star configuration; after startup is complete, the stator windings are connected in a delta configuration to enable stable operation. This is because during star connection starting, the voltage across each phase winding is 1/√3 of the rated voltage, and the line current is 1/3 of the line current during direct starting. 184. What is self-inductance?——When the current in a closed circuit changes, the magnetic flux passing through the circuit itself, generated by this current, also changes. As a result, an induced electromotive force is generated within the circuit. This phenomenon is known as self-inductance, and such an induced electromotive force is called a self-induced electromotive force. 185. What is mutual inductance—if two coils are placed close to each other, part of the magnetic flux generated by the current in the first coil links with the second coil. When the current in the first coil changes, the magnetic flux linked with the second coil also changes, thereby generating an induced electromotive force in the second coil. This phenomenon is called mutual inductance. 186. What is inductive reactance?——When alternating current flows through a circuit that contains an inductor, the inductor resists the flow of this alternating current; this resistance is known as inductive reactance, denoted by Lx. The formula for Lx is Lx=2πfL. 187. What is capacitive reactance?——When alternating current flows through a circuit that contains a capacitor, the capacitor resists the flow of this alternating current; this resistance is known as capacitive reactance, denoted by Cx. The formula for Cx is Cx=1/2πfc. 188. What is electromagnetic induction?—When the magnetic flux passing through a conductor enclosed by a loop changes, an electromotive force is generated in the conductor; this phenomenon is called electromagnetic induction. 189. What is the skin effect? Also known as the surface effect, it occurs when high-frequency current flows through a conductor; the current concentrates on the surface of the conductor. This phenomenon is called the skin effect. 190. What are eddy currents? Eddy currents are circular currents that are generated within a solid iron core (or aluminum disc) in an alternating magnetic field due to the induction of electric potential. The direction of these currents is always perpendicular to the main magnetic flux. 191. Reasons for overheating or burning out of contactors: 1. Damage to the coil insulation or mechanical damage that causes inter-turn short circuits or contact with the ground. 2. An excessive air gap between the armature and the core when they come together, which reduces the impedance of the coil and results in an increased current at the rated voltage. 3. Frequent starting and stopping operations, which cause the coil to be subjected to high-current surges each time it closes. 4. An excessively low supply voltage, which leads to poor engagement of the components, resulting in vibration or even failure to engage, thereby causing excessive current flow. 192. What are the common faults of the electromagnetic coils in low-voltage electrical appliances, and how to repair them? Answer: Common faults are open circuits and short circuits. An infinite resistance value indicates an open circuit; if there are only individual breaks in the circuit ； Just ensure proper insulation is in place after welding. If there are many damaged sections or the affected area is large, use wire of the same gauge to rewind the wires or replace them entirely. Identify the cause of the short circuit; if poor contact leads to high currents that damage the insulation, check whether the voltage is too low or if the contact surfaces of the moving core are not flat, and carry out repairs accordingly. In cases of mechanical damage or moisture damage, a few damaged turns can be removed and replaced with wire of the same type. For larger areas, wind new wires using the same gauge, then proceed with standard treatments such as drying, impregnation, and baking at 100–120 degrees Celsius. 193. What does the speed of a synchronous motor depend on? Answer: It is proportional to the power supply frequency, inversely proportional to the number of pole pairs, and independent of the magnitude of the rotor’s DC excitation current. When its rated torque is greater than that of the load, the speed has no relation to the size of the load either. 194. Can voltage be applied to the windings after removing the rotor? Answer: No! Under normal conditions, the air gap between the stator and rotor is small, resulting in low magnetic resistance; consequently, the current required to generate magnetic flux is also low. However, when that gap is reduced, the magnetic resistance increases, which in turn leads to an increase in current. And too high a magnetomotive force can burn out the windings. Working principle of capacitor-started single-phase asynchronous motors – Since the capacitor is connected in series with the starting winding, two-phase currents with a 90-degree phase difference are generated in the stator windings, thereby creating a two-phase rotating magnetic field. This rotating magnetic field cuts across the rotor conductors, inducing an electromotive force and induced current in those conductors. The interaction between this current and the magnetic field produces an electromagnetic torque, causing the rotor to rotate in the direction of the rotating magnetic field. 