After AC contactor is electrified, press start button, what is the reason of contactor's continuous pull in and disconnect? In my opinion, most cases are caused by the wrong coil voltage of the replaced AC contactor, or the working voltage of the replaced AC contactor is not understood by the buyer, and the replacement operator is also caused by a beginner electrician. Simply speaking, since it can pull in and release, it means that the control circuit is a closed circuit. As long as the start button is not released, it is equivalent to an electric button. Here, we will not talk about whether the self-locking auxiliary contact of the AC contactor works, because the hand is not loose.

The most common one is that the AC contactor with the voltage of 220 V in the original control circuit has been replaced with the AC contactor with the pull-in coil voltage of 380V. Therefore, the magnetic force generated by the contactor coil parameters can not overcome the tension of four springs, and it is always in a half dead working state.

If the contactor in the original circuit is the pull-in voltage of 380V, and the coil voltage of the replaced AC contactor is 220V, then it can be pulled in temporarily, but the time is a little longer, the contactor coil will heat up immediately due to the high voltage, which is because the supply voltage is far beyond the normal working pull-in voltage range.

Here I use the simplest motor start and stop circuit to talk about. See the figure below.

The working principle of the control circuit is: first close the main circuit breaker QF1 and the control circuit breaker QF2 to provide the preparation conditions for the whole circuit work. Start: press the start button SB2 (3-5), the KM coil of AC contactor is powered on and closed, and the auxiliary normally open contact (3-5) of the contactor is closed and self-locking, KM three-phase main contact is closed, the electric operation of the motor, and the driving equipment starts to work.

Stop: press the stop button SB1 (1-3), the KM coil of AC contactor is de energized and released, KM three-phase main contact is disconnected, the motor loses power, the motor stops running, and the driven equipment stops working.

In the process of line debugging, first disconnect the main circuit breaker QF1, close the control circuit breaker QF2, and debug the control circuit. Press the start button SB2, and the KM coil of AC contactor shall pull in action. Release SB2, KM will not release, press stop button SB1, AC contactor km coil power off release. Repeat several times, if there is no abnormal situation, you can debug the main circuit. Close the main circuit breaker QF1, press the start button SB2, AC contactor km coil is powered on and self-locking, its three-phase contact is closed, and the motor operates normally (at this time, observe whether the motor steering meets the operation requirements, if not, stop the motor, and change the direction of two phases in the three-phase power supply arbitrarily). Press the stop button SB1, the KM coil of AC contactor is de energized and released, KM three-phase main contact disconnects the motor power supply, and the motor stops running after power loss.

When debugging the control circuit, if the circuit breaker QF2 is closed and the KM coil of AC contactor acts, it means that the button line No. 1 or no. 3 is wrongly connected to No. 5, resulting in direct starting without pressing the start button. In case of this problem, the circuit breaker QF2 should be disconnected and the money should be connected correctly according to the drawing. Here is a little experience for readers. Just remember one of the three starting wires connected to the terminal No. 5 of the distribution board in the button, and the other two wires can be connected at will.

Then debug the overload protection circuit. First, set the fr current setting knob of the thermal relay to a lower level, which is much lower than the rated current of the motor. Press the start button SB2. At this time, the KM coil of the AC contactor is powered on and self-locking, and the electric operation of the motor is carried out. Since the current set by the thermal relay is far less than the rated current of the motor, the fr of the thermal relay will act in a short time The KM coil of the current contactor is de energized and released, which plays an overload protection role, indicating that the thermal relay FR is good and the control circuit wiring is correct. At this time, adjust the current setting knob of thermal relay to the rated current of controlled motor about 1.6A.

 common faults and troubleshooting. (1) As soon as the control circuit breaker QF2 is closed, the KM coil of AC contactor will be closed immediately and the motor will run. The possible causes of this fault are: Start button SB2 is short circuited, so SB2 button can be replaced; wiring error, power line No. 1 or self-locking Line No. 3 is wrongly connected to terminal No. 5, which can be correctly connected according to the circuit diagram; main contact of KM AC contactor needs to be replaced due to fusion welding of main contact of AC contactor, and oil and rust are found on the polar surface of KM iron core of AC contactor, which makes AC contactor delay release (with different delay time), Remove the AC contactor and clean the iron core pole surface; for mixed line or touching line, find the mixed line or contact line and deal with it.

(2) Press the start button SB2, the KM coil of AC contactor does not pull in. The possible causes of this fault are: button sb, 2 is damaged, and it can be solved by replacing with a new one; the control wire falls off and reconnects; if the stop button is damaged or has poor contact, the damaged button SB1 should be replaced; if the normally closed contact of the thermal relay fr is not reset or damaged, it can be reset manually, if not, it can be replaced with a new one; the KM coil of AC contactor is open circuit and needs to be replaced with a new coil.

(3) Press the stop button sb? And the KM coil of AC contactor does not release. In this case, the control circuit breaker QF2 can be disconnected immediately, and then the circuit breaker QF1 can be disconnected to repair the control circuit. The reason may be that the SB1 button is damaged, so it is necessary to replace the new one. In addition, the AC contactor itself will also have the above problems, which can be distinguished and treated according to fault (1).

(4) Shortly after the motor runs, the thermal relay fr acts and trips. Possible origin