Beware the big bad brownouts

A long duration of undervoltage can toast your motors, but there are ways to prevent the problem.

By Chuck Yung

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Ongoing energy shortages can mean big trouble for electric motors. The dangerously low voltage that characterizes brownouts and power interruptions not only reduces motor efficiency, but also may cause a rash of motor failures in the days, weeks and months that follow. The absolutely best defense — shutting down every motor during a brownout — is seldom an option. Rarely are production personnel eager to scrap a perfectly good batch of product just to protect an electric motor. Still, knowing how to respond to a brownout or power interruption can minimize the effects of the problem in ways that may be more acceptable to production.

Why brownouts pose problems

Brownouts or power interruptions are, by definition, periods during which line voltage is significantly lower than normal. Although operating on reduced voltage extends the life of an incandescent bulb, it can be hard on motors. Reviewing the relationship between motor torque and applied voltage shown in Figure 1 makes the reason clear. The torque that an electric motor develops changes as the square of the applied voltage. A 10% voltage increase, for example, boosts torque by 21% (1.1 x 1.1 = 1.21). Conversely, a 10% voltage decrease during a brownout reduces torque by 19% (0.9 x 0.9 = 0.81). A motor operating on line voltage 20% below normal produces only 64% of its rated torque (0.8 x 0.8 = 0.64).

Unless the load drops immediately at the onset of the brownout, the motor will, in effect, be subjected to a 156% overload. It will overheat and fail. Then, when output torque decreases and the motor stalls, the total amount of input electrical energy is being converted to heat. Now, the winding temperature increases even faster.

So what?

First, excess heat is a problem for motors because every 10° C increase in operating temperature halves insulation life. Second, the winding temperature rises 10° C to 15° C for each 10 percent drop in voltage. Taken together, these mean a motor operating on 10% lower voltage will exhibit an insulation life decrease of 50% to 75%. Figure 2 shows a temperature increase of 10° C cuts a motor's theoretical life expectancy from 30,000 hours to 15,000 hours or less, and an increase of 20° C shortens it to 7,500 hours. The winding temperature of a severely overloaded motor can rise 50° C or more in a matter of minutes, reducing its life expectancy to less than 940 hours.

If you take away nothing else from this article, take this. The theoretical effects on insulation life shown in Table 1 apply to motors exposed to elevated temperatures for any appreciable period, not just those operating continuously at elevated temperatures. That means brownouts lasting less than an hour can damage motor insulation and set up these drivers for an expected, but unpredictable, failure after the brownout ends. A sustained brownout, on the other hand, destroys the insulation on every electric motor running fully loaded during the event.

While brownout voltages damage every running motor, some designs are less susceptible than others. The more robust motor models include energy-efficient units that meet EPACT regulations, NEMA premium-efficiency motors, motors designed for variable frequency drives and older U-frame motors. These designs withstand increased operating temperatures a little better because they have more copper in their windings, better insulation systems, or simply more iron in the stator. Unless they have upgraded insulation systems, pre-EPACT T-frame motors or metric motors are more likely to sustain damage from under-voltage conditions. Ultimately, the only immune motor is one that's turned off during the brownout.

 Motor Size (hp)  Temp (deg C) -10% voltage  Temp (deg C) nominal voltage  Efficiency (%) -10% voltage  Efficiency (%) nominal voltage
 10  66  56  90.0  91.4
 20  84  70  90.4  91.8
 50  84  69  91.9  93.1
 100  82  72  94.2  94.8
 200  90  77  94.9  95.5

What about efficiency?

Another potentially expensive result of operating during a brownout is increased energy consumption. Compared with normal operation, an overloaded motor converts more of the supplied power to heat and less to doing work, so efficiency drops and power bills rise —sometimes dramatically. As Table 1 shows, operating at 10% voltage decrease reduces efficiency 0.5% to 1.0%. This seemingly small change could have a big effect on the bottom line, given the incremental cost of electricity during peak periods that foster brownouts can soar from around $95 per MWh to more than $1,000. Figures 3 and 4 illustrate the effects of low-voltage operation on efficiency and winding temperature.

Some good news

It's clear that brownouts and power outages have expensive repercussions, both in terms of damaged motors and higher power bills. Fortunately, proactive management can reduce or even eliminate the effects of low-voltage events.

Basic safeguards include monitoring the supply voltage and training personnel to respond quickly if it drops to a trigger level. For critical applications, install thermal protectors or condition-monitoring devices that detect abnormally high winding temperatures and shut down the motor. If shutting down during a brownout isn't an option, it's sometimes possible to reduce the load by throttling back a fan or partially closing a valve. The alternative is to rewind or replace motors at an alarming rate.

Finally, when having a motor rewound, be sure to specify class H insulation and maximum slot fill. The greater the ratio of circular mils of wire cross-section to motor current, the better able the motor will be to withstand under-voltage conditions (and the more efficient the motor will be).

Power interruptions and brownouts probably will be with us for some time, especially in California and on the East Coast. Because electric motors account for 70% of industrial electricity use, it makes sense to be proactive in guarding against the effects of low-voltage events. The benefits for end users include avoiding unnecessarily high power bills and substantial motor repair and replacement costs. Even motor service centers would benefit because there would be fewer unjustified warranty claims.

Chuck Yung is a Technical Support Specialist at the Electrical Apparatus Service Association (EASA), St. Louis, Mo. He can be reached at 314-993-2220.

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