Low-voltage AC contactors are mainly used to switch power supplies on and off in electrical equipment, enabling remote control of power equipment and preventing personal injury during power connection and disconnection. The selection of AC contactors is crucial for the normal operation of power equipment and power lines. I. Structure and Parameters of AC Contactors Generally, AC contactors are required to have a compact structure, be easy to use, have a good magnetic blow-out mechanism for the moving and stationary contacts, good arc extinguishing effect (ideally zero arc flash), and low temperature rise. They are classified by arc extinguishing method into air-type and vacuum-type, and by operating method into electromagnetic, pneumatic, and electromagnetic-pneumatic types. Rated voltage parameters for contactors are divided into high voltage and low voltage, with low voltage typically being 380V, 500V, 660V, 1140V, etc. Current is classified by type into AC and DC. Current parameters include rated operating current, rated thermal current, making current and breaking current, rated thermal current of auxiliary contacts, and short-time withstand current of the contactor. Generally, the contactor model parameters provide the rated thermal current, which corresponds to several rated operating currents. For example, the CJ20-63 main contactor has rated operating currents of 63A and 40A. The 63A in the model parameters refers to the agreed-upon thermal current, which is related to the contactor's casing insulation structure. The rated operating current is related to the selected load current and voltage level. AC contactor coils are classified by voltage as 36V, 127V, 220V, 380V, etc. The number of poles in the contactor is classified as 2, 3, 4, 5, etc. The number of auxiliary contacts depends on whether they are normally open or normally closed, and is selected according to control requirements. Other parameters include the number of making and breaking cycles, mechanical life, electrical life, maximum permissible operating frequency, maximum permissible wire diameter, and overall dimensions and installation dimensions. The classification of contactors is shown in Table 1. Table 1: Common Contactor Types, Usage Category Code, Applicable Typical Loads, Examples, Typical Equipment. AC-1: Non-inductive or slightly inductive loads, resistive loads, resistance furnaces, heaters, etc. AC-2: Starting and stopping wound-rotor induction motors, cranes, compressors, hoists, etc. AC-3: Starting and stopping squirrel-cage induction motors, fans, pumps, etc. AC-4: Starting, reverse braking, or close-connection switching of squirrel-cage induction motors, fans, pumps, machine tools, etc. AC-5a: Switching on/off of discharge lamps, high-voltage gas discharge lamps such as mercury lamps, halogen lamps, etc. AC-5b: Switching on/off of incandescent lamps, incandescent lamps. AC-6a: Switching on/off of transformers, welding machines. AC-6b: Switching on/off of capacitors. AC-7a: Low-inductive loads for household appliances and similar uses, microwave ovens, hand dryers, etc. AC-7b: Household motor loads, refrigerators, washing machines, etc. Power switching. AC-8a: Motors of hermetically sealed refrigeration compressors with manual reset overload trip devices, compressors. AC-8b: Motors of hermetically sealed refrigeration compressors with manual reset overload trip devices, compressors. II. Selection Principles for AC Contactors As a device for switching power supply to and from a load, the selection of a contactor should be based on meeting the requirements of the controlled equipment. In addition to the rated operating voltage being the same as the rated operating voltage of the controlled equipment, the load power, usage category, control method, operating frequency, service life, installation method, installation size, and economy of the controlled equipment are the basis for selection. The selection principles are as follows: (1) The voltage level of the AC contactor should be the same as that of the load, and the type of contactor selected should be compatible with the load. (2) The calculated current of the load should conform to the capacity level of the contactor, that is, the calculated current should be less than or equal to the rated operating current of the contactor. The contactor's connecting current should be greater than the load's starting current, and the breaking current should be greater than the current required for breaking the load during operation. The calculated current of the load should take into account the actual working environment and operating conditions. For loads with long starting times, the peak current in half an hour should not exceed the agreed heating current. (3) Verify the dynamic and thermal stability of the short-term circuit. The three-phase short-circuit current of the circuit should not exceed the dynamic and thermal stability current allowed by the contactor. When using the contactor to break the short-circuit current, the breaking capacity of the contactor should also be verified. (4) The rated voltage and current of the contactor's attraction coil, as well as the number and current capacity of auxiliary contacts, should meet the wiring requirements of the control circuit. The length of the line connected to the contactor's control circuit should be