Zero and under voltage protection

Update:14-02-2020
Summary:

When the motor is working normally, if the motor stops […]

When the motor is working normally, if the motor stops due to the disappearance of the power supply voltage, the motor may start automatically when the power supply voltage is restored. The self-starting of the motor may cause personal or equipment accidents. For the power grid, the self-starting of many motors can also cause unallowable overcurrents and voltage drops. The protection that prevents the motor from starting when the voltage is restored is called zero voltage protection. When the motor is working, excessively reducing the power supply voltage will cause the motor's speed to drop or even stop; under constant load, it will cause the motor current to increase, causing the winding to overheat and damage;

   In addition, the decrease in voltage will cause the release of some electrical appliances, causing the circuit to malfunction and an accident may occur. Therefore, the motor power needs to be cut off when the voltage drops to the minimum allowable value. This protection is called undervoltage protection. Buttons and contactors, voltage relays, etc. are generally used for zero and under voltage protection. The suction voltage of a voltage relay is usually set to 0.8 to 0.85 U RT, and the release voltage of a voltage relay is usually set to 0.5 to 0.7 U RT.