How to choose the type of AC contactor?

AC contactor working principle

When the coil is energized, the static iron core generates an electromagnetic suction force, and the moving iron core is attracted. Since the contact system is linked with the moving iron core, the moving iron core drives three moving contact pieces to operate at the same time, and the contact is closed, thus connecting power supply. When the coil is de-energized, the suction force disappears, and the moving part of the moving iron core separates due to the reaction force of the spring, so that the main contact is disconnected and the power is cut off.

The selection of AC contactors should be based on the type of load and the operating parameters. Specifically divided into the following steps:

1. Select the type of contactor

AC contactors are generally classified into Class 1, Class 2, Class 3, and Class IV according to load types, which are denoted as AC1, AC2, AC3, and AC4, respectively. The corresponding control object of a type of AC contactor is a non-inductive or micro-inductive load, such as an incandescent lamp, an electric resistance furnace, etc.; and the second type of AC contactor is used to start and stop the induction motor of the wire wound type; the typical use of the three types of AC contactor is The squirrel-cage asynchronous motors are operated and disconnected during operation; four types of AC contactors are used for starting, reverse braking, reversing and jogging of accommodative asynchronous motors.

2. Select the rated parameters of the contactor

The contactor's rated parameters are determined based on the controlled object and operating parameters such as voltage, current, power, frequency, and work system.

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

2) The operating frequency of the motor is not high, such as compressors, pumps, fans, air conditioners, punches, etc. The rated current of the contactor is greater than the rated load current. Contactor type can choose CJl0, CJ20 and so on.

3) For heavy duty type motors, such as machine main motors, lifting equipment, winches, crushers, etc., the average operating frequency is more than 100 times/min, and it operates on starting, jogging, forward and reverse braking, reverse braking, etc. In the state, contactors of type CJl0Z and CJl2 can be used. In order to ensure the electrical life, the contactor can be derated. When selected, the contactor rated current is greater than the rated motor current.

4) For extremely heavy-duty motors, such as printing presses, boring machines, etc., the operating frequency is very high, up to 600 to 12,000 times/h, and it is often operated in the state of starting, reverse braking, reversing, etc., and the contactor can generally press the power. Life and starting current selection, contactor model selection CJl0Z, CJl2 and so on.

5) When the capacitor in the AC circuit is put into the grid or cut off from the grid, the contactor selection should take into account the closing impulse current of the capacitor. In general, the rated current of the contactor can be selected according to 1.5 times the rated current of the capacitor, and the type is CJ10, CJ20, etc.

6) When controlling the transformer with a contactor, the inrush current should be taken into account. For example, AC arc welding machines, resistance welding machines, etc., can generally select contactors according to 2 times of the transformer's rated current, and the models are selected as CJ10, CJ20, etc.

7) For electric equipment, such as electric resistance furnaces, electric heaters, etc., the cold resistance of the load is small, so the starting current is correspondingly larger. When selecting a contactor, it is not necessary to consider (starting current), and it is directly selected according to the load rated current. Models can choose CJl0, CJ20 and so on.

8) Since the gas discharge lamp has a large starting current and a long starting time, AC contactors can be selected according to the rated current of 1.1 to 1.4 times for the control of the lighting equipment. The models can be selected from CJ10, CJ20, etc.

9) The rated current of the contactor refers to the maximum allowable current of the contactor under long-term operation, the duration is ≤8h, and it is installed on the open control board. If the cooling condition is poor, when the contactor is selected, the rated current of the contactor is 110% to 120% of the rated load current is selected. For motors that work for long periods of time, there is no chance that their oxide film will be removed, which will increase the contact resistance and cause the contact to heat more than the allowable temperature rise. When actually selected, the rated current of the contactor can be reduced by 30%.