Motor Protection : Phase failure and reversal

What happen if one of a cable broken or fuse blow while the electric motor is running?The answer is motor will run in two phase supply that can generate the high draw current due to unbalance of phase.

Why the current value increase?From the theory, the current in the two phase would increase by 1.7 times due to negative phase and generate heat in the rotor area.If this situation continue in a long time period,it can cause serious damage.

Another phenomena would happen is the motor are interchanged the motor will reverse its direction of rotation.We called is phase reversal and it can cause serious injury and motor damage.

How to protect motor from phase failure and reversal?

For motor protection phase failure and reversal,we can choose a many types of phase failure and reversal relay.A famous type is that have both protection built in one relay.

This type consists of four magnetic poles the coil of which are excited by the currents of the two stator phases of the
motor. These coils set up a rotating flux which intersects a disc pivoted at the centre.

A torque is produced due to interaction of flux and eddy currents induced in the disc. Torque is made of two components.A clockwise rotating torque is produced by the normal polyphase supply and a counter clockwise torque is produced due to the action of the shaded poles.

Another type of relay that used is trigger based on negative current phase sequence during unbalance of phase.This relay has two built in current transformers(CT) which sense currents of the motor.

The secondary of CTs feeds a negative sequence filter. The output of this filter is proportional to the negative sequence
component of currents. This output is fed to a sensor which detects the level of negative sequence components of the currents and thus trips the motor starter by opening its control magnetic contactor.