What is Phase Loss? What Happens if a 3-phase Motor Loses a Phase?

What is Phase Loss?

Phase loss is the operation of a three-phase motor on only two phases due to the loss of voltage on one phase. Phase loss is the maximum condition of voltage unbalance. This occurs when one fuse blows when there is a mechanical failure within the equipment, a broken power line, open supply transformer winding, or a lightning strike.

When a phase is lost, the motor does not start or even if it operates, it does not do so smoothly and as a result, the motor does not provide its specified output and the motor itself may overheat and burn out.

A phase loss (phase failure) may occur because of a blown fuse in some part of the power distribution system, a mechanical failure within the switching equipment or if one of the power lines opens.

A three-phase motor running on a single phase draws all of its currents from the remaining two lines. Attempting to start a three-phase motor on a single phase will cause the motor to draw a locked-rotor current and the motor will not start. The reaction time of thermal overload units may be too slow to provide effective protection from the excessive heat generated in the motor windings when a phase failure occurs.

What happens if a 3-phase motor loses a phase?

Normally, three-phase motors will continue to rotate even if one phase is open. The three-phase voltage will be induced at the motor terminals. The diagram below shows the voltage induction at the motor terminals when phase R has been lost with a load applied to a three-phase motor. The horizontal axis shows the motor load as a percentage of the rated load and the vertical axis shows the voltage as a percentage of the rated voltage. The solid line in the graph shows the voltage that is induced at the motor terminals when a phase loss occurs while the motor is operating under various loads. The figure below shows how a phase loss that occurs while the motor is operating causes an imbalance in the voltage across each motor terminal.

How do you determine phase loss?

A phase loss in a running three-phase motor (with line regeneration) can be detected with the help of a phase loss monitoring device.