195. Why cannot electromagnetic flow transmitters be used under negative pressure? Answer: The electromagnetic flowmeter transmitter has a lining provided by the manufacturer, which ensures pressure resistance but not tensile strength. If used under negative pressure, the lining is prone to peeling off, especially with polytetrafluoroethylene linings. 196. What is the task of regulator parameter tuning? What are the several methods commonly used in engineering for tuning regulator parameters? Answer: The task of tuning regulator parameters is to determine the optimal values for these parameters—namely the proportional band δ, integral time TI, and derivative time TD—based on the established control strategy, so that the system can achieve good control quality. The engineering tuning methods for regulator parameters mainly include the critical proportionality method, the attenuation curve method, and the empirical trial-and-error method. 197. In what situations should proportional (P), proportional-integral (PI), and proportional-integral-derivative (PID) control laws be used? Answer: The proportional control law is suitable for control systems where the load changes are small, the pure lag is not significant, the process requirements are not high, and a margin of error is acceptable. The proportional-integral control law is appropriate for control systems where the time constant of the control loop is small, the system load changes significantly and it is necessary to eliminate errors caused by disturbances, the pure lag is not large and the time constant is not too high, and it is not permissible for the controlled parameter to deviate from the set value. The proportional-integral-derivative control law is suitable for systems with large capacity lag, relatively small pure lag, and no tolerance for error. 198. Write down the conversion relationships between 5 non-standard units and the standard pressure unit Pa. Answer: 1 millimeter of water column, mmH2O = 9.8066 Pa ≈ 9081 Pa. 1 millimeter of mercury, mmHg = 133.322 Pa ≈ 1.333×10² Pa. 1 engineering atmosphere, kgf/cm² = 9.80665×10⁴ Pa ≈ 9.81×10⁴ Pa. 1 physical atmosphere, atm = 101325 Pa ≈ 1.0133×10⁵ Pa. 1 bar = 1000 mbar = 10⁵ Pa. 199. Why do the rotors in remote-transmission rotor flow meters tend to get stuck? How to improve? Answer: The remote transmission part of the remote-transmission rotor flowmeter relies on magnetism to couple with the rotor. Although the medium under test is mechanically filtered, some of the magnetic particles present in it inevitably end up inside the cone tube and get attracted by the magnets at the upper end of the rotor. Over time, the gap between the moving parts and the cone tube gradually decreases, resulting in friction and causing stagnation. Improvement methods: First, regularly disassemble and clean the rotor to remove any ferromagnetic materials that have stuck to it; second, install an additional magnetic filter in the pipeline in front of the instrument. 200. What is the measurement principle of orifice plate flow meters? Answer: When the fluid being measured, whether gas or liquid, fills the pipe and flows through a throttling device, the flow stream undergoes local contraction at the throttling point; this causes the kinetic energy and potential energy of the fluid to be converted into one another, resulting in an increase in flow velocity and a decrease in static pressure. Thus, a pressure difference is created before and after the throttling element. Assuming that the fluid in the pipe is continuous and incompressible, the volumetric flow rate and mass flow rate passing through the pipe can be determined by measuring the pressure difference ΔP before and after the throttling device. To ensure the accuracy of the measurements, what requirements must be met regarding the installation of the temperature sensing elements? Answer: 1. When installing the temperature sensing element in the pipeline, it is necessary to ensure full contact between the element and the fluid; the element should face the flow direction of the medium being measured, and at least be in flow alignment with that medium ； 2. The working end of the temperature sensing element should be located at the point in the pipeline where the flow velocity is highest ； 3. There must be sufficient insertion depth. 4. The junction box cover should face upward. 202. What are negative pressure and vacuum level? What type of elastic element is commonly used to measure the negative pressure of a medium? Answer: 1. When the absolute pressure of the medium under test is lower than standard atmospheric pressure, it is referred to as negative pressure. 2. Relative vacuum degree indicates the level of negative pressure; it is expressed as a percentage, which is the ratio of the difference between atmospheric pressure Patm and absolute pressure Pabx to atmospheric pressure. In other words, relative vacuum degree or absolute vacuum degree = 3. The elastic element commonly used to measure the negative pressure of the fluid under consideration is the diaphragm micromanometer. 