When It’s Time To Replace The Motor

March 14, 2002
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Most of the time it’s not a question of whether or not to replace a motor. By the time a contractor is called to the scene, the motor has already failed or is in its death throes.

Replacement is usually a viable solution, because repairing a motor can be fairly expensive. Indeed, it may be the case that the cost to repair the motor in a typical air handler (15 to 40 hp) is greater than the cost of a new motor.

Then there’s the question of whether or not the customer can wait for a motor to be repaired. It always seems that the motor fails on either the hottest or coldest day of the year, and chances are the customer is going to want a replacement right away.

A failing motor is often identified by excessive bearing noise and vibration, according to information from Fasco Motors, Springfield, MO. In addition, a failing motor may not always start, or it may trip on and off due to its overload device receiving excessive amperage resulting from tight bearings.

“To determine whether or not the motor is working correctly, measure amperage and rpm to make sure it is within the equipment specifications. The shaft of a de-energized motor can be manually rotated to make sure it is not binding, tight, and free of axial play,” says Todd Greenwell, market manager — Distribution, Fasco Motors.

He adds that it is possible to service the motor. For example, bearings can be replaced and damaged lead wires can be repaired. In addition, capacitors on permanent-split capacitor (PSC) motors can be replaced and some sleeve bearings can be changed out if the shaft has not been damaged from a dry bearing. However, Greenwell notes that a qualified motor repair technician should perform the services.

Jim Nanney, Baldor Electric Co., Fort Smith, AR, adds that poor service can cause the single biggest problem he has seen: belts that are overtightened, causing premature bearing failure. “In addition, regreasing with the wrong grease or using too much grease is a problem,” he says. “The bearing will begin to ‘sing’ for all to hear in the building as it sends it down the ductwork.”

Age isn’t always a factor in motor failure, either. Indeed, Greenwell says that one of the primary reasons for motor failure is excessive heat generated by the lack of cooling air, mechanical overload, excessive ambient temperature, or blocked cooling vents. Moisture, stemming from rain entering the motor or condensation buildup with blocked motor drain holes, can also lead to failure.

“Other factors that contribute to motor failure are harsh application, loads that have vibration, lack of re-lubing where required, and high on-off operating cycles. Premature motor failure is usually due to misapplication of a motor through the conditions just mentioned,” says Greenwell.

FACTORS TO CONSIDER

If the motor is close to failing or, indeed, has failed and a replacement is necessary, the question is whether to swap it out for a like motor, or to replace it with a more efficient motor.

A more efficient motor should be considered if the motor runs a lot, as there may be potential energy savings, says Nanney. “However, there is a down side and that is the more efficient the motor, the faster it will tend to run. This can actually cause the motor to use more energy if the speed changes too much without a little trimming of the load. Also, the fan and coils were matched at the factory for good performance. Be sure the contactor and circuit breaker are rated for the new load.”

And don’t ever be tempted to change out one type of motor for another; for example, using a PSC for a shaded pole. “That it is not recommended,” says Greenwell. “The tolerance the equipment has is usually low for replacement motors with different operating characteristics. Each type of motor has specific characteristics needed by the equipment it is operating in, and should not be altered.”

Sometimes contractors think it’s necessary to replace a motor with one having higher horsepower than the original; that’s not a good idea either. As Nanney notes, the manufacturer (operating under guidelines from UL and ETL) spends a great deal of time making sure all the motors, contactors, overloads, wiring, and circuit breakers are coordinated. Using a different-sized motor could really cause some problems.

Greenwell advises that there are 12 basic things a motor distributor will want to know in order to provide a satisfactory replacement:

1. What is the application?
2. How is the motor mounted?
3. What type of motor is it?
4. What ventilation does it have?
5. How many speeds does it have?
6. What is the direction of rotation?
7. What are the horsepower, rpm, and voltage?
8. What is the shaft length and diameter?
9. Does the motor have any special features?
10. What is the motor diameter?
11. What type of bearings does it have?
12. What is the oem part number on it?

Once you come up with that information, chances are the distributor will be able to steer you in the right direction.

The Department of Energy’s free software, MotorMaster+3.0, can help you find electric motors for your particular application and compare the cost of an energy-efficient motor to a standard-efficiency motor. To download MotorMaster, go to http://mm3.energy.wsu.edu/mmplus/default.stm (website).

For more information on motor replacement, check out Fasco’s 54-page brochure, “Time-Saving Motor Replacement Tips.” It can be found at www.fasco.com/fasfacts1.asp (webpage). Information on motor installation and operation can also be found on Baldor’s website at www.baldor.com.

Publication date: 03/18/2002

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