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This article, which is part of the essential technical content on electrical engineering fundamentals for electricians and instrument technicians, teaches them to start with basic knowledge such as the principle and structure of contactors, their basic classifications, key technical parameters, and parameters of control circuits. It then guides them to gradually gain a deeper understanding of methods for calculating current in contactors as well as the expertise required for selecting AC contactors. Calculation of contactor current: yunrun.com.cn/tech/2854.html. A contactor is an electrical device used to automatically connect or disconnect circuits under load. It can repeatedly switch AC and DC circuits, enable remote operation and control, and can be used in conjunction with relays to achieve timed operations, interlock functions, various types of quantitative control, as well as protection against voltage loss and low voltage. The main control target of an AC contactor is electric motors, but it can also be used to control other electrical loads, such as electric heaters, lighting fixtures, capacitors, etc. AC contactors feature large control capacity, strong overload resistance, long service life, and simplicity and cost-effectiveness; they are among the most widely used low-voltage electrical devices in power drive and automatic control circuits. http://yunrun.com.cn/upload/201912/29/201912290050574555.png Figure 1: Schematic diagram of the AC contactor. In Figure 1, we can see that the main contacts of the contactor have a bridge-type structure with double breaks. When the coil is energized, the armature moves downward, causing the moving contact to make contact with the stationary contact. Since the direction of current in the moving and stationary contacts is opposite, an electric repulsive force is generated between them. The pressure spring leaf of the moving contact is used to eliminate the effect of electrostatic repulsion. When the coil loses power, the moving and stationary main contacts return to their released position under the combined action of the buffer springs, contact springs, and electrodynamic repulsion. At the moment when the main contacts open, an arc is generated between the moving and stationary contacts, and the purpose of the arc extinguishing cover is to extinguish this arc. The rated making capacity of an AC contactor refers to the current value that can be switched on under specified conditions, without the contacts suffering from welding, significant burnout, or excessive arcing. The rated breaking capacity of a contactor refers to the current value that it can break under specified conditions, without the contacts being damaged to the point where they can no longer function, nor without excessive arcing occurring. The electrical life of an AC contactor is a parameter that indicates the contactor’s ability to withstand electrical wear, and it is expressed in terms of the number of on-off cycles under load. When measuring the electrical life of an AC contactor, maintenance and part replacement are not allowed. http://yunrun.com.cn/upload/201912/29/201912292258444179.png Figure 2: Graphic symbol of a contactor (the upper part represents the primary contacts, the middle part the secondary contacts, and the lower part the drive coil). Classification of AC contactors: 1. Classified by the number of poles of the main contacts – single-pole, double-pole, three-pole, four-pole, and five-pole contactors. Single-pole contactors are mainly used for single-phase loads, such as lighting circuits and welding machines ； Bipolar contactors are used in the rotor circuit of wound-rotor asynchronous motors, where they serve to short-circuit the starting windings during motor startup. Tripole contactors are used for the control of three-phase motors. Quadrupole contactors are employed in three-phase four-wire lighting circuits as well as in two-speed motors. Pentapole contactors are used to form the autotransformer starting circuit of motors, and they also serve to control the control circuit of two-speed motors in order to change the winding connection. 2. Classification by arc-extinguishing medium: AC contactors can be classified into air-insulated contactors, vacuum contactors, etc., based on their arc-extinguishing medium. Contactors with air insulation are used for general loads, while those with vacuum insulation are employed in special environments such as coal mines, the petrochemical industry, and in applications with voltages of 660V and 1140V. 3. Classification by the presence or absence of contacts: AC contactors can be classified into contactors with contacts and contactors without contacts, based on whether they have contacts or not. Common contactors are those with contacts, while contactors without contacts use thyristors as the elements for switching the circuit on and off; they are often used in flammable and explosive environments. Basic technical parameters of contactors 1. Rated voltage of AC contactors: The rated voltage of an AC contactor refers to the rated operating voltage of its main contacts, and it should be equal to the rated operating voltage of the load. AC contactors generally come in several rated voltage values, and the corresponding rated current or control power is specified in the technical specifications. Typically, the