202. For fluids at high temperatures and pressures, what method is usually used to measure their flow rate? Try to briefly describe its working principle. Answer: Principle: A throttling element is placed inside the pipeline; as the fluid flows through this element, its flow rate is reduced, which creates a pressure difference before and after the throttling element. The flow rate can be determined by measuring this pressure difference. 203. What precautions should be taken during the installation, commissioning, and operation of pressure instruments? Answer: Before putting the gauge into use, the pressure-taking pipeline should first be flushed under pressure; once the pipeline has cooled down, the pressure gauge can be installed and put into use. Before putting the 1/2 differential pressure gauge into use, first open the balance valve, then open the secondary needle valve on the high-pressure side, close the balance valve again, and then open the secondary needle valve on the low-pressure side; this helps to prevent the instrument from being overloaded by static pressure or from losing condensate water. After the 1.3-pressure gauge is installed, its sensitivity in transmitting pressure should be checked; otherwise, the pipeline should be flushed again before starting it up. 1.4 For pressure gauges whose mechanical zero point has not been adjusted in advance, care should be taken to account for the correction due to the height of the liquid column when reading them during use. For pressure gauges with some important parameters at 1 minute and 5 seconds, in order to facilitate reading during operation, the height correction value is pre-adjusted using the mechanical zero adjustment method of the gauge; thus, no further correction is needed during operation. 1 point 6: For precise pressure measurement, it is necessary to take into account factors such as atmospheric pressure readings and instrument correction values. 1 point 204: What are the main factors that affect the accuracy of water level measurement in differential pressure type water level gauges? What are the main measures taken to reduce errors? Answer: There are two factors that affect measurement error: changes in the ambient temperature of the balance vessel. Changes in the operating pressure of the 2-stage steam drum. The main measures taken to achieve a 2-point reduction are to insulate the balance vessel and the steam-water connection pipes, as well as to compensate for the pressure in the steam drum through appropriately designed structural dimensions. More ideal compensation can be achieved by measuring the drum pressure and ambient temperature, as well as using calculation formulas for their effects on the density of saturated water, saturated steam, and condensed water. That is, full compensation 205. According to system structure, what are the two main categories of control systems? Explain them separately. Answer: Open-loop control system: A control system is considered an open-loop control system if there is only a forward action between the controller and the controlled object, with no reverse action. Without output feedback, the error cannot be corrected. Closed-loop control system: A control system is considered a closed-loop control system if there are both forward and reverse actions between the controller and the controlled object. Also known as a feedback control system. 206. What are absolute pressure, atmospheric pressure, gauge pressure, and vacuum? What is their relationship to each other? Answer: Absolute pressure refers to the total pressure exerted by the medium under measurement on a unit area of the container ； The average pressure generated by the column of air at the ground surface is atmospheric pressure ； The difference between absolute pressure and atmospheric pressure is called gauge pressure ； If the absolute pressure of the fluid being measured is lower than atmospheric pressure, the pressure indicated by the gauge is negative, and this value is referred to as vacuum level. 207. What is the function of the belt scale speed sensor? Answer: The speed sensor converts the rotational speed of the roller, which is proportional to the belt transmission speed, into a frequency signal f. 208. What precautions should be taken when using a gravity sensor? Answer: Be aware of zero drift and temperature drift. 209. What inspections should be carried out on transformers that are in operation? Answer: ① Is the sound normal? ； ②Check whether the transformer has any oil seepage or leakage, and whether the color of the oil and its level are normal ； ③Whether the current and temperature of the transformer exceed the allowable values ； ④Whether the transformer bushing is clean, and whether there are any damages, cracks, or signs of discharge ； ⑤, Is the transformer properly grounded? 210. How to conduct a no-load test on a motor? Answer: Before the test, the motor is inspected; once no issues are found, a three-phase power supply is connected to make the motor spin idly without any load. Next, it is necessary to check the operating condition of the audio equipment, the operation of the bearings, and the three-phase currents. Generally, the no-load current of large-capacity, high-speed motors is 20–35% of their rated current, while that of small-capacity, low-speed motors is 35–50% of their rated current. The no-load current should not be too high or too low, and the three phases must be balanced. The no-load test should last at least 1 hour; meanwhile, the temperature rise of the motor also needs to be measured, with this rise not exceeding the allowable limits based on the insulation class. 