How does the frequency converter work?

　(frequency changer / frequency converter) is an electrical device used to change the frequency of alternating current. In addition, it has an auxiliary function to change the AC voltage. 　　In the past, frequency converters were generally included in electrical equipment such as motor generators and rotary converters. With the advent of semiconductor electronics, it has become possible to produce completely independent frequency converters. 　　Frequency converters usually consist of 2 components: Rectifier and Inverter. Among them, the rectifier converts the input alternating current into direct current, and the inverter converts the direct current into alternating current at the required frequency. In addition to these 2 parts, the inverter may also contain a transformer and battery. Among them, the transformer is used to change the voltage and can isolate the input/output circuit, and the battery is used to compensate for the energy loss on the internal circuit of the frequency converter. 　　Different frequency converters can handle different power supplies, ranging from a few watts to several megawatts. Composition of the frequency converter The frequency converter is mainly composed of the main circuit and the control circuit. 　　The main circuit is the power conversion part that provides voltage and frequency modulation power to the asynchronous motor. The main circuit of the frequency converter can be roughly divided into two categories: the voltage type is a frequency converter that converts the DC voltage source into AC, and the filter of the DC circuit is a capacitor. The current type is an inverter that converts DC from a current source into AC, and its DC loop filter is an inductor. It consists of three parts: a "rectifier" that converts commercial frequency power into DC power, a "smoothing circuit" that absorbs voltage pulsations generated by converters and inverters, and an "inverter" that converts DC power into AC power. 　　(1) Rectifier: Recently, diode converters are widely used, which convert power frequency power into DC power. Two sets of transistor converters can also be used to form a reversible converter. Since its power direction is reversible, it can perform regenerative operation. 　　(2) Smoothing loop: The DC voltage rectified by the rectifier contains a pulsating voltage six times the frequency of the power supply. In addition, the pulsating current generated by the inverter also causes the DC voltage to fluctuate. In order to suppress voltage fluctuations, inductors and capacitors are used to absorb pulsating voltage (current). When the device capacity is small, if the power supply and main circuit components have margin, the inductor can be omitted and a simple smoothing circuit can be used. 　　(3) Inverter: Contrary to the rectifier, the inverter converts DC power into AC power at the required frequency. By turning on and off six switching devices at a determined time, a three-phase AC output can be obtained. Taking a voltage-type PWM inverter as an example, the switching time and voltage waveform are shown. 　　The control circuit is a circuit that provides control signals to the main circuit that supplies power to the asynchronous motor (voltage and frequency are adjustable). It consists of an "operation circuit" for frequency and voltage, a "voltage and current detection circuit" for the main circuit, a "speed detection circuit" for the motor, a "drive circuit" that amplifies the control signal of the operation circuit, and a "protection circuit" for the inverter and motor. 　　(1) Arithmetic circuit: The external speed, torque and other instructions are compared with the current and voltage signals of the detection circuit to determine the output voltage and frequency of the inverter. 　　(2) Voltage and current detection circuit: Isolated from the main circuit potential to detect voltage, current, etc. 　　(3) Drive circuit: The circuit that drives the main circuit devices. It is isolated from the control circuit to turn on and off the main circuit components. 　　(4) Speed ​​detection circuit: The signal of the speed detector (tg, plg, etc.) installed on the asynchronous motor shaft is used as the speed signal and sent to the calculation loop. According to the instructions and calculations, the motor can run at the command speed. 　　(5) Protection circuit: detects the voltage, current, etc. of the main circuit. When overload or overvoltage and other abnormalities occur, in order to prevent the inverter and asynchronous motor from being damaged, the inverter is stopped or the voltage and current values ​​are suppressed. The types of frequency converters are based on international practice and our country’s * * According to the standard classification of voltage levels, when the power supply voltage is ≥10KV, it is called high voltage, and when the power supply voltage is 1kV ~ 10KV, it is called medium voltage. us * Motors with rated voltages of 6kV or 3kV are also traditionally called "high-voltage motors". (1) Classification according to different converter links: 1. The AC-DC-AC frequency converter first "rectifies" the AC power with a fixed frequency into DC power, and then "inverts" the DC power into a three-phase AC power with an adjustable frequency. 