A Phase Imbalance Fault indication on a motor monitor means that the voltage and / or current in one phase winding of the motor is not the same as the other two phases by an amount that exceeds a settable maximum.This means something is wrong and need to be corrected. The most common reason for this fault to appear is from loss of a single phase in the supply to the motor. 

Three phase motors will continue to run during a loss of phase, a condition called  single phasing. If left running without one of the phases being powered, the motor will overheat and be damaged.

It is also possible for the phases in our  power to be at different voltages. Often this is caused by the power company having more load on one phase. Power companies split off their phases to different geographic areas. We may be that phase A has lots of commercial customers hooked to it while phase B has more residential customers. In the late afternoon, the commercial customers close decreasing the load on phase A. Shortly after this all the workers come home and start using power on phase B and makes the problem worse. Power companies spend a lot of time and effort trying to balance everything as best they can but things happen, situations change. A factory may close or expand and change things.

A third reason for a Phase imbalance Fault is from a motor winding going bad. The insulation might have failed in a winding and shorted part of the turns or broken causing an open circuit.Motor monitors are stand alone CPUs inside an enclosure with indicator lights and switches used to setup the monitor. They will be connecting to current transformers and potential transformers that measure current and voltage for each phase. Monitors also usually also have optional connections for temperature sensors on the two bearings. It might have other options for things like vibration sensors. They are often part of a larger distributed control system and have different types of COM ports to connect to a remote computer. The more expensive and critical the motor is, the more elaborate the motor monitor will be. Everything will be constantly monitored so a failure can be detected before it does more damage. Often an upcoming failure can be predicted by recording all this information and noting changes, such as increasing phase differentials or a rising bearing temperature.

A typical setting for a phase imbalance alarm is 3–5% voltage / current differential.

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