211. How to conduct a motor short-circuit test? Answer: The short-circuit test involves using a braking device to prevent the motor rotor from rotating, while gradually increasing the output voltage of the three-phase voltage regulator from zero. The voltage rise stops when the current reaches the motor’s rated current; this voltage is known as the short-circuit voltage. For motors with a rated voltage of 380 volts, their short-circuit voltage is generally between 75 and 90 volts. A high short-circuit voltage indicates too large leakage reactance. 2 A too low short-circuit voltage indicates that the leakage reactance is too small. Both of these are detrimental to the proper operation of the motor. 212. What are the advantages of wound-rotor asynchronous motors compared to squirrel-cage asynchronous motors? Answer: The advantage of wound-rotor asynchronous motors is that an external resistor can be connected in series in the rotor circuit through slip rings and brushes, thereby improving starting performance; moreover, the speed can be adjusted by varying the value of this external resistor within a certain range. However, wound-rotor asynchronous motors have a more complex structure than squirrel-cage asynchronous motors, are more expensive, and have lower reliability in operation. 213. What should be noted in the operation and maintenance of asynchronous motors? Answer: ① The area around the motor should be kept clean ； ②Use instruments to monitor changes in power supply voltage and current. Generally, for motors, the allowable voltage fluctuation is ±5% of the rated voltage; the difference between the three-phase voltages should not exceed 5%, and the imbalance in current across the phases must not be more than 10%. It is also important to determine whether the motor is operating with a missing phase ； ③Regularly check the temperature rise of the motor; a thermometer is commonly used to measure this rise, and it should be ensured that the temperature rise does not exceed the maximum allowable value ； ④Listen for any abnormal noises from the bearings; the seals must be in good condition, and the lubricating oil should be replaced regularly. The typical replacement interval is 1000 hours for sliding bearings and 500 hours for rolling bearings ； ⑤Pay attention to the sound, odor, vibration of the motor, as well as the condition of the transmission mechanism. Under normal operation, the motor should produce a even sound, without any noise or unusual noises. 214. How to determine the cause of a fault based on abnormal vibration and noise in an asynchronous motor? Answer: Abnormal vibration and unusual noises in asynchronous motors are mainly caused by mechanical and electromagnetic factors. Mechanical reasons: ① The motor blades are damaged or the screws that hold them in place are loose, causing the blades to collide with the blade cover; the sound produced as a result varies in intensity depending on the severity of these collisions ； ②If severe eccentricity of the motor rotor occurs due to wear of the bearing or improper shaft alignment, the stator and rotor will come into contact with each other, causing intense vibrations in the motor as well as uneven rubbing noises ； ③The motor experiences abnormal vibrations due to loose anchor bolts or an unstable foundation as a result of long-term use, which in turn causes electromagnetic torque to act on the motor ； ④Motor motors that have been in use for a long time produce abnormal hissing or groaning sounds inside their bearing housings, as a result of a lack of lubricant in the bearings or damage to the steel balls within them. Electromagnetic-related reasons: ① If a motor operating normally suddenly emits abnormal noises and its speed drops significantly while under load, with a low-frequency roaring sound, it may be due to uneven three-phase current, excessive load, or operation in single phase ； ②In a motor that is operating normally, if there is a short-circuit fault in the stator or rotor windings, or if the bars in the squirrel-cage rotor are broken, the motor will emit a humming sound of varying intensity. The fuselage vibrates as well. 215. What are the regulations regarding the control cables for imported cabinets? Answer: ① The cables entering the cabinet should be arranged neatly, without crossing, and fixed so that all terminal boards are not under stress ； ②Cables should not be placed inside the panel; the cut ends of the steel strips should be tied tightly ； ③Control cables used for transistor protection and control; when shielded cables are used, their shielding must be grounded, and if unshielded cables are used, one of the spare conductors must be grounded ； ④Rubber-insulated wires should be covered with an insulating tube for protection ； ⑤The cable cores in the cabinets should be arranged neatly, horizontally and vertically, without any intersections; appropriate space should be left for the spare cores. 