Subscribe to our

Pumps & Systems, July 2008

Most of us have seen the graph shown in Figure 1, which shows how a motor's various operating characteristics change with a corresponding increase or decrease in supply voltage. NEMA motors are designed to accommodate a voltage variation of +/-10 percent of their nameplate voltage.

Will a variation of this magnitude affect motor life? It depends on the motor loading. For example, the graph shows that a 10 percent decrease in supply voltage will result in a 10 percent increase in current draw. As long as this increase does not exceed the nameplate current, no damage will occur. In fact, a small voltage drop on lightly loaded motors can actually improve efficiency (the Nola effect). However, if the motor is already loaded to its nameplate amperage, a 10 percent voltage drop will definitely adversely affect motor life due to the additional heat that will be generated.

Figure 1

Figure 1 shows a similar increase in starting current when voltage increases by 10 percent. Some stator designs also undergo a much larger increase in full load amps than shown on the right side of the graph. This increase is due to magnetic saturation and can be even greater on lightly loaded (<50 percent) motors. In the end, the motor's service factor (SF) defines its ability to handle the additional current due to voltage variation (or varying loads). SF is intended to provide protection against short periods of rises and sags that occur in many circuits. If low or high supply voltage is continuous, it should be corrected at the source. The closer to nameplate voltage a motor is operated, the longer its life.

Voltage Unbalance

A rise or sag in supply voltage may or may not have an adverse effect, but another type of voltage variation will almost always shorten the life of a fully loaded motor - -: voltage unbalance. Voltage unbalance occurs when the individual phase voltages in a three-phase circuit are not equal. The major effect of this unbalance is an increase in stator and rotor I2R losses, which results in an even larger phase current unbalance. A voltage unbalance among the phases of just 1 percent can result in a current unbalance of 6 to 10 percent (even higher when a VFD is involved).

Figure 2 is a close approximation of the percent current unbalance generated by unbalanced phase voltage at various motor loads. The ratio of current to voltage unbalance increases dramatically as motor loading decreases. In the case of a fully loaded motor, a 2 percent voltage unbalance will typically result in a current unbalance of about 15 percent. NEMA motors are designed to accommodate a maximum phase voltage unbalance of 1 percent.

Figure 2

The net effect of voltage and current unbalance is a substantial increase in motor operating temperature. This increase can be estimated by the following equation: % Increase = 2(% Voltage Unbalance)2. For example, if unbalance is 2 percent, the expected temperature increase would be 8 percent. If a motor normally operates at 130-deg C, a 2 percent unbalance would raise the operating temperature to 140-deg C. This may not seem like a huge increase, but insulation life is reduced by 50 percent for each 10-deg rise in operating temperature. A worst case scenario occurs when there is a combination of significant supply voltage variation and voltage unbalance.

Sources

Unbalanced voltage can originate with the utility or within your own distribution system. (The "Fixes" section will identify a number of the potential causes.) Another source of current unbalance is voltage distortion. This condition, due to the harmonics produced by non-linear loads, is characteristic of solid state devices. These harmonics may not lead to measurable voltage unbalance but can cause a significant current unbalance.

Current unbalance can also be due to problems on the motor side of the circuit that usually do not result in voltage unbalance. Since current unbalance can exist without an accompanying voltage unbalance, an accurate system analysis will require measurement of both voltage and current in the circuit.