Phase Loss and Phase Sequence Monitoring Relays

A phase loss is a very damaging event for a three-phase motor. It will cause a reduction in the power output from the motor which can cause stalls or an inabilty to start the motor - all while the motor draws excess current. This will cause the motor to overheat and quickly burn out. A phase loss monitoring relay will detect the loss of phase and provide a signal to stop the motor.

Another common issue is voltage unbalance which is usually caused by poorly distributed single-phase loads. While the motor will still run, there will be more stress on the motor. A 5% voltage unbalance reduces the maximum load on the motor to just 75% of its rated value. Over time, the motor will have a reduced life span. A phase asymmetry monitoring relay has an adjustable unbalance setting to suit the application as well as a time delay to prevent nuisance tripping.

In instances where a line is lost or there is a disconnection at the mains, it is essential that the lines are re-connected to the correct phases. A phase sequence monitor checks the phase sequence and if it is reversed, it will prevent the motor from turning in the wrong direction. This is critical on elevators, conveyors and pumps - anywhere where direction of rotation is important.