Electric Motor Tech Tip: Voltage and Current Troubleshooting

Even with modern MCA and ESA technologies available, we often will not have them at our disposal. Instead, we must rely upon our age-old tools such as the voltmeter and ammeter.

One of the issues that commonly comes up has to do with how much current unbalance is too much?

The answer is a qualified: ‘it depends.’

If you have selected the appropriate true RMS meter and you are testing a system that has low to no harmonic content, then you can utilize the guidance shown in NEMA MG1-2006. In this case, for an AC induction motor of any size, NEMA defines current unbalances of 6 to 10 times that of the voltage unbalance. So, for a 1% voltage unbalance, your current could be 6 to 10%, for a maximum 5% voltage unbalance, your current unbalance could be 30 to 50%. Significant variation!

If you have a circuit impedance unbalance, voltage vector issues, high harmonics, or a failed power factor correction capacitor, the current unbalance may be significant in relation to the voltage unbalance. How do you tell the difference if the unbalance is in the motor or the supply?

One method is to ‘rotate the phases.’ This can be done right at the starter and requires that the power is turned off and all appropriate safety measures are taken:

1. Note the high current leg. Let’s use the example that Phase A (A) is the high leg;
2. At the output of the starter, move A to B, B to C, and C to A. This prevents the motor from running in the opposite direction. Now C is A, A is B, and B is C;
3. Take your measurements from the output of the starter.
a. If the high current stays on the A position, then it is related to the supply;
b. If the high current moves to B, then it is related to the motor.
From this point, you can focus either on the motor or supply.