Low-voltage AC contactors are mainly used to switch power supplies on and off electrical equipment, enabling remote control of power equipment and preventing personal injury when switching power on and off. The selection of AC contactors is crucial for the normal operation of power equipment and power lines. [b]1. Structure and Parameters of AC Contactors[/b] Generally, AC contactors are required to have a compact structure, be easy to use, have good magnetic blow-out devices for 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-type, pneumatic-type, and electromagnetic-pneumatic-type. 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, conventional heating current, connecting current and breaking current, conventional heating current of auxiliary contacts, and short-time withstand current of the contactor. Generally, the contactor model parameters provide the conventional heating 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 and off discharge lamps, high-pressure gas discharge lamps such as mercury lamps, halogen lamps, etc. AC-5b: Switching on and off incandescent lamps, incandescent lamps. AC-6a: Switching on and off transformers, welding machines. AC-6b: Switching on and off capacitors. AC-7a: Low-inductive loads for household appliances and similar applications, microwave ovens, hand dryers, etc. AC-7b: Household electric motor loads, refrigerators, washing machines, etc. Power switching. AC-8a: Hermetically sealed refrigeration compressor motors with manual reset overload trip devices. AC-8b: Compressors. [b]2. Selection Principles of AC Contactors[/b] As a device for switching the power supply of the load, the selection of contactors 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 meet 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 is greater than the starting current of the load, and the breaking current is greater than the breaking current required when the load is running. 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 in a short time. The three-phase short-circuit current of the line should not exceed the dynamic and thermal stability current allowed by the contactor. When using the contactor to disconnect the short-circuit current, the breaking capacity of the contactor should also be checked. (4) The rated voltage, current and auxiliary contact quantity and current capacity of the contactor's attraction coil should meet the wiring requirements of the control circuit. The length of the line connected to the contactor control circuit should be considered. Generally, the recommended operating voltage value is that the contactor should be able to work at 85 to 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; the line capacitance may not respond to the tripping command due to the large line capacitance. (5) Check the allowable operating frequency of the contactor according to 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 parameters of the contactor. When selecting, refer to the sample manual. The sample manual generally provides a matching table of contactors and fuses. The matching of the contactor and the air circuit breaker should be determined according to 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. In this way, the circuit breaker can protect the contactor. In practice, the ratio of the rated heating current to the rated operating current of the contactor at a voltage level is between 1 and 1.38. However, the inverse time overload coefficient parameter of the circuit breaker is relatively large and different for 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. [b]3. Selection of AC contactors under different loads[/b] In order to prevent the contactor from sticking and burning and to 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, the loads on which the contactor operates should be analyzed, and the starting and stopping currents of different loads should be calculated and adjusted according to the electrical characteristics of the load and the actual situation of the power system. 3.1 Selection of AC Contactors for Controlling Electric Heating Equipment This type of equipment includes resistance furnaces, temperature control equipment, etc. The winding resistors used in their heating element loads can draw up to 1.4 times the rated current. If the power supply voltage increases, the current will be even greater. The current fluctuation range of this type of load 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 electric heating equipment. 3.2 Selection of Contactors for Controlling 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, the 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 lower, the 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 | 15 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 Factor Compensation | 20 Ie | 0.5～0.6 | | 5～10 | 3.3 Selection of Contactors for Welding Transformers: When a low-voltage transformer load is connected, a short-term, steep, large current occurs due to a short circuit on the secondary side electrodes, resulting in a large current on the primary side, reaching 15 to 20 times the rated current. This is related to the transformer's winding arrangement and core characteristics. When welding machines frequently generate sudden, strong currents, the primary side switch of the transformer is subjected to enormous stress and current. Therefore, the contactor must be selected based on the primary side short-circuit current and welding frequency under the transformer's rated power, 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 category AC-6a. 3.4 Selection of Contactors for Motors: Contactors for motors can be selected from AC-2 to AC-4 depending on the motor's usage and category. 