Since about one-fifth of electric motor failures are caused by an overheating problem that leads to insulation failure, any suspicion of an overheated motor should be taken seriously. Bearing damage and other problems can also result from overheating, so if you've recently had a bearing failure, you should include overheating on your list of possible causes. And if checking your engine's draw does indicate overheating, the next step is to find out why. Here are three steps to troubleshoot common causes of overheating in electric motors.
1. Check the conditions of operation.
How is the engine being used? Stopping and starting an engine too often results in many temperature changes in quick succession. The heating and cooling cycles that result can prematurely damage insulation and windings, eventually ending in failure when a short is created. Environmental factors can play a part as well; high altitude (above 3300 feet), extremely high environmental temperature, and improper airflow can all place additional stress on an engine, causing it to heat up faster. Any of these circumstances point to an easily explainable cause for overheating.
2. Check that correct voltage is being used.
Trying to use a higher voltage to reduce current is an ineffective shortcut because it results in excess buildup of heat despite the lower current. This can especially be a problem if you're relying on stator current readings as an indicator of load. Stator current is an unpredictable and variable indicator and may not show the true state of your motors until it's too late. Low voltage can be just as much of a problem as high voltage, because the motor will have to draw higher current to compensate for the lower voltage. The higher current can then cause winding temperatures to rise, which can also result in overheating. If you find that the voltage is either too high or too low, it should be a prime suspect on your list of possible causes.
3. Calculate the service factor.
You can check the service factor of your motor with the help of a technician or by comparing the load your motor is running (use a dynamometer to take a reading of the load) with its respective NEMA derating. Any result that indicates your motor has a service factor higher than 1.0 should be cause for concern, since it means that the motor is unduly stressed. Your motor may be labeled as having the capacity for greater than 1.0 service factor. This means that it was designed to be able to take the pressure of a higher service factor for a little while if necessary. It's just a safety feature and doesn't mean that the motor can operate constantly under these conditions.
By following these three steps, you can either identify or eliminate several common causes of electric motor overheating. Let your repair technician know what data you've gained about the cause of the problem. For example, if you've discovered that the voltage to your motor was ten percent too high, let him know so that he can apply any special considerations regarding the repair of an overheating due to over-voltage. Your repair technician can also advise you on how to prevent motor overheating in the future.
For more information, contact Boise Electric Motor Company or a similar organization.
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