216. In a three-phase four-wire power supply system, what is the role of the neutral wire? Why is it not allowed for the neutral wire to be disconnected? Answer: The neutral wire is the common return path in a three-phase circuit. The neutral wire ensures that the three-phase load forms three independent circuits that do not affect each other ； Regardless of whether the loads in each phase are balanced or not, the loads in each phase can withstand symmetric phase voltages; in the event of a fault in one phase, the other two phases can continue to operate normally. If the neutral wire is disconnected, the impedance between the neutral point and the load’s neutral point becomes infinite. In this case, the displacement of the neutral point is at its maximum. The phases with higher power ratings experience a voltage across them that is lower than the rated phase voltage, resulting in dimmer light from the bulbs ； In the phase with a lower wattage rating, the voltage actually applied to the load is higher than the rated voltage, causing the bulb to emit too much light and eventually get damaged. Therefore, the neutral wire must be installed firmly; switches and fuses must not be installed on it to prevent circuit breaks. 217. What are the regulations regarding the operating voltage of asynchronous motors? Why? Answer: The operating voltage of an asynchronous motor shall not be higher than or lower than 5% of the rated value. As the voltage increases, the magnetic flux increases, which leads to an increase in the excitation current. As a result, not only does the iron loss increase and the core heat up, but the windings also experience overheating. When the voltage decreases, it causes the speed to drop and the current to increase. This correspondingly reduces copper loss and causes the windings to heat up. At the same time, efficiency decreased by 218. What are the reasons for the electrification of the motor casing? What are the reasons for the decrease in insulation resistance? Answer: The reasons why the motor casing becomes electrified are: ① Damage to the insulation of the lead wires or junction boxes, resulting in grounding. ②The end of the winding is too long and touches the casing. ③The insulation at the ends of the grooves is damaged. ④The groove insulation is damaged. ⑤The enclosure does not have a reliable ground neutral. The reasons for the decrease in insulation resistance include: ① Moisture penetrating in or rainwater dripping into the motor. ②There is too much dust and dirt on the windings. ③The insulation of the lead wires or junction box contacts is about to fail. ④The insulation ages when the motor overheats. 219. What phenomena occur if the six leads of an asynchronous motor are connected incorrectly? Answer: When one phase of the six leads of an asynchronous motor is connected incorrectly, the currents in each phase become unbalanced, which reduces the motor’s speed and causes a rumbling noise. Furthermore, this will cause the motor to overheat; if the protection device does not activate, the motor will burn out on site. To make full use of the total power output by the power supply, it is necessary to find ways to improve the power factor of the circuit; one method is to connect capacitors in parallel across the inductive load. 220. Using (parallel capacitors) to improve the power factor of a circuit allows the current in that circuit to be reduced, thereby enhancing the load-carrying capacity of the power supply. 221. When a capacitor is connected in parallel with an inductive load, the energy exchange between the power supply and the load is reduced; at this time, most of the reactive power required by the inductive load is supplied by the capacitor. 222. A symmetric three-phase electromotive force refers to three electromotive forces that have equal magnitudes and the same frequency, with phases differing from each other by (120)°. 223. In China, the line voltage for industrial use is mostly 380 volts; when the rated voltage of a three-phase load is 380 volts, the load should be connected in a (triangle) configuration ； If the rated voltage of a three-phase load is 220 volts, the load should be connected in (star) configuration. 224. Motors can be connected in delta or star configuration depending on their rated voltage, whereas lighting circuits can only be connected in star configuration; since the lighting fixtures are divided into three groups based on their power levels, they still represent an asymmetric load, which requires the use of a three-phase four-wire system. 225. Outside a magnet, the direction of magnetic field lines is always from the (N) pole to the (S) pole, while inside the magnet the magnetic field lines go from the S pole to the N pole. 226. As long as the magnetic flux passing through the coil changes, an induced electromotive force is generated in the coil, and the magnitude of this induced electromotive force is proportional to the rate of change of the magnetic flux within the coil. 227. The magnitude of the self-induced electromotive force is (directly proportional) to the rate of change of current in the coil, and the direction of the self-induced electromotive force is always such as to oppose the change in current. 228. In a magnetic circuit, the greater the excitation current, the more magnetic flux is generated ； The more turns in the coil, the greater the magnetic flux generated. Therefore, the product of the excitation current I and the number of coil turns N is called (magnetic motive force). 