It’s impossible to balance line-to-line voltages perfectly in three-phase circuits, so they typically differ by a few volts or more. However, if voltage unbalance exceeds 1%, it can markedly decrease the performance and energy efficiency of three-phase motors while increasing the likelihood of premature failure.

What is voltage variation? Voltage variation is the deviation of voltage from the rated voltage; NEMA MG1 Section 12 allows a plus or minus 10% variation from rated voltage. That rating assumes balanced voltages and acknowledges that motor performance will not necessarily be the same as at rated voltage. "Table 1: Typical effects of voltage change" explains numerically the relationships between various motor performance parameters and a dynamic power supply subject to variation. It was featured in a paper by P. Pillay at an IEEE Petroleum and Chemical Industry Committee (PCIC) conference in 1995 and is shown on Page 24 in EASA's Electrical Engineering Pocket Handbook. The table was a subject of discussion at the recent meeting of the EASA Technical Services Committee in St. Louis. One purpose of this article is to better utilize the information in this table.

Three of the most common motor problem calls received from members by EASA's Technical Support Department are:

• "The motor is drawing high no-load current."
• "The current of the three line leads is not balanced."
• "The motor is running hot."

If you have ever faced one or more of these issues, and it's almost certain you have, read on.