Freedom Is Not Licence

%The Highs and Lows of Motor Voltage

May 1, 2008 12:00 PM, By Ed Cowern, P.E.

Operating a motor at the "outer limits" of its voltage requirements reduces its efficiency and causes premature failure.

The economic loss from premature motor failure is devastating. In most cases, the price of the motor itself is trivial compared to the cost of unscheduled shutdowns of processes. Both high and low voltages can cause premature motor failure, as will voltage imbalance. Here, we'll look at the effects of low and high voltage on motors and the related performance changes you can expect when you use voltages other than those noted on the nameplate.

Effects of low voltage. When you subject a motor to voltages below the nameplate rating, some of the motor's characteristics will change slightly and others will change dramatically. To drive a fixed mechanical load connected to the shaft, a motor must draw a fixed amount of power from the line. The amount of power the motor draws has a rough correlation to the voltage 2current (amps). Thus, when voltage gets low, the current must increase to provide the same amount of power. An increase in current is a danger to the motor only if that current exceeds the motor's nameplate current rating. When amps go above the nameplate rating, heat begins to build up in the motor. Without a timely correction, this heat will damage the motor. The more heat and the longer the exposure to it, the more damage to the motor.

The existing load is a major factor in determining how much of a decrease in supply voltage a motor can handle (see sidebar, below). For example, let's look at a motor that carries a light load. If the voltage decreases, the current will increase in roughly the same proportion that the voltage decreases. For example, a 10% voltage decrease would cause a 10% amperage increase. This would not damage the motor, if the current stays below the nameplate value.

Now, what if that motor has a heavy load? In this case,...