Single Phase Preventer Working Principle | Induction Motor Protection

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Single Phase Preventer Working Principle:

Single phase preventer is used to protect the induction motor from single phasing fault. Single phasing is a very dangerous fault to the electrical motor and Which damages the motor stator winding rapidly. Generally single phasing is nothing but a motor runs when one of the supply is disconnected due to open circuit or improper contact in switch or other electrical equipment failure. Normally, the motor runs with the three phase supply and which takes balance current in each phase winding. Consider one of the fuse has blown. But the motor still in rotating position which tries to rotates in the same speed. At that same time the absence phase current will be shifted to the remaining live phases. Therefore, the current in the other phases increases upto 3 times its normal value instantly. This is called single phasing fault.


The single phasing leads to unbalanced current in the motor stator. The component which is present in this unbalanced current called negative sequence component. These negative sequence component creates the magnetic flux opposite to the main flux. This results in double frequency currents to induce in the rotor to cause its heating.

Current sensing Single Phase Preventer:

Current sensing single phase preventer works like negative sequence relay. It gives protection up to motor terminals. It Operates on the principle of sensing negative sequence components of the system. Different types of relay manufacturer give addition function such as reverse phasing, unbalance supply and under voltage /over voltage trip with that single phase preventer.

Voltage sensing single phase preventer:


The relay sense the negative sequence components of voltage in the motor power supply.

Single Phasing Preventer Working Principle:

Single phase preventer
Single phase preventer

The three number of current transformer is placed in each phase of the power supply. The output of the current transformer is given to the negative sequence filter circuit and which sense the magnitude of unbalance. The filter circuit will be connected to the control circuit. The control circuit sends the trip command to the circuit breaker, if the negative sequence current exceeds the preset value. If the failure of single phase the unbalance current flow in the motor and the current will be sensed by the negative sequence filter. Then the control unit send the trip command to the circuit breaker or other motor isolating mechanism.

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