One-way rotation circuit of motor controlled by contactor

The figure below is a one-way rotation circuit of a motor controlled by a contactor. Q is a three-phase knife switch, FU1 and FU2 are fuses, KM is a contactor, FR is a thermal relay, M is a cage induction motor, SB1 is a stop button, and SB2 is a start button.

Close the knife switch Q, introduce the power supply, press the start button SB2, the KM coil is energized, its main contact is closed, and the motor is powered on. At the same time, the auxiliary contact of the contactor KM connected in parallel with the start button SB2 is also closed, short-circuiting SB2. When SB2 is released, the KM coil continues to be energized through its own auxiliary contact, achieving self-locking. Press the stop button SB1, the contactor coil KM loses power, the main contact and auxiliary contact of KM are disconnected, cutting off the motor circuit and control circuit, and the motor stops.

(1) Short-circuit protection of fuse.

The fuses FU1 and FU2 are used to implement short-circuit protection for the main circuit and the control circuit respectively. The fuse element is connected in series in the protected circuit. When a short circuit or severe overload occurs in the circuit, it will automatically melt, thereby cutting off the circuit and achieving the purpose of protection.

(2) Overload protection.

Since the fuse has the characteristics of inverse time protection and dispersion, it is difficult to achieve long-term overload protection for the motor. Therefore, FR is used to achieve long-term overload protection for the motor. When the motor is at rated current, the motor is at rated temperature rise, and the thermal relay does not operate; when the overload current is small, the thermal relay takes a long time to operate; when the overload current is large, the thermal relay will operate quickly. The bimetallic strip connected in series in the motor stator circuit is deformed due to overheating, causing the moving contact connected in series in the control circuit to disconnect, cutting off the KM coil circuit, and the motor stops running, achieving overload protection.

(3) Loss of voltage and undervoltage protection.

When the motor is running, if the power supply disappears for some reason, then when the power supply voltage is restored, if the motor starts automatically, it may cause damage to production equipment or even cause personal injury. For the power grid, if many motors and other electrical equipment start automatically at the same time, it will cause unacceptable overcurrent and instantaneous network voltage drop.

The protection to prevent the motor from starting when the power is restored is called zero voltage or loss of voltage protection.

When the motor is running normally, an excessive drop in power supply voltage will cause the motor speed to drop or even stop.

Therefore, it is necessary to cut off the power supply when the power supply voltage drops below a certain allowable value. This is undervoltage protection.　　

By using the automatic recovery function of the button and the self-locking function of the contactor, there is no need to add zero voltage or undervoltage protection. In the circuit below, when the power supply voltage is too low or the power is off, the contactor KM is released. At this time, the main contact and auxiliary contact of the contactor KM are opened at the same time, so that the power supply of the motor is cut off and the self-locking is lost. However, when the power supply returns to normal, the operator must press the start button SB2 again to start the motor. Therefore, such a circuit with a self-locking link itself has both zero voltage and undervoltage protection functions.