Selection criteria for ac contactor types and rated parameters

- Jan 20, 2020-

Type of contactor


Ac contactors are generally divided into one, two, three and four categories according to load types, which are denoted as AC1, AC2, AC3 and AC4 respectively.

A class of ac contactor corresponding control objects are non-inductive or micro-inductive load, such as incandescent lamp, resistance furnace, etc.Type ii ac contactor is used for starting and stopping winding asynchronous motor.The typical use of the three types of ac contactors is the running of squirrel-cage asynchronous motor and the breaking during running.Four types of ac contactors are used for starting, reverse braking, reversing and inching of cage asynchronous motors.

Rated parameters of contactor


The rated parameters of the contactor are determined according to the controlled object and operating parameters such as voltage, current, power, frequency and operating system.

1) the coil voltage of the contactor should be generally lower, so that the insulation requirements of the contactor can be reduced, and it is safer to use.But in order to facilitate and reduce equipment, often according to the actual power grid voltage selection.

2) the operating frequency of the motor is not high, such as compressor, water pump, fan, air conditioner, press, etc. The rated current of contactor is greater than the rated current of load.Contactor type can choose CJl0, CJ20 and so on.

3) for the heavy-duty motor, such as the main motor of machine tool, lifting equipment, capstan, crusher, etc., its average operating frequency is more than 100 times/min, and it runs in starting, inching, forward and backward braking, reverse braking and other states. CJl0Z, CJl2 contactors can be selected.In order to ensure the electrical life, the contactor can be used at reduced capacity.When selected, the contactor rated current is greater than the motor rated current.

4) for extra-heavy task motors, such as printing machines, boring machines, etc., the operating frequency is very high, up to 600 ~ 12000 times/h, often running in starting, reverse braking, reverse and other states, contactor can be roughly selected according to the electrical life and starting current, contact type and number choose CJl0Z, CJl2, etc.

5) when the capacitor in the ac circuit is put into or removed from the power grid, the closing shock current of the capacitor shall be considered in the contactor selection.Generally, the rated current of the contactor can be 1.5 times the rated current of the capacitor, and the model is CJ10, CJ20, etc.

6) the size of surge current should be considered when the transformer is controlled by contactor.For example, ac arc welding machine, resistance welding machine and so on, generally can choose the contactor according to the rated current of the transformer 2 times, the model choose CJl0, CJ20 and so on.

7) for electric heating equipment, such as resistance furnace and electric heater, the cold resistance of the load is small, so the starting current is correspondingly larger.When selecting contactor, it is not necessary to consider (starting current), but directly select according to load rated current.Model can choose CJl0, CJ20 and so on.

8) due to the large starting current and long starting time of the gas discharge lamp, the ac contactor can be selected according to the rated current of 1.1 ~ 1.4 times for the control of the lighting equipment, and the model can be selected as CJl0, CJ20, etc.

9) the rated current of the contactor refers to the maximum allowable current of the contactor under long-term operation, whose duration is ≤8h and is installed on the open control panel. If the cooling condition is poor, the rated current of the contactor shall be selected according to 110% ~ 120% of the rated current of the load.

For the motor working for a long time, the oxidation film has no chance to be removed, which increases the contact resistance and causes the contact heating to exceed the allowable temperature rise.In practice, the rated current of the contactor can be reduced by 30%.