maximum operating voltage is the rated voltage, such as 220-230V, 230-240V, 380-400V, and 400-415V, etc. 2. Rated current: The rated current of a contactor refers to the rated current value of its main contacts. The commonly used rated current levels are 9A, 12A, 16A, 26A, 30A, 40A, 50A, 63A, 75A, 95A ； 110A, 145A, 150A, 175A, 210A, 260A ； 300A, 375A, 550A, 1000A, 1350A, 1650A, and 2000A, etc. 3. Making and breaking capacity of the contactor: The making and breaking capacity of a contactor includes two parameters: the maximum making current and the maximum breaking current. The maximum making current refers to the maximum current value at which the contacts are closed without causing welding of the contacts, while the maximum breaking current is the maximum current at which arcing can be reliably extinguished when the contacts are separated. The normal on-off capacity is 5-10 times the rated current. The switching capacity is related to the voltage level; the higher the voltage level, the lower the switching capacity. For example, AC contactors operating under AC-2, AC-3, and AC-4 should be able to withstand overcurrents up to 8 times the maximum rated operating current for category AC-3. The holding time for contactors of 630A and below is 10s, while it is slightly reduced for contactors above 630A. The application categories and on/off conditions of AC contactors are shown in the table. Table 1 Usage categories and on/off conditions of AC contactors http://yunrun.com.cn/upload/201912/29/201912292250218584.png Note 1: In Table 1, I represents the current during operation ； In is the rated current ； Ib is the breaking current ； U is the voltage before switching on ； Un is the rated voltage ； Ur is the recovery voltage. Note 2, AC-1: The error for cosΦ is ±0.05, and the error for L/R is ±15% ； Note 3, AC-2: The minimum value of I or Ib is 1000A ； Note 4, AC-3: The minimum value of Ib is 800A ； Note 5, AC-4: The minimum value of I is 1200A. 4. Operating values: The operating values of a contactor are divided into the closing voltage and the releasing voltage. The closing voltage refers to the minimum voltage required to cause an AC contactor to close by gradually increasing the coil voltage before the contactor closes ； The release voltage refers to the maximum voltage at which the coil voltage is slowly reduced to cause the AC contactor to release. General rule: The holding voltage shall not be lower than 85% of the coil’s rated voltage, while the releasing voltage shall not be higher than 70% of the coil’s rated voltage. 5. Operating frequency: The operating frequency of a contactor refers to the maximum number of operations allowed per hour. The number of operations allowed per hour can be: 1 time/h, 3 times/h, 12 times/h, 30 times/h, 120 times/h, 300 times/h, 600 times/h, 1200 times/h, and 3000 times/h. The operating frequency affects the electrical life of the AC contactor, as well as the temperature rise of its coil. 6. Operating Modes: Contactors come in four operating modes, namely an 8-hour operating mode, continuous operation mode, intermittent periodic operation mode, and short-duration operation mode. The 8-hour operating mode is the basic operating mode for contactors, and the parameters regarding the heating current are determined based on this 8-hour operating mode ； The continuous operation regime is much more demanding than an 8-hour work schedule; the contacts of the contactors are prone to oxidation while the coils are prone to overheating. Under continuous operation, the contactor needs to be used at a reduced capacity ； The load factors for the intermittent periodic operation mode are 15%, 25%, 40%, and 60% of the standard values respectively ； Under the short-hour working system, the standard values for the energization time of contacts are 10 min, 30 min, 60 min, and 90 min, among others. 7. Application categories: Contactors have four standard application categories, namely AC-1, AC-2, AC-3, and AC-4. Among them, AC-3 is used for the direct starting and operation of the motor, while AC-4 is used for the reversible starting, back-driving braking, and operation of the motor. 8. Mechanical life and electrical life: The mechanical life of a contactor refers to the number of no-load cycling operations it can withstand before normal maintenance and replacement of mechanical parts are required. The electrical life of a contactor refers to the number of load-operating cycles it can perform under standard conditions, without the need for repairs or part replacement. In the absence of other specified conditions, the electrical life cycle count for contactor AC-3 in its designated usage category shall be no less than 1/20 of the corresponding mechanical life cycle count. Control circuit parameters: Rated voltage of the attraction coil: The voltage applied to the coil when the contactor is operating normally. When an AC contactor is in operation, the voltage applied to its coil is often the same as the voltage in the main circuit, but it may also differ; sometimes a DC power source is used instead. This is determined by the site conditions and design. The voltage applied to the coil of the AC contactor is a standard value, as shown in Table 2. Table 2: Voltage standard data for AC contactor coils http://yunrun.com.cn/upload/201912/29/201912292255490236.png Selection of AC contactors: There are 7 principles for selecting AC contactors, as follows: 1. Choose the number of poles of the contactor. 