2. AC-AC frequency converter directly converts AC power with fixed frequency into AC power with adjustable frequency (the number of phases before and after conversion is the same). (2) Classification according to the energy storage link (filtering method) of the DC circuit: 1. The energy storage component of voltage-type inverter is a capacitor. Medium and small-capacity inverters are mainly voltage-type inverters. 2. The energy storage element of the current-type inverter is an inductor coil. (3) Classification according to voltage modulation method: 1. The voltage of the pulse width modulation (SPWM) inverter is achieved by adjusting the pulse duty cycle. Almost all medium- and small-capacity general-purpose inverters use this type of inverter. 2. The voltage of the pulse amplitude modulation (PAM) inverter is achieved by adjusting the DC voltage amplitude. (4) Classification according to the number of phases of the input power supply: 1. Three-input three-output inverter The input side and output side of the inverter are three-phase alternating current. Most frequency converters fall into this category. 2. Single-input three-output inverter The input side of the inverter is single-phase alternating current, and the output side is three-phase alternating current. Frequency converters in household appliances fall into this category and usually have smaller capacities. This post was last written by no1_pp edited on 2009-1-7 09:24 ]

Frequency converter is a power control device that uses the on-off function of power semiconductor devices to convert industrial frequency power into another frequency. The frequency converters we use now mainly use AC-DC-AC mode (VVVF frequency conversion or vector control frequency conversion). First, the industrial frequency AC power is converted into DC power through a rectifier, and then the DC power is converted into AC power with controllable frequency and voltage to supply the motor. The circuit of the frequency converter generally consists of four parts: rectifier, intermediate DC link, inverter and control. The rectifier part is a three-phase bridge uncontrollable rectifier, the inverter part is an IGBT three-phase bridge inverter, and the output is a PWM waveform. The intermediate DC link is filtering, DC energy storage and buffering reactive power. Frequency converter selection: The following points should be determined when selecting an inverter:: 1) The purpose of using frequency conversion ； Constant voltage control or constant current control, etc. 2) Load type of inverter ； For example, vane pumps or positive displacement pumps, etc., pay special attention to the performance curve of the load. The performance curve determines the application method. 3) Matching problem between inverter and load ； I. Voltage matching ； The rated voltage of the inverter matches the rated voltage of the load. II. Current matching ； For ordinary centrifugal pumps, the rated current of the frequency converter matches the rated current of the motor. For special loads such as deep water pumps, you need to refer to the motor performance parameters and determine the inverter current and overload capacity based on the maximum current. III.Torque matching ； This situation may occur when there is a constant torque load or a reduction device. 4) When using a frequency converter to drive a high-speed motor, due to the small reactance of the high-speed motor, the increase in high-order harmonics results in an increase in the output current value. Therefore, the capacity of the frequency converter used for high-speed motors is slightly larger than that of ordinary motors. 5) If the frequency converter needs to run with a long cable, measures must be taken to suppress the impact of the long cable on the ground coupling capacitance to avoid insufficient output of the frequency converter. Therefore, in this case, the frequency converter capacity must be enlarged by one level or an output reactor must be installed at the output end of the frequency converter. 6) For some special applications, such as high temperature and high altitude, the frequency converter will be derated, and the frequency converter capacity must be enlarged by one gear. Frequency converter control schematic design: 1) First confirm the installation environment of the inverter ； I. Working temperature. There are high-power electronic components inside the inverter, which are easily affected by the operating temperature. The product generally requires a temperature range of 0 to 55°C. However, in order to ensure safe and reliable work, allowance should be made when using it. It is best to control it below 40°C. In the control box, the frequency converter should generally be installed on the upper part of the box, and the installation requirements in the product manual must be strictly followed. Heating elements or components that are prone to heat are absolutely not allowed to be installed close to the bottom of the frequency converter. II. Ambient