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 table lookup and selection curve methods, based on samples and manuals, without further calculation. Wound-rotor motors have a starting current 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 classified as AC-2, and a rotary contactor can be used. When the motor is in jogging, needs to reverse, or brakes, the starting current is 6Ie, and the classification is AC-4, which is much stricter than AC-3. The motor power can be calculated based on the current values ​​listed under AC-4. The formula is as follows: Pe=3UeIeCOS￠η, where Ue is the rated current of the motor, Ie is the rated voltage of the motor, COS￠ is the power factor, and η is the motor efficiency. If a shorter contact life is permissible, the AC-4 current can be appropriately increased, and at very low switching frequencies, it can be changed to AC-3. According to the requirements of 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, so the switching and closing locked-rotor currents must be considered when selecting a contactor. The standard stipulates that when the motor operates under AC-3 conditions and the contactor's rated current does not exceed 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, and generally, a contact capacity greater than 1.25 times the motor's rated capacity is sufficient. For motors operating under special conditions, the actual working conditions must be considered. For example, electric hoists are impact loads, with frequent heavy-load starts and stops, and reverse braking, so the calculated operating current must be multiplied by the corresponding factor. Due to frequent heavy-load starts and stops, 4 times the motor's rated current is selected. Typically, the reverse braking current under heavy load is twice the starting current, so 8 times the rated current should be selected for this condition. 3.5 Selection of Contactors for Capacitors When a capacitor is switched on, it undergoes a transient charging process, 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 the contact closing process. 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 capacitor is switched on in the actual power system. This ensures correct and safe operation. When selecting a general-purpose AC contactor, the inrush current multiple when switching on the capacitor bank must be considered, along with the grid capacity, transformer, circuit and switching equipment impedance, discharge state of the parallel capacitor bank, and closing phase angle. The rated current is generally 50 to 100 degrees, making the calculations quite complex; refer to reference 1. If the capacitor bank lacks a discharge device, a dedicated contactor with a forced discharge resistor circuit can be used, 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. When selecting a capacitor, refer to the sample and consider the provisions in the reactive power compensation device standard. 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 (JB7113-1993 Low Voltage Parallel Capacitor Device Specification); the operation of the capacitor under the maximum steady-state current should also be considered. The harmonic voltage of the capacitor bank during operation, plus the power frequency overvoltage of 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 the root mean square value generated by the rated sinusoidal voltage not exceeding 1.3 times the rated current. Since the actual capacitance value of the capacitor may reach 1.1 times the rated capacitance value, 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. [b]4. Selection of AC Contactors under Special Requirements[/b] 4.1 Anti-Voltage-Slip AC Contactors The power supply system may experience voltage slippage due to lightning strikes, reclosing after short circuits, and automatic recovery after a single-phase short-term human-caused ground fault. The voltage slippage time is generally less than a few seconds. In situations requiring continuous production, where the process cannot tolerate equipment tripping due to short-term power interruptions (power dips), a new type of electrical control equipment can be used: the FS series anti-power dip AC contactor. The FS series anti-power dip 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 exhibits no harmful jitter during engagement and release, avoiding arc welding caused by contact jitter when the power grid fails, thus reducing contact wear. The contactor coil has an energy storage mechanism. When a power dip occurs, the contactor coil releases with a delay, and its auxiliary contacts delay sending a disconnection control signal, thereby avoiding the power dip duration. The power dip duration is determined by the load characteristics and the length of the power outage; the contactor delay time is adjustable. 4.2 Energy-saving AC Contactors: The energy saving of AC contactors refers to the use of various energy-saving technologies to reduce the active and reactive power consumed during the operation of the electromagnetic system. The operating electromagnetic system of AC contactors generally uses AC control power. Currently, AC contactors with an operating current of 63A or higher 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 holding or current-limiting holding methods, 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, resulting in noiseless electromagnetic circuits and low temperature rise, and solves the problem of release delay associated with energy-saving devices, such as the domestically produced CJ40 series. 4.3 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%, so selecting electrical devices such as circuit breakers and contactors with phase loss protection is essential. Contactors with auxiliary modules 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 main circuit of the motor. If powered by other power sources, the control power should be automatically disconnected when the main circuit loses voltage. [b]5. Installation of AC Contactors[/b] AC contactors vibrate significantly during engagement and disengagement. During installation, avoid installing them in the same cabinet as electrical equipment with strict vibration requirements; otherwise, anti-vibration measures should be taken. Generally, it is best to install them at the bottom of the cabinet. The installation environment of AC contactors must meet product requirements, and the installation dimensions should comply with electrical safety distances and wiring regulations, and should be convenient for maintenance. [b]6. Conclusion[/b] The selection of AC contactors 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 AC contactors, 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.