2. Select the parameters of the main circuit, including the rated operating voltage, rated operating current, rated switching capacity, and overload tolerance. 3. Select appropriate control circuit parameters. 4. Select the appropriate electrical life and usage category. 5. For contactors used in motors, considerations must be made based on the operating conditions of the motor. For motors that operate in one direction, such as fans and water pump loads, AC contactors of the AC-3 category can be used ； For reversible motors, the current drawn during reverse operation, jogging, and back-emf braking can exceed 8 times the rated current; therefore, AC contactors of the AC-4 category should be used. When the motor’s power is not greater than 630 kW, the contactor should be able to withstand 8 times the rated current for at least 10 seconds. There is an empirical formula for selecting the rated current of the AC contactor to be used in the motor circuit: Ie = (PM×103)/(K×UN), where PM represents the power of the motor, in kW ； Rated voltage of UN ； Ie is the rated current of the AC contactor ； K is an empirical coefficient, typically taking values between 1 and 1.4. For ordinary motors, the operating current is always less than the rated current. Although the starting current of a motor can reach 4 to 8.4 times the rated current, this occurs for a short period of time, so it has little effect on the wear of the main contacts of the contactor. Therefore, a K factor of 1.25 for the rated current of the AC contactor is sufficient. For example, if the power of the motor is 30 kW, using the formula Ie = (PM×10³)/(K×UN), we get Ie = (30×1000)/(1.25×380) ≈ 63.2 A. Therefore, the rated current for the AC contactor should be set at 63 A. It should be noted that the rated making and breaking capacity of a contactor must be higher than the current values that may occur in the circuit during making and breaking operations, while its ability to withstand overload currents must be greater than the overload current values that may appear in the circuit. Since all these parameters in the circuit can be determined by using the category and operating mode, it is reasonable to select contactors based on the category and operating mode. This is also the basis for using the contactor selection table provided by the contactor manufacturer. For wound-rotor asynchronous motors, both the current applied and the current interrupted are 2.5 times the rated current; therefore, AC contactors of category AC-2 can be used. 6. Principles for selecting AC contactors for electric heating equipment: They can be chosen according to the AC-1 classification. When selecting a contactor, ensure that its rated current is 1.2 times or greater than the rated current of the electric heating device. 7. Principles for selecting switching capacitor contactors: Since the charging current of a capacitor can reach 1.43 times its rated current, when choosing such contactors, it is necessary to take 1.5 times the capacitor’s rated current into account. Physical wiring of AC contactors and motors 1. Schematic diagram of the physical connection for controlling a three-phase asynchronous motor: http://yunrun.com.cn/upload/201912/28/201912282253436947.png http://yunrun.com.cn/upload/201912/28/201912282259432697.png 2. Physical wiring diagrams for forward/reverse control plus limit control of motors: http://yunrun.com.cn/upload/201912/28/201912282254182065.png http://yunrun.com.cn/upload/201912/28/201912282258323110.png Questions and answers about contactors 1. What are the functions of contactors A1 and A2? Answer: An AC contactor uses a coil to attract and close the contacts; it is only when the contacts are closed that the power supply can be connected, allowing L1, L2, L3 at the input terminals to be connected to T1, T2, T3 at the corresponding output terminals. A1 and A2 are the power interfaces for the contactor coils, and they are the source that controls the operation of the coils. 2. How should the voltage for the contactor coil be selected? Answer: The coil voltage of the AC contactor can be selected based on the output voltage of the controller that controls the opening and closing of the contactor. For example: if the controller at the higher level that controls the contactor operates at 220V, then the coil voltage should be selected as 220V; the actual voltage of the contactor coil is determined based on specific circumstances. A1 and A2 are the power supply terminals for the contactor coil; if it is 220V, A1 and A2 are connected to one live wire and one neutral wire respectively, while if it is 380V, they are connected to two live wires. 3. What is the difference between **10 and **01 on the contactor? Answer: 10 indicates that the contactor has a pair of normally open auxiliary contacts ； 01 indicates that the contactor has a pair of normally closed auxiliary contacts ； 11 indicates that the contactor has a pair of normally open auxiliary contacts and a pair of normally closed auxiliary contacts. Readings related to contactors: How to distinguish between intermediate relays and contactors; a comparison table of parameters for motors along with their associated circuit breakers/contactors/thermal relays; how to improve the breaking capacity of the contacts in relays and contactors