temperature. When the temperature is too high and the temperature changes greatly, condensation will easily occur inside the inverter, and its insulation performance will deteriorate. * * Reduction may even cause a short circuit accident. If necessary, desiccant and heater must be added to the box. In the water treatment room, water vapor is generally relatively heavy. If the temperature changes greatly, this problem will be more prominent. III. Corrosive gases. If the concentration of corrosive gas is high in the use environment, it will not only corrode the leads of components, printed circuit boards, etc., but also accelerate the aging of plastic components and reduce the insulation performance. IV. Vibration and Shock. When the control cabinet equipped with a frequency converter is subjected to mechanical vibration and impact, poor electrical contact may occur. Huaian Thermal Power has such a problem. At this time, in addition to improving the mechanical strength of the control cabinet and keeping it away from vibration and impact sources, anti-seismic rubber pads should also be used to fix vibration-generating components such as electromagnetic switches outside and inside the control cabinet. After the equipment has been running for a period of time, it should be inspected and maintained. V. Electromagnetic interference. Due to the rectification and frequency conversion of the frequency converter during operation, a lot of interfering electromagnetic waves are generated around it. These high-frequency electromagnetic waves have certain interference on nearby instruments and instruments. Therefore, the instruments and electronic systems in the cabinet should use metal casings to shield the interference from the frequency converter on the instruments. All components should be reliably grounded. In addition, shielded control cables should be used for connections between electrical components, instruments and meters, and the shielding layer should be grounded. If electromagnetic interference is not handled well, the entire system will often fail to work, causing the control unit to malfunction or be damaged. 2) The distance between the inverter and the motor determines the cable and wiring methods ； I. The distance between the inverter and the motor should be as short as possible. This reduces the cable's capacitance to ground and reduces the sources of interference. II. Use shielded cables for control cables, shielded cables for power cables, or use conduit shielding from the inverter to the motor. III. The motor cable should be routed independently from other cables, with a minimum distance of 500mm. At the same time, long-distance parallel routing of motor cables and other cables should be avoided to reduce electromagnetic interference caused by rapid changes in the inverter output voltage. If control cables and power cables cross, they should be crossed at a 90-degree angle whenever possible. The analog signal lines related to the frequency converter must be routed separately from the main loop lines, even in the control cabinet. IV. It is best to use shielded twisted pairs for analog signal lines related to the inverter, and shielded three-core cables for power cables (the specifications of which are larger than those of ordinary motor cables) or follow the user manual of the inverter. 3) Inverter control schematic diagram ； I.Main loop: The function of the reactor is to prevent the high-order harmonics generated by the frequency converter from returning to the power grid through the input circuit of the power supply and thus affecting other powered equipment. It is necessary to decide whether to add a reactor according to the capacity of the frequency converter. ； The filter is installed at the output end of the frequency converter to reduce the high-order harmonics output by the frequency converter. When the distance between the frequency converter and the motor is long, the filter should be installed. Although the frequency converter itself has various protection functions, the phase loss protection is not perfect. The circuit breaker provides overload and phase loss protection in the main circuit. The selection can be based on the capacity of the frequency converter. The thermal relay can be replaced by the overload protection of the frequency converter itself. II. Control loop: It has manual switching of power frequency conversion, so that when the frequency conversion fails, it can be manually switched to power frequency operation. Since voltage cannot be applied to the output end, the fixed power frequency and frequency conversion must be interlocked. 