229. The cores of electrical devices such as motors and transformers are made up of steel sheets that are insulated from one another; this is done in order to reduce (eddy current) losses. 230. Both (hysteresis) losses and eddy current losses can cause the core to heat up. 231. To reduce hysteresis loss, magnetic materials with (narrow hysteresis loops) should be used to manufacture the core. 232. In a DC magnetic circuit, as the air gap in the magnetic circuit increases, the magnetic flux decreases. In an AC magnetic circuit, when the air gap in the magnetic circuit changes, the magnetic flux remains unchanged. 233. The heat resistance grades of insulating materials are divided into seven levels. Answer: They are both classified as Grade Y, with a maximum allowable operating temperature of 90℃ ； Class A, whose maximum allowable operating temperature is 105℃ ； Class E, whose maximum allowable operating temperature is 120℃ ； Class B, whose maximum allowable operating temperature is 130℃ ； Class F, with a maximum allowable operating temperature of 155℃ ； Class H, whose maximum allowable operating temperature is 180℃ ； Class C, whose maximum allowable operating temperature is 180°C. 234. As for so-called constant power speed control, the answer is: essentially, it means that the maximum power that the motor can deliver remains unchanged at different operating speeds. However, the torque changes with the speed of rotation: the higher the speed, the lower the torque output, and the lower the speed, the greater the torque output. Constant torque speed control essentially means that the maximum torque that the motor can deliver remains unchanged at different operating speeds. However, the power varies with the speed of rotation; the higher the speed, the greater the output power, and the lower the speed, the smaller the output power. 235. The Y‑△ reduced-voltage starting method is used for three-phase squirrel-cage asynchronous motors. At startup, the stator windings should be connected in a Y configuration, with each phase winding receiving a voltage of 220 volts. During normal operation, the stator windings should be connected in a Δ configuration, with each phase winding receiving a voltage of 380 volts. When a three-phase AC asynchronous motor is driven using variable frequency speed control, the motor’s speed increases as the power supply frequency rises, and it decreases as the power supply frequency falls. 236. Variable-pole speed control for three-phase AC asynchronous motors involves changing the number of pole pairs in the rotating magnetic field. As the number of pole pairs increases, the speed of the motor decreases ； As the number of pole pairs decreases, the speed of the motor increases. This speed control method is only applicable to (squirrel-cage) asynchronous motors. 237. Three-phase wound-rotor asynchronous motors are speed-controlled by adjusting the (resistance) connected in series in the rotor circuit. 238. For three-phase AC asynchronous motors operating within the region of stable mechanical characteristics, when the load increases, the electromagnetic torque produced by the motor increases, as does its slip rate. 239. Single-phase AC asynchronous motors are classified into two types based on their starting method: split-phase motors and shaded-pole motors. 240. The insulation class of a three-phase AC asynchronous motor indicates the grade of the (insulation material) used in the motor. 241. The efficiency of a three-phase AC asynchronous motor varies with the load. When the load is low, the efficiency is very low ； Efficiency increases as the load increases. Generally, the efficiency is highest when the load is at (0.75~0.8) of the rated power. 242. The requirements for the starting characteristics of three-phase AC asynchronous motors are mainly a sufficiently large (starting torque) and as small as possible (starting current); the starting equipment should be simple, economical, and easy to operate. 243. Three-phase AC asynchronous motors must not operate with one phase missing for an extended period, as this results in excessive current in the other two phases, which can easily lead to the burnout of the motor windings. 244. At the moment of startup, the slip rate S of a three-phase AC asynchronous motor is (S=1) ； When the motor is in operation, the slip rate S is (0 < S < 1) ； When the motor is in synchronization, the slip rate S is (S=0) ； When the motor is in reverse braking mode, the slip rate S is (S＞1). 245. The grounding resistance of the protective grounding wire for the enclosure of a three-phase AC asynchronous motor should be less than (0.10Ω). 246. The methods of reduced-voltage starting used for three-phase squirrel-cage asynchronous motors include starting with stator resistance in series for reduced voltage, star-delta reduced-voltage starting, starting using an autotransformer for reduced voltage, and extended delta reduced-voltage starting. 247. When a three-phase squirrel-cage asynchronous motor is braked, the braking current is generally about (10) times the rated current. 248. Substances with electrical conductivity between that of conductors and insulators are called (semiconductors). 249. To make a diode conduct in the forward direction, the applied forward voltage must be greater than the diode’s (dead zone) voltage. 