Use a thermal relay to measure whether the current exceeds the rated current of the motor and adjust the setting current. When the motor is running, the auxiliary head of the thermal relay, the normally closed point is opened, and the normally open point is closed for protection. In the relay control, the normally closed point and the stop button are connected in series. When the overload occurs, the motor operation is stopped and an alarm signal is given. For example, in a control system with a PLC, a normally open or normally closed action signal is introduced to the signal input terminal of the PLC, and the start and stop of the motor is controlled by a program. If the normally open is introduced as the stop signal X input, the normally closed point is used, otherwise it is not.

When the motor is protected by temperature, the built-in thermal sensor is used for protection. When the temperature is too high, a signal is provided for protection through an external control loop. Or use thermistor, thermoelectric measuring the temperature of the motor through the temperature controller to give an overheating signal to protect the motor.

Use the current transformer to measure whether the three-phase zero sequence of the motor incoming line is zero. It is generally used for motors with higher power. The specific method is: select a suitable current transformer according to the power of the motor and install it at the input end of the motor control box. Phase A, phase B, and phase C are all connected. Connect the L1 end of the A B C current transformer to the incoming end of a thermal relay, connect the L2 end of the A B C current transformer into a star, and introduce the star point into the neutral line. When the three phases are unbalanced or the current is too large, the thermal relay will act, normally open, normally closed. The action signal is introduced into the control loop to cut off the current to protect the motor.