considered. Generally, the recommended operating voltage value is that the contactor should be able to operate at 85-110% of the rated voltage value. If the line is too long, the contactor coil may not respond to the closing command due to the large voltage drop; and it may not respond to the tripping command due to the large line capacitance. (5) Verify the allowable operating frequency of the contactor based on the number of operations. If the operating frequency exceeds the specified value, the rated current should be doubled. (6) The parameters of the short-circuit protection components should be selected in conjunction with the contactor parameters. When selecting, refer to the sample manual, which generally provides a matching table for contactors and fuses. The matching of the contactor and the air circuit breaker should be determined based on the overload factor and short-circuit protection current factor of the air circuit breaker. The rated heating current of the contactor should be less than the overload current of the air circuit breaker, and the connecting and disconnecting current of the contactor should be less than the short-circuit protection current of the circuit breaker, so that the circuit breaker can protect the contactor. In practice, the ratio of the heating current to the rated operating current of a contactor at a voltage level is between 1 and 1.38. However, the inverse time overload coefficient parameter of a circuit breaker is more varied and varies between different types of circuit breakers. Therefore, it is difficult to have a standard for coordination between the two and a coordination table cannot be formed. Actual calculation is required. (7) The installation distance between the contactor and other components should comply with relevant national standards and specifications, and maintenance and wiring distances should be considered. III. Selection of AC contactors under different loads In order to prevent contactor contact sticking and burning and extend the contactor's life, the contactor should avoid the maximum starting current of the load and also take into account adverse factors such as the length of the starting time. Therefore, it is necessary to analyze the loads that the contactor operates on and off, and calculate and match the starting and stopping currents of different loads based on the electrical characteristics of the load and the actual situation of the power system. 1. Selection of AC contactors for controlling electric heating equipment Such equipment includes resistance furnaces, temperature control equipment, etc. The winding resistors used in the electric heating element loads can have a current of up to 1.4 times the rated current. If the power supply voltage is increased, the current will also increase. For this type of load, the current fluctuation range is very small, and it belongs to the AC-1 category. Operation is infrequent. When selecting a contactor, simply ensure that the contactor's rated operating current Ith is equal to or greater than 1.2 times the operating current of the heating equipment. 2. Selection of Contactors for Lighting Equipment: There are many types of lighting equipment, and different types have different starting currents and starting times. This type of load belongs to the AC-5a or AC-5b category. If the starting time is very short, its heating current Ith can be selected to be equal to 1.1 times the operating current of the lighting equipment. If the starting time is longer and the power factor is low, its heating current Ith can be selected to be slightly larger than the operating current of the lighting equipment. Table 2 shows the selection principles for contactors used in different lighting equipment. Table 2. Selection Principles for Contactors for Different Lighting Equipment | No. | Lighting Equipment Name | Starting Power Supply | Power Factor | Starting Time | Contactor Selection Principle | |---|---|---|---|---| | 1 | Incandescent Lamp | 1.5 Ie | 1 | Ith ≥ 1.1 Ie | | 2 | Mixed Lighting | 1.3 Ie ≈ 1.3 | Ith ≥ 1.1 × 1.3 Ie | | 3 | Fluorescent Lamp | ≈ 2.1 Ie | 0.4～0.6 | Ith ≥ 1.1 Ie | | 4 | High-Pressure Mercury Lamp | ≈ 1.4 Ie | 0.4～0.6 | 3～5 | Ith ≥ 1.1 × 1.4 Ie | | 5 | Metal Halide Lamp | 1.4 Ie | 0.4～0.5 | 5～10 | Ith ≥ 1.1 × 2 Ie | 6. Lamp with power compensation 20Ie 0.5～0.65～10 Selected according to the starting current of the compensation capacitor. 3. Selection of contactors for controlling welding transformers: When a low-voltage transformer load is connected, the transformer experiences a short-term steep large current due to a short circuit in the electrodes on the secondary side, resulting in a large current on the primary side, reaching 15 to 20 times the rated current. This is related to the winding arrangement and core characteristics of the transformer. When the welding machine frequently generates sudden strong currents, the primary side switch of the transformer is subjected to enormous stress and current. Therefore, the contactor must be selected according to the short-circuit current on the primary side when the electrodes are short-circuited under the rated power of the transformer and the welding frequency, i.e., the connecting current must be greater than the primary side current when the secondary side is short-circuited. This type of load uses AC-6a. 