250. When the reverse voltage applied to a diode exceeds a certain value, the reverse current increases suddenly, and the diode loses its one-way conductivity; this phenomenon is known as diode reverse breakdown. 251. Silicon Zener diodes must operate in the reverse breakdown region in order to exert their (voltage-stabilizing) function. Silicon Zener diodes function as voltage stabilizers in circuits by taking advantage of the fact that, even when the current flowing through them varies over a wide range, the voltage across the diodes remains almost constant. 252. A diode rectifier circuit utilizes the one-way conductivity of diodes to convert alternating current into a unidirectional pulsating (direct current) voltage. 253、In a single-phase full-wave bridge rectifier circuit, the average value of the rectified voltage is (0.9) times the effective value of the AC voltage. 254. An inductor is able to filter because when the current flowing through it changes, an induced electromotive force is generated across the inductor, which acts to prevent that change in current, thereby serving a (filtering) function. 255. The voltage after rectification and filtering tends to (change) due to fluctuations in the AC supply voltage and changes in the load current. At this time, a voltage stabilization circuit can be constructed using silicon Zener diodes to stabilize the output voltage. 256. Both PNP and NPN transistors have (two) PN junctions, which are referred to as the emitter junction and the collector junction respectively ； They all have three regions as well; the one in the middle is called the base region, while those on the sides are called the emitter region and the collector region respectively. 257. Whenever the signal voltage or current at the output of an amplification circuit is partially or entirely fed back to the input through certain means, it is referred to as feedback of the amplification circuit. If the feedback signal amplifies the input signal, it is called positive feedback ； When the feedback signal weakens the input signal and reduces the amplification factor, it is called (negative feedback). 258. To make a thyristor conduct, two conditions must be met simultaneously: a (forward voltage) must be applied between the anode and the cathode, and an appropriate (reverse voltage) must be applied between the control electrode and the cathode. 259. After the thyristor is turned on, even if the voltage applied to the control terminal is removed, the thyristor continues to remain in the on state; this shows that once the thyristor is turned on, the control terminal loses its control function. 260. Once the thyristor is turned on, it remains in the on state regardless of whether there is voltage applied to the control terminal; therefore, during normal operation, a trigger pulse voltage is generally applied to the control terminal. 261. In a single-phase half-wave controlled rectifier circuit with an inductive load, the larger the inductance, the longer the thyristor remains conductive during the negative half-cycle of the supply voltage. As a result, the proportion of negative voltage across the load increases, and the average values of the output voltage and current decrease. To this end, a diode can be connected in parallel across the load; this component is known as a freewheeling diode. 262. In a single-phase bridge half-controlled rectifier circuit, connecting two thyristors in series can prevent loss of control. In this circuit, the conduction angle of the thyristor is (π–α), and the conduction angle of the diode is (π+α). 263. A bridge-type semi-controlled rectifier circuit with thyristors in series can do without a freewheeling diode; however, since the cathodes of the two thyristors do not share a common point, when using a single triggering circuit, a (pulse transformer) with two secondary coils must be used for power supply. 264. In a three-phase half-wave controlled rectifier circuit with a resistive load, if a trigger pulse is applied before the natural commutation point, phase loss occurs. To avoid this phenomenon, the (phase shift range) of α should be controlled, or the trigger pulse should have a certain (width). 265. In a three-phase half-controlled bridge rectifier circuit, the maximum conduction angle of the thyristor is (120)°, the maximum phase-shifting range of the trigger pulse is (180)°, and the interval between trigger pulses is (120)°. 266. Under high-voltage conditions, several thyristors can be used in series. In high-current situations, several thyristors can be used in parallel. 267. The protection measures against thyristor overvoltage generally include RC protection and selenium stack protection. When there is a high risk of overvoltage at the high-power rectifier output connected directly to the main power grid, thyristors can be used for (overvoltage) protection. 268. The three types of dangerous lines that are prohibited from use are (crossing) lines, (ground-crawling) lines, and (meeting) lines. 269. The actual direction of electromotive force is (opposite) to the actual direction of voltage. 270. If the actual directions of voltage and current in a certain component are opposite, it indicates that this component has the properties of a (power source).