May 1, 2000 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, you already have a high current draw, so voltage is already lower than it would be without the load. You may even be close to the nameplate's lower limit for voltage. When you have a voltage reduction, the current would rise to a new value, which may exceed the full-load rated amps.
Low voltage can lead to overheating, shortened life, reduced starting ability, and reduced pull-up and pullout torque. The starting torque, pull-up torque, and pullout torque of induction motors all change, based on the applied voltage squared. Thus, a 10% reduction from nameplate voltage (100% to 90%, 230V to 207V) would reduce the starting torque, pull-up torque, and pullout torque by a factor of .92.9. The resulting values would be 81% of the full voltage values. At 80% voltage, the result would be .82.8, or a value of 64% of the full voltage value. What does this translate to in real life? Well, you can now see why it's difficult to start "hard-to-start" loads if the voltage happens to be low. Similarly, the motor's pullout torque would be much lower than it would be under normal voltage conditions.
On lightly loaded motors with easy-to-start loads, reducing the voltage will not have any appreciable effect, except that it might help reduce the light load losses and improve the efficiency under this condition. This is the principle behind some add-on equipment whose purpose is to improve efficiency.
Effects of high voltage. An assumption people often make is that since low voltage increases the amperage draw on motors, then high voltage must reduce the amperage draw and heating of the motor. This is not the case. High voltage on a motor tends to push the magnetic portion of the motor into saturation. This causes the motor to draw excessive current in an effort to magnetize the iron beyond the point where magnetizing is practical.
Motors will tolerate a certain change in voltage above the design voltage. However, extremes above the design voltage will cause the amperage to go up with a corresponding increase in heating and a shortening of motor life.
For example, manufacturers previously rated motors at 220/440V, with a tolerance band of 510%. Thus, the voltage range they can tolerate on the high-voltage connections is 396V to 484V. Even though this is the so-called tolerance band, the best performance would occur at the rated voltage. The extreme ends (either high or low) put unnecessary stress on the motor.
Don't fall into the trap of thinking you're okay just because your supply voltage falls within these tolerance bands. The purpose of these bands is to accommodate the normal hour-to-hour swings in plant voltage. Operation on a continuous basis at either the high or low extreme will shorten the life of the motor.
Such sensitivity to voltage is not unique to motors. In fact, voltage variations affect other magnetic devices in similar ways. The solenoids and coils you find in relays and starters tolerate low voltage better than they do high voltage. This is also true of ballasts in fluorescent, mercury, and high-pressure sodium light fixtures. And it's true of transformers of all types. Incandescent lights are especially susceptible to high voltage. A 5% increase in voltage results in a 50% reduction in the life of the lamp. A 10% increase in voltage above the rating reduces incandescent lamp life by 70%.
Overall, it's definitely better for the equipment if you change the taps on incoming transformers to optimize the voltage on the plant floor to something close to the equipment ratings. In older plants, you may need to make some compromises because of the differences in the standards on old motors (220/440V) and the newer "T-frame" standards (230/460V). A voltage in the middle of these two voltages (something like 225V or 450V) will generally result in the best overall performance. High voltage will always tend to reduce power factor, thus increasing the losses in the system. This results in higher operating costs for the equipment and the system.
A standard figure (found in motor books and in the original print version of this article) illustrates the general effects of high and low voltage on the performance of "T-frame" motors. This graph is in wide use in a variety of reference materials. But, it's only representative and does not give precise information that applies to all motors. Instead, it represents only a single motor type, with much variation from one motor design to the next. For example, the lowest point on the full-load amp line does not always occur at 21/2% above rated voltage. On some motors, it might occur at a point below rated voltage. Also, the rise in full load amps at voltages above rated tends to be steeper for some motor winding designs than others. The sidebar, on page 78, offers some guidelines for determining the effects of voltage variations on individual motor designs and frames.
Don't place stress on your electric motors and other electrical equipment as a result of operating a power system at or near the ends of voltage limits. The best life and most efficient operation usually occur when you operate motors at voltages very close to the nameplate ratings. When supplying voltage to motors, stay away from the "outer limits."
This text is an adaptation of The Cowern Papers, courtesy Baldor Electric Co., Wallingford, Conn., edited by Mark Lamendola, EC&M Technical Editor. Cowern is an Application Engineer for Baldor.