# Seven basic principles of AC contactor selection

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There are seven standards for selection of AC contactor

1. Very few contactors are selected.

2. The main parameters of the main power supply circuit are selected, including rated value, standard voltage, rated value I as electric flow, rated value conduction capacity and load bearing capacity.

3. Select suitable main parameters of control circuit.

4. Select the appropriate electrical service life and application type.

5. The contactor for motor should be considered according to the operation condition of motor. AC contactor can be selected according to AC-3 type for unilateral operation motor, such as centrifugal fan and centrifugal water pump; AC contactor can be selected according to AC-4 type for reversible motor, when it runs in the opposite direction, starts the light and reverses the brake system, the connected current reaches 8 times of the rated current. When the output power of the motor does not exceed 630kw, the contactor should be able to bear 8 times of the rated current and operate for at least 10s.

There is an empirical formula for selecting the rated current of AC contactor applied in motor control circuit, I E = (pmx103) / (kxun). In the formula calculation, PM is the output power of motor, the enterprise is kW; UN is the rated current of motor; ie is the rated current of AC contactor; K is the work experience index, generally assigned as 1-1.4

For general motors, the electric flow in operation is lower than the rated current. Although the starting current of the motor is 4-8.4 times of the rated current, the time is short, and the burning effect on the main circuit breaker of the contactor is not big. Therefore, the K index of the rated current of AC contactor is 1.25.

For example, the output power of the motor is 30kW, from the formula ie = (pmx103) / (kxun), ie = (30x1000) / (1.25x380) ≈ 63.2a, Therefore, the rated current of AC contactor is 63A. It must be emphasized that the rated conduction capacity of contactor should be higher than the current that will appear in power circuit when conducting, and the capacity of contactor to bear load current should be higher than the load current that will appear in power circuit. Because the data information in the power circuit can be determined according to the application type and working hour system, it is effective to select the contactor according to the application type and working hour system. This is also the basis of contactor selection report obtained by contactor manufacturer. Winding motor rotor multithreaded motor access current and breaking current are 2.5 times the rated current, AC-2 type of AC contactor can be selected.

6. Standard of AC contactor selection for electric heating equipment: ac-1 application type can be selected. When selecting contactor, the rated current of contactor shall be greater than or equal to 1.2 times the rated current of electric heating equipment.

7.Selection standard of switching power capacitor contactor: since the current of power capacitor reaches 1.43 times of rated current, 1.5 times of rated current of power capacitor should be considered when selecting switching power capacitor contactor.