4. Selection of Contactors for Electric Motors: Contactors for electric motors can be selected from AC-2 to AC-4 depending on the motor's application and type. For motors with a starting current of 6 times the rated current and a breaking current equal to the rated current, such as fans and pumps, AC-3 can be used. This can be done using a lookup table or selection curve method, referring to samples and manuals, without further calculation. Wound-rotor motors have a starting and breaking current of 2.5 times the rated current. Generally, a resistor is connected in series with the rotor during startup to limit the starting current and increase the starting torque. This is AC-2 type, and a rotary contactor can be selected. When the motor is in jogging, needs to reverse, or brakes, the starting current is 6Ie, and the application category is AC-4, which is much more stringent than AC-3. The motor power can be calculated based on the current values ​​listed under application category AC-4. The formula is as follows: Pe = 3UeIeCOS￠η, where Ue: rated current of the motor, Ie: rated voltage of the motor, COS￠: power factor, η: motor efficiency. If a shorter contact life is permissible, the AC-4 current rating can be appropriately increased, and it can be changed to AC-3 at very low switching frequencies. According to the requirements for motor protection coordination, currents below the locked-rotor current should be switched on and off by the control electrical equipment. Most Y-series motors have a locked-rotor current ≤7Ie, therefore, the switching and closing locked-rotor current must be considered when selecting a contactor. The standard stipulates that when the motor is operating under AC-3 conditions and the contactor's rated current is not greater than 630A, the contactor should be able to withstand 8 times the rated current for at least 10 seconds. For motors used in general equipment, the operating current is less than the rated current. Although the starting current reaches 4 to 7 times the rated current, the time is short, and the damage to the contactor contacts is minimal. This factor is already considered in the contactor design; generally, a contact capacity greater than 1.25 times the motor's rated capacity is sufficient. For motors operating under special conditions, the actual operating conditions must be considered. For example, electric hoists are impact loads, frequently start and stop under heavy loads, and use reverse braking, so the calculated operating current must be multiplied by a corresponding factor. Due to frequent start and stop under heavy loads, a current four times the rated motor current should be selected. Typically, the reverse braking current under heavy loads is twice the starting current, so for this condition, a current eight times the rated current should be selected. 5. When selecting a capacitor contactor, the capacitor undergoes a transient charging process upon connection, resulting in a large inrush current accompanied by high-frequency current oscillations. This current is determined by the grid voltage, capacitor capacity, and reactance in the circuit (i.e., related to the feeder transformer and connecting wires). Therefore, severe contact erosion may occur during contact closure. The contactor should be selected based on the calculated maximum steady-state current in the capacitor circuit and the maximum inrush current peak value that may occur when the actual power system is connected. This ensures correct and safe operation. When selecting a general-purpose AC contactor, the inrush current factor when connecting the capacitor bank, grid capacity, transformer, circuit and switching equipment impedance, discharge state of the parallel capacitor bank, and closing phase angle must be considered. The rated current is generally 50 to 100 degrees, making the calculation quite complex. Refer to reference 1 for further details. If the capacitor bank lacks a discharge device, a dedicated contactor with a forced discharge resistor circuit can be selected, such as the ABB B25C and B275C series. The domestically produced CJ19 series capacitor switching contactor is specifically designed for capacitors and also employs series resistors to suppress inrush current. Refer to the catalog when selecting a contactor, and also consider the regulations in the reactive power compensation device standards. The peak inrush current generated at the moment of capacitor connection should be limited to less than 20 times the rated current of the capacitor bank (as specified in JB7113-1993 Low-voltage Parallel Capacitor Devices). The operation of the capacitor under maximum steady-state current should also be considered. The harmonic voltage during capacitor bank operation, combined with the power frequency overvoltage reaching up to 1.1 times the rated operating current, will generate a large current. The equipment and components in the capacitor bank circuit should be able to operate continuously at the rated frequency and with the root mean square value of the rated sinusoidal voltage not exceeding 1.3 times the rated current. Since the actual capacitance of the capacitor may reach 1.1 times the rated capacitance, this current can reach 1.43 times the rated current. Therefore, the rated heating current of the selected contactor should not be less than this maximum