Bad Power is No Power

Motors used in building systems—despite their degree of efficiency—are all subject to poor power quality, which can cause overheating and render a motor useless. But there are steps to follow to protect your motors from the problems—spikes, harmonics, single-phasing and tripping—incurred from poor power quality.

10/01/2003


Motors used in building systems—despite their degree of efficiency—are all subject to poor power quality, which can cause overheating and render a motor useless. But there are steps to follow to protect your motors from the problems—spikes, harmonics, single-phasing and tripping—incurred from poor power quality.

For Jack Hudson, P.E., senior associate with CRB Consulting Engineers, Inc., Kansas City, Mo., adhering to the code is a good place to start. His firm follows NEC Article 430, which covers motor overload and short circuit protection. "If you follow this guideline, it provides for a relatively safe level of motor protection," he says.

However, Hudson explains that from one company to the next, there is a difference of opinion as to what level within the guideline to go with. For example, if a circuit breaker goes up to 250% of the full load rating of the motor, some firms will run the motor at 250%. Others, including Hudson's firm, believe that's just too high. "We take a little more conservative approach to sizing motor short circuit protection, which results in using smaller frame sizes for circuit breakers. This saves the client money and space on panels," he says.

Proper system design and specification is another way to minimize motor problems. "Good motor protection starts with good mechanical design in your own office," says Pete Bohn, P.E., chief electrical engineer for the Austin Company's Midwest Region.

Once the system is designed to code adherance, sizing overcurrent protection is key. "Frequently, we'll select the overcurrent device based on what the actual amperage draw is for that motor," Hudson says. "We'll do a field test on the motor, take an amperage rating and then select the overload protection based on the results of the test. Otherwise, because the actual loading conditions on a specific motor may vary, we base it on the nameplate amperage rating of the motor."

Variable Frequency Drives

When it comes to motors powered by variable frequency drives, the risk of failure increases. "A VFD is harder on a motor, but a motor can be specified to withstand that assault, and so you can still wind up with a quality installation," Bohn says.

Hudson expresses that most VFD issues can be avoided by placing the VFD closer to the motor. If the distance is more than 50 ft., his firm will employ a dv/dt (change in voltage per change in time) filter on output of the drive to attenuate spikes. If the distance is under 50 ft., they don't use the filter and typically don't experience problems.

Bohn also explains that when a VFD is used with a motor in a hazardous application, it requires a great deal of care in design; minimum speed, proper operating temperatures and temperature monitoring should all be used for motors driven by VFDs in hazardous areas.

Above all, "just do it" is Bohn's most important advice. "When you apply any of the available overload protection techniques, whether it's alloy, bimetallic or the electronic forms, your chances of having overload and motor difficulty become very, very slim."



Motor Protection Tips

Start with proper design and specification

Adhere to the code

Size overcurrent protection appropriately

Minimize distance between VFD and motor when possible

Make special considerations for VFD applications in hazardous areas



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