Phase To Phase

Question: Phase To Phase
We have a compressor motor at 480v 217fla.
L1=135A
L2=126A
L3=144A
Is it a problem having this much difference phase to phase on L2/L3?



Answer:
The likely mechanisms of a stator winding fault are either a turn-to-turn or phase-to-phase short, or an insulation to ground fault. A turn-to-turn short is identified as a shorting of one or more windings in a coil. This can develop into a very low impedance loop of wire, which acts as a shorted secondary of a current transformer. This results in excessive current flow through this shorted loop, creating intense heat and possible insulation damage. Due to the nature of the low voltage random wound design a shorted turn could occur with much higher impedance, allowing the motor to run for extended periods of time before eventually burning up the coil with the high currents. As a result it is not unexpected to find low voltage motors still running with bad stator windings. Form wound coils however, do not exhibit high turn impedances and will therefore burn up quickly following the presence of a turn-to-turn short. A phase-to-phase short is identified as a shorting of one or more phases to another phase. This fault can be quite damaging due to the possibility of very large voltage potential existing between phases at the location of the short. In your case we have an inductive imbalance of 6.67% which is not considered abnormal for a 480v motor. This imbalance may be caused by the rotor by design or as a result of a rotor anomaly. Performing a Rotor Influence Check will allow a quick root cause analysis of the 6.67% imbalance identifying either the rotor or stator as the cause. Once you have identified the source you can better focus your analysis and trending.