Utility Voltage Unbalance

[jtester]

There are motor derating curves for voltage unbalance. 3% causes a 90% derating. 4.5% causes an 80% derating factor. If voltage is low, current increases, causing added heat. 5% low voltage causes 5% increase in current or a 10% increase in stator heating.

In my experience a 1% voltage imbalance can cause a 6-10% current imbalance, it is not a 1 to 1 ratio. A DOE paper on motor imbalance reports a 2.5% voltage imbalance in the feed to a 100 hp motor resulted in a 27% current imbalance.

I usually identify a voltage imbalance in excess of 2% as very problematic. Note that we are talking of imbalance between phases not just a voltage swing.

Jim T