4) Grounding of the frequency converter ； Correct grounding of the frequency converter is an important means to improve system stability and suppress noise. The grounding resistance of the grounding terminal of the frequency converter should be as small as possible. The cross-section of the grounding wire should not be less than 4mm and the length should not exceed 5m. The grounding point of the frequency converter should be separated from the grounding point of the power equipment and cannot be grounded together. One end of the shielding layer of the signal line is connected to the ground terminal of the frequency converter, and the other end is left floating. The frequency converter and the control cabinet are electrically connected. Frequency converter control cabinet design: The frequency converter should be installed inside the control cabinet. When designing the control cabinet, attention should be paid to the following issues 1) Heat dissipation issues: The heat generated by the frequency converter is generated by internal losses. Among the losses in various parts of the frequency converter, the main circuit is mainly the main one, accounting for about 98%, and the control circuit accounts for 2%. In order to ensure the normal and reliable operation of the frequency converter, the frequency converter must be cooled. We usually use fans to dissipate heat. ； The built-in fan of the frequency converter can dissipate heat away from the inside of the frequency converter box. If the fan cannot work properly, the frequency converter should be stopped immediately. ； High-power inverters also need to add fans to the control cabinet. The air ducts of the control cabinet must be reasonably designed. Dust screens must be installed at all air inlets to ensure smooth exhaust to avoid the formation of eddy currents in the cabinet and dust accumulation in fixed locations. ； Select a matching fan according to the ventilation volume in the inverter manual. Pay attention to shockproof issues when installing the fan. 2) Electromagnetic interference problem: I. Due to the rectification and frequency conversion of the inverter during operation, a lot of interfering electromagnetic waves are generated around it. These high-frequency electromagnetic waves have certain interference with nearby instruments and instruments, and will produce high-order harmonics. This high-order harmonics will enter the entire power supply network through the power supply loop, thereby affecting other instruments. If the power of the frequency converter is very large and accounts for more than 25% of the entire system, anti-interference measures for the control power supply need to be considered. II. When there are high-frequency impact loads in the system, such as welding machines and electroplating power supplies, the inverter itself will be protected due to interference, and the power quality of the entire system must be considered. 3) The following points need to be noted for protection issues: I. Waterproof and anti-condensation: If the frequency converter is placed on site, you need to pay attention to the fact that there are no pipe flanges or other leaking points above the frequency converter cabinet, and there must be no splashing water near the frequency converter. In short, the protection level of the on-site cabinet must be above IP43. II. Dustproof: All air inlets should be equipped with dust-proof nets to block the entry of flocculent debris. The dust-proof nets should be designed to be removable to facilitate cleaning and maintenance. The grid of the dust-proof net is determined according to the specific conditions of the site, and the connection between the dust-proof net and the control cabinet must be handled tightly. III. Protection against corrosive gases: This situation is more common in the chemical industry. At this time, the frequency conversion cabinet can be placed in the control room. Frequency converter wiring specifications: Signal lines and power lines must be routed separately: When using analog signals to remotely control the inverter, in order to reduce the interference of the analog signal from the inverter and other equipment, please separate the signal lines that control the inverter from the strong current loop (main loop and sequence control loop). The distance should be more than 30cm. Even in the control cabinet, such wiring specifications must be maintained. The longest control loop line between this signal and the inverter must not exceed 50m. Signal lines and power lines must be placed inside different metal pipes or metal hoses.: If the signal line connecting the PLC and the frequency converter is not placed in a metal pipe, it will be easily interfered by the frequency converter and external equipment. ； At the same time, since the frequency converter does not have a built-in reactor, the input and output stage power lines of the frequency converter will cause strong interference to the outside. Therefore, the metal tube or metal hose where the signal line is placed must be extended to the control terminal of the frequency converter to ensure that the signal line and the power line are completely separated. 1) The analog control signal line should use twisted pair shielded wire, and the wire specification is 0.75mm2. Be sure to pay attention when wiring. The cable stripping should be as short as possible (about 5-7mm). At the same time, the shielding layer after stripping should be wrapped with insulating tape to prevent the shielding wire from contacting other equipment and causing interference. 