One of the most frequent customer complaints is “low voltage.” Like “defective wiring” in the fire investigation field, “low voltage” takes the blame for many problems. Where motors are concerned, what is “low,” and how are motors affected?
Let’s distinguish between two conditions —reduced voltage for a few cycles (or a few seconds) — and the same condition lasting for minutes or even hours. For both a-c motors (single-phase or three phase) and their controllers, NEMA standards tell us that the second of those conditions is OK provided the apparatus terminal voltage doesn’t go below 90% of its nameplate rating. The motor will operate “successfully” — no overheating, enough torque to carry the load — but not with its normal performance, such as efficiency. A contactor will remain closed.
If the equipment is initially de-energized, closing the circuit at no less than 90% voltage will close the contactor and successfully start the motor (provided the driven machinery torque and inertia are within NEMA standard limits).
When the voltage reduction is only a fleeting transient, a more serious dip will be safe for the motor, able to ride through such a brief disturbance. There’s no standard. But during either starting or running a drop to 70% of rated voltage is usually tolerable.
Starting contactors, however, will drop open if they see 65-75% voltage for more than 3 to 8 cycles — a fraction of a second. Solid state electronic motor starters, micro-processor controllers, or adjustable frequency power supplies will typically quit working at 80-85% voltage for a similar time. Even an electronic computer isn’t normally that sensitive. And remember, we’re not talking “interruption” here; power is never lost completely.
Deliberately lowered voltage on a motor carrying at least half rated load — the 5% “brownout” some utilities have used during system peaks — will not reduce motor load or energy usage; because of higher copper loss in running motors, energy usage will probably actually increase. As long as the horsepower output remains unchanged, low motor voltage means higher motor current — and slightly greater electric power input. But the motor isn’t likely to be harmed.
Remember that whether motor voltage is “low” or “high” can only be judged at the motor terminals not at the control center, the bus, or the transformer secondary. Standard voltage tolerances apply only to the equipment nameplate value which, in turn, must be measured at the equipment terminals.
From time to time, various equipment vendors have declared that their apparatus requires line voltage variation to be limited to “1 2%,” or “2 to 3” volts on a 120-volt basis. Not only is such close control impractical, it is also not prescribed by national standards for power system operation and is not required by any standard motor or control apparatus.
Richard L. Nailen, P. E