Single Phasing of Three Phase Induction Motor

Single Phasing:

If one line of a three phase line is accidentally opened or if a fuse blows while the motor is running, the machine will continue to run on a single –phase. It simply means one of the phases is open.


Causes of single phasing are given below:

  1. Open winding in motor
  2. Any open circuit in any phase anywhere between the secondary of the transformer and the motor
  3. Primary fuse open.


Single phasing represents the worst case of an unbalance voltage condition the effects are similar to the unbalanced voltage condition.

  • The torque and speed characteristic is seriously affected when a 3-phase motor operates on a single phase.
  • The breakdown torque decrease to about 40% of its original value, and the motor develops no starting torque at all.

An additional affect is the current drawn from the remaining two lines will almost double, and the motor will begin to overheat.

Action needed:

  • Single phasing is a hazardous condition and steps should be taken to de-energize the motor.
  • The thermal relays, if provided, will protect the motor from overheating.