2) In order to improve the simplicity and reliability of wiring, it is recommended to use crimping bar terminals on signal lines. Operation of the frequency converter and setting of related parameters: The frequency converter has many setting parameters, and each parameter has a certain selection range. During use, it is often encountered that the frequency converter cannot work properly due to improper settings of individual parameters. Control method: That is, speed control, torque control, PID control or other methods. After the control method is adopted, static or dynamic identification is generally required based on the control accuracy. Minimum operating frequency: That is, the minimum speed at which the motor runs. When the motor runs at low speed, its heat dissipation performance is very poor. If the motor runs at low speed for a long time, it will cause the motor to burn out. Moreover, at low speeds, the current in the cable will also increase, which will also cause the cable to heat up. Maximum operating frequency: The maximum frequency of the general inverter is 60Hz, and some even reach 400Hz. The high frequency will make the motor run at high speed. For ordinary motors, the bearings cannot run at over-rated speed for a long time. Whether the rotor of the motor can withstand such centrifugal force. Carrier frequency: The higher the carrier frequency is set, the greater the high-order harmonic component will be, which is closely related to factors such as the length of the cable, motor heating, cable heating, and inverter heating. Motor parameters: The inverter sets the power, current, voltage, speed, and maximum frequency of the motor in parameters. These parameters can be obtained directly from the motor nameplate. frequency hopping: At a certain frequency point, resonance may occur, especially when the entire device is relatively high ； When controlling the compressor, avoid compressor surge points. Common fault analysis: 1) Overcurrent fault: Overcurrent faults can be divided into acceleration, deceleration, and constant speed overcurrent. It may be caused by the inverter's acceleration and deceleration time being too short, load mutation, uneven load distribution, output short circuit, etc. At this time, it is generally possible to extend the acceleration and deceleration time, reduce sudden changes in load, add energy-consuming braking components, carry out load distribution design, and inspect the line. If the overcurrent fault persists after disconnecting the load inverter, it means that the inverter circuit has gone into a loop and the inverter needs to be replaced. 2) Overload fault: Overload faults include frequency conversion overload and motor overload. It may be caused by the acceleration time being too short, the grid voltage being too low, the load being too heavy, etc. Generally, you can extend the acceleration time, extend the braking time, check the grid voltage, etc. The load is too heavy and the selected motor and frequency converter cannot drag the load. It may also be caused by poor mechanical lubrication. If the former is the case, high-power motors and frequency converters must be replaced. ； If the latter is the case, the production machinery will need to be inspected and repaired. 3) Undervoltage: It means there is a problem with the power input part of the inverter and it needs to be checked before it can be run.

Factory motors mostly use induction AC motors (hereinafter referred to as motors), and their rotation speed is approximately determined by the number of poles and frequency of the motor. The number of poles of the motor is determined by the working principle of the motor. Since the pole value is not a continuous value (it is a multiple of 2, for example, the number of poles is 2, 4, 6), it is not suitable to change the value to adjust the speed of the motor. In addition, the frequency is the electrical signal of the motor power supply, so this value can be adjusted outside the motor and then supplied to the motor, so that the rotation speed of the motor can be freely controlled. Therefore, the frequency converter with the purpose of controlling frequency is the preferred equipment for motor speed regulation equipment. n = 60f/p, n: synchronous speed, f: power frequency, p: number of motor poles, changing frequency and voltage is the optimal motor control method. If you only change the frequency, the motor will be burned out. Especially when the frequency is reduced, this problem is very prominent. In order to prevent motor burnout accidents, the inverter must change the voltage at the same time when changing the frequency, for example: In order to halve the rotation speed of the motor, the output frequency of the frequency converter must be changed from 60Hz to 30Hz. At this time, the output voltage of the frequency converter must be changed from 200V to about 100V.

So why is it that a motor with a rated voltage of 380 can still run at 200v after frequency conversion? I am assuming here that if the power required by the outside world remains unchanged, the current will inevitably increase when the voltage decreases. Is this not conducive to the operation of the motor? Will there be overcurrent and burn the motor?

