What is Single Phasing Fault on Induction motors

Single phasing fault is the worst case of voltage imbalance in a three-phase supply where there is a break on one of the phases in a 3- phase supply hence the remaining phases draw excess current which quickly overheats the motor windings and reduces the power output of the motor drastically.

Causes Of voltage imbalance and single phasing on an induction motor.

1) Faulty Circuit breaker – This is a situation where there is an open pole in the circuit breaker and that phase becomes absent.

2) Blown fuse – One of the three fuses use for protection if the circuit melts and supply is current is lost on that phase.

3) Faulty contactor- One of the phases of a contactor could be damaged or coated due to rust and not conducting.

4) Partial Contact – also known as high resistance connection. A loose connection anywhere on the power circuit could create a very high resistance joint which will cause a severe voltage drop at that point. This could cause complete loss of voltage or a  much lower voltage on one phase leading to grossly imbalance voltage.

Effects of single phasing on the electric motor

When a single-phase fault occurs the motor still tries to deliver its full horsepower causing the remaining two phases to pull excess current in an attempt to compensate for the missing phase this leads to heat buildup and failure of the winding insulation. It is documented that  44% of all motor failures are caused by heat.

The output horsepower of the motor also drops in the event of a single phase (or voltage imbalance) NEMA recommends that the maximum voltage imbalance between phases be 1% because if the imbalance is above 1 % the motor is de-rated i.e. delivers less power and heats up. A voltage imbalance of up to 5% could reduce the power output for the motor by 75%.

Calculating the % imbalance

1)    Take the voltage reading between RY RB YB phases let’s assume its 415, 400, 406

2)   Add all the readings and find average i.e. (415+400+406) / 3  = 407

3)   Subtract the average from the highest voltage reading to get the difference ie 415 – 407 = 8

4)   Divide the difference by the average i.e.  8 / 407 = 0.0197

5)   Multiply by 100 to get percentage i.e. 0.0197 * 100 = 1.97% approx. 2% voltage imbalance.

A practical case of how single phasing drops the horsepower of an electric motor was the event when an induction motor was used to drive a sewing machine, all of a sudden the operator noticed that the motor stopped and began to hum. He removed the belt that coupled the motor to the sewing machine and the motor ran a bit slower. He oiled the sewing machine on suspicion of a mechanical restriction but as he tried to couple the belt the motor shaft stopped rotating. The power the motor delivered was so low that you could stop the shaft rotation with your bare hands. Long story short; the electrical team was called in and the voltage readings across phases of the  400V power circuit were RY – 192V, RB – 400V, YB – 201V. And after troubleshooting, we found out that the severe voltage drop on one phase was caused by a loose connection on the Yellow(Y) phase.

The most common Symptom you will notice when there is a single-phase fault is humming and vibration of the motor. If the motor starts and you can visibly notice that RPM is lower there is a voltage imbalance greater than 7% between phases. 

NOTE -  it is possible to get a balanced voltage reading when the motor isn't running but the voltage on one phase drops immediately the motor is energized. This could also happen due to a loose connection or termination.