steady-state current. IV. Selection of AC Contactors under Special Requirements 1. Anti-Power Slip AC Contactors Power systems experience power slippage due to lightning strikes, reclosing after short circuits, and automatic recovery after a single-phase short-term human-caused ground fault. The slippage time is generally less than a few seconds. In situations requiring continuous production, where the process cannot allow equipment to trip due to a short power interruption (power slip), a new type of electrical control equipment can be used: the FS series anti-power slip AC contactor. The FS series anti-power slip contactor does not rely on auxiliary power supplies or auxiliary mechanical devices. It is small in size and highly reliable. It uses a powerful engaging device and a double-winding coil. The contactor has no harmful jitter during engagement and release, avoiding arc welding caused by contact jitter when the power grid loses voltage, thus reducing contact wear. The contactor coil has an energy storage mechanism. When a power slip occurs, the contactor coil releases with a delay, and its auxiliary contacts delay sending a disconnection control signal, thereby avoiding the power slip time. The power slip time is determined by the load characteristics and the length of the power outage; the contactor delay time is adjustable. 2. Energy-Saving AC Contactors: Energy saving in AC contactors refers to using various energy-saving technologies to reduce the active and reactive power consumed during the holding process of the operating electromagnetic system. The operating electromagnetic system of an AC contactor generally uses an AC control power supply. Currently, AC contactors with an A rating of 63A or higher in my country consume tens to hundreds of watts of active power and tens to hundreds of VARs of reactive power during holding. Generally, the active power consumed is approximately 65-75% in the iron core, 25-30% in the short-circuit ring, and 3-5% in the coil. Therefore, changing the AC holding current to DC holding, or using mechanical structure holding, current-limiting holding, etc., can save most of the power loss in the iron core and short-circuit ring, and also eliminate or reduce noise, improving the environment. Based on their principles, they are generally divided into three categories: energy-saving devices, node coils, and energy-saving AC contactors. The electromagnetic system uses energy-saving devices to make the electromagnetic system noiseless and with low temperature rise, and solves the disadvantage of release delay when using energy-saving devices, such as the domestically produced CJ40 series. 3. The application of electronic technology in AC contactors with additional functions allows for the convenient addition of main circuit protection functions, such as undervoltage and overvoltage protection, phase loss protection, and leakage protection. In motor burnout accidents, poor contact in one phase of the contactor accounts for 11%, making the selection of circuit breakers, contactors, and other electrical devices with phase loss protection essential. Adding auxiliary modules to contactors can meet some special requirements. Adding mechanical interlocks can create reversible contactors, enabling reversible rotation of the motor, or two contactors with mechanical interlocks can achieve electrical interlocking of the main circuit, which can be used for frequency conversion/power frequency switching in frequency converters; adding air delay heads and auxiliary contact groups can achieve star-delta starting of the motor; adding air delay heads can create time-delay contactors. The electromagnetic coil of an AC contactor can be used for low-voltage protection of the motor. Its control circuit should preferably be powered by the motor's main circuit. If powered by other power sources, the control power should be automatically disconnected when the main circuit loses voltage. V. Installation of AC Contactors The AC contactor vibrates significantly when it engages and disengages. Therefore, it should not be installed in the same cabinet as electrical equipment with strict vibration requirements. Otherwise, anti-vibration measures should be taken. Generally, it should be installed at the bottom of the cabinet. The installation environment of the AC contactor should meet the product requirements. The installation dimensions should comply with electrical safety distances and wiring regulations, and maintenance should be convenient. Conclusion: The selection of an AC contactor is not only related to the load being switched, but also to the impedance parameters of the power system in the circuit where the contactor is located, as well as the control method, operating environment, and usage requirements. Therefore, when selecting an AC contactor, a comprehensive consideration should be given, and the values ​​of each parameter should be calculated step by step to achieve reasonable selection and convenient use. References: [1] Electric World, 2001 bound edition. P387. Shanghai. 2001. [2] Industrial and Civil Power Distribution Design Manual, 2nd Edition. Edited by China Aviation Industry Planning and Design Institute. 1999. [3] Industrial and Civil Power Distribution Design Manual, 2nd Edition. Edited by China Aviation Industry Planning and Design Institute. 1999.