6#cgy_When the 5437 motor is driven by industrial frequency power supply, the current increases when the voltage drops; for inverter drive, if the voltage also drops when the frequency drops, does the current increase? When the frequency decreases (low speed), if the same power is output, the current will increase, but under the condition of constant torque, the current will almost remain unchanged. When running with a frequency converter, what are the starting current and starting torque of the motor? Using a frequency converter to operate, the frequency and voltage increase accordingly as the motor accelerates, and the starting current is limited to less than 150% of the rated current (125% to 200% depending on the model). When starting directly with industrial frequency power supply, the starting current is 6~7 times, so there will be mechanical and electrical impact. Using frequency converter drive can start smoothly (starting time becomes longer). The starting current is 1.2~1.5 times the rated current, and the starting torque is 70%~120% of the rated torque. ； For inverters with automatic torque enhancement function, the starting torque is above 100% and can be started with full load.

In general, most loads are not constant. In order to facilitate smooth control, frequency conversion control is now often used. The simplest understanding is that when AC power supply - DC - AC output changes the corresponding frequency, thereby changing the speed of the motor. The frequency converter is mainly composed of rectification (AC to DC), filtering, re-rectification (DC to AC), braking unit, drive unit, detection unit, microprocessing unit, etc. Principle of Frequency Converter Frequency converter is an electric energy control device that uses the on-off function of power semiconductor devices to convert industrial frequency power into another frequency. The frequency converters we use now mainly use AC-DC-AC mode (VVVF frequency conversion or vector control frequency conversion). First, the industrial frequency AC power is converted into DC power through a rectifier, and then the DC power is converted into AC power with controllable frequency and voltage to supply the motor. The circuit of the frequency converter generally consists of four parts: rectifier, intermediate DC link, inverter and control. The rectifier part is a three-phase bridge uncontrollable rectifier, the inverter part is an IGBT three-phase bridge inverter, and the output is a PWM waveform. The intermediate DC link is filtering, DC energy storage and buffering reactive power.

The speed of the motor depends on the frequency, so frequency conversion is basically equivalent to speed change. Frequency converters mainly include current type and voltage type. Currently, small and medium-sized motors mainly use voltage type.

In the past, I only knew how to adjust the flow rate of the pump by adjusting the Hertz number, but for some reason, I understand everything after reading this. Thank you, sir.

Frequency converter is a power control device that uses the on-off function of power semiconductor devices to convert industrial frequency power into another frequency. The frequency converters we use now mainly use AC-DC-AC mode (VVVF frequency conversion or vector control frequency conversion). First, the industrial frequency AC power is converted into DC power through a rectifier, and then the DC power is converted into AC power with controllable frequency and voltage to supply the motor. The circuit of the frequency converter generally consists of four parts: rectifier, intermediate DC link, inverter and control. The rectifier part is a three-phase bridge uncontrollable rectifier, the inverter part is an IGBT three-phase bridge inverter, and the output is a PWM waveform. The intermediate DC link is filtering, DC energy storage and buffering reactive power. Classification of ordinary frequency converters: (1) Classification according to different converter links: 1. The AC-DC-AC frequency converter first "rectifies" the AC power with a fixed frequency into DC power, and then "inverts" the DC power into a three-phase AC power with an adjustable frequency. 2. AC-AC frequency converter directly converts AC power with fixed frequency into AC power with adjustable frequency (the number of phases before and after conversion is the same). (2) Classification according to the energy storage link (filtering method) of the DC circuit: 1. The energy storage component of voltage-type inverter is a capacitor. Medium and small-capacity inverters are mainly voltage-type inverters. 2. The energy storage element of the current-type inverter is an inductor coil. (3) Classification according to voltage modulation method: 1. The voltage of the pulse width modulation (SPWM) inverter is achieved by adjusting the pulse duty cycle. Almost all medium- and small-capacity general-purpose inverters use this type of inverter. 2. The voltage of the pulse amplitude modulation (PAM) inverter is achieved by adjusting the DC voltage amplitude. (4) Classification according to the number of phases of the input power supply: 1. Three-input three-output inverter The input side and output side of the inverter are three-phase alternating current. Most frequency converters fall into this category. 2. Single-input three-output inverter The input side of the inverter is single-phase alternating current, and the output side is three-phase alternating current. Frequency converters in household appliances fall into this category and usually have smaller capacities.