Some principles and methods for contactor selection

Some principles and methods for contactor selection:

The contactor is used as the equipment for switching off the load power supply. The contactor should be selected according to the requirements of the controlled equipment. Except the rated working voltage is the same as the rated working voltage of the controlled equipment, the load power, usage category and control of the controlled equipment. Mode, operating frequency, working life, installation method, installation size and economy are the basis for selection. The selection principles are as follows:

(1) The voltage level of the AC contactor should be the same as the load, and the type of contactor to be used should be compatible with the load.

(2) The calculated current of the load must conform to the capacity level of the contactor, that is, the calculated current is less than or equal to the rated operating current of the contactor. The contact current of the contactor is greater than the starting current of the load. The breaking current is greater than the current required for the breaking of the load. The calculated current of the load should take into account the actual working environment and working conditions. For a load with a long starting time, the peak current of half an hour cannot exceed the agreed heat. Current.

(3) According to short-term dynamic and thermal stability check. The three-phase short-circuit current of the line should not exceed the dynamic and thermal stable current allowed by the contactor. When the contactor is used to disconnect the short-circuit current, the breaking capacity of the contactor should also be verified.

(4) The rated voltage, current and the number of auxiliary contacts and current capacity of the contactor attracting coil should meet the control circuit wiring requirements. To consider the length of the line connected to the contactor control loop, the recommended operating voltage value, the contactor should be able to work at 85 ~ 110% of the rated voltage. If the line is too long, the contactor coil may not reflect the closing command because the voltage drop is too large; it may not work for the trip command because the line capacitance is too large.

(5) Verify the operating frequency allowed by the contactor based on the number of operations. If the operating frequency exceeds the specified value, the rated current should be doubled.

(6) The short circuit protection component parameters should be selected in conjunction with the contactor parameters. For the selection, please refer to the sample manual. The sample manual generally gives the matching table of the contactor and the fuse.

The cooperation of the contactor and the air circuit breaker is determined according to the overload factor of the air circuit breaker and the short circuit protection current coefficient. The agreed heating current of the contactor should be less than the overload current of the air circuit breaker. The contact 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 contactor agrees that the ratio of the heating current to the rated working current is between 1 and 1.38 at a voltage level, and the reverse time overload factor of the circuit breaker has more parameters. Different types of circuit breakers are different, so it is difficult to cooperate between the two. There is a standard that does not form a matching table and requires actual accounting.

(7) The installation distance of the contactor and other components must comply with the relevant national standards and specifications, and the maintenance and wiring distance should be considered.

Selection of AC contactors under different loads

In order to prevent contact cracking and ablation of the contactor, prolong the life of the contactor, the contactor should avoid the load to start the maximum current, and also take into account the unfavorable factors such as the length of the start-up time, so the load on the contactor is required to be turned on and off. Analyze and calculate and match different load start and stop currents according to the electrical characteristics of the load and the actual situation of the power system.

Selection of AC contactors for controlling electric heating equipment

Such equipments include electric resistance furnaces, temperature control equipment, etc., and the winding resistance components used in the heating element load can reach 1.4 times the rated current. If the power supply voltage is increased, the current will become larger. The current fluctuation range of such a load is small, and it belongs to AC-1 according to the use category, and the operation is not frequent. When the contactor is selected, the rated working current Ith of the contactor is equal to or greater than 1.2 times the operating current of the electric heating device.

Selection of contactors for controlling lighting equipment

There are many types of lighting equipment, and different types of lighting equipment, starting current and starting time are also different. This type of load usage is AC-5a or AC-5b. If the startup time is short, the heating current Ith can be selected to be equal to 1.1 times the operating current of the lighting device. The startup time is longer and the power factor is lower, and the heating current Ith can be selected to be larger than the operating current of the lighting device.

Selection of contactors for controlling welding transformers

When the low-voltage transformer load is turned on, the transformer has a short-term steep high current due to the short-circuit of the secondary side electrode, and a large current appears on the primary side, which can reach 15 to 20 times of the rated current, and it is arranged with the winding of the transformer. The core characteristics are related. When the welding machine frequently generates sudden high current, so that the switch on the primary side of the transformer is subjected to huge stress and current, it is necessary to select the contact according to the short-circuit current and welding frequency of the primary side when the electrode is short-circuited under the rated power of the transformer. The current is greater than the primary current when the secondary current is shorted. This type of load usage is AC-6a.

Selection of contactors for motors

The contactor for the motor can be AC-2~4 according to the motor usage and the motor type. AC-3 can be used for the starting current at 6 times of rated current and the breaking current is rated current. For example, fan and water pump can be used. The method and the curve method are selected according to the sample and the manual, and no further calculation is required.

The winding motor's on-current and breaking current are both 2.5 times rated current. Generally, a resistor is connected in the rotor to limit the starting current and increase the starting torque. The category AC-2 is used, and the rotary contactor can be used.

When the motor is jogging, requires reverse operation and braking, the on current is 6Ie, and the use category is AC-4, which is much harsher than AC-3. The power of the motor can be calculated based on the magnitude of the current listed under category AC-4. The formula is as follows:

Pe=3UeIeCOS¢η

Ue: rated motor current, Ie: rated voltage of the motor, COS¢: power factor, η: motor efficiency.

If the contact life is allowed to be shortened, the AC-4 current can be appropriately increased and changed to AC-3 at a very low switching frequency.

According to the requirements of motor protection, the current below the stall current should be switched on and off by the control unit. Most Y series motors have a stall current of ≤7Ie, so when selecting a contactor, it is necessary to consider the split and closing current. The specification stipulates that when the motor is running under AC-3 and the rated current of the contactor is not more than 630A, the contactor should be able to withstand 8 times of rated current for at least 10 seconds.

For the general equipment motor, the working current is less than the rated current, although the starting current reaches 4 to 7 times of the rated current, but the time is short, the contact of the contactor is not damaged, the contactor has been considered in the design, generally used The contact capacity is greater than 1.25 times the rated capacity of the motor.

For motors that work under special conditions, consider the actual operating conditions. For example, the electric hoist is an impact load, the heavy load starts and stops frequently, and the reverse braking is performed. Therefore, the calculated working current is multiplied by the corresponding multiple. Since the heavy load starts and stops frequently, the rated current of the motor is 4 times, usually reversed under heavy load. The braking current is twice the starting current, so 8 times the rated current is used for this condition.