Motor Overheating

November 2, 2004
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Our final installment of motor troubleshooting basics will look into possible causes of HVACR motor overheating. We will look at several potential overheating culprits, from the obvious to the not-so-obvious.

Some motor problems occur due to a service or maintenance issue with the motor. Some occur because the motor was not suited for the job. Some require basic maintenance, and still others require a change in the facility's power infrastructure. Those will require that you bring in another professional: a qualified electrician.

The first thing to check is whether or not the motor is overloaded. Check the motor's amp draw using a good ammeter. If the amp draw is 110 percent or more of that marked on the motor nameplate, the motor is definitely overloaded. You will need to replace the motor with a higher hp motor, or one designed with a service factor.

If the motor is a belt-drive motor, a tight belt can also produce an overloaded condition. The belt should not be taut, that is, it shouldn't have too much play, but it should not be so tight that there is no give at all. If you cannot easily cause a deflection of 1 inch halfway between the two pulleys, loosen the belt and retighten it to the manufacturer's specs.

Power Problems

One of the big culprits on three-phase motors is unbalanced voltage. When you are troubleshooting a three-phase motor, it is necessary to measure the voltage of each phase. Voltages on all three phases should be within one percent of the average. If they are not, contact a qualified electrician to help correct the problem.

You may also encounter a three-phase motor running on single-phase power. This is the severest form of unbalanced voltage. One phase has no voltage at all.

Check any fuses and reset breakers if they have tripped. If the fuses and breakers are good and you still have a severe unbalance, the problem could be at the transformer Call an electrician to determine if the transformer is the problem.

Is the voltage too high or too low? Voltage must be within 10 percent of the nameplate value.

If voltage is low, you can use a higher horsepower motor or one with a service factor. If voltage is high, use a specially rated motor (certain motors are more tolerant of high voltage). You may also want to contact an electrician to find the reason for the high or low voltage.

Physical Problems

Once you have checked out the motor for potential power problems, you need to check for other maintenance-related problems. You should check for some of these even if you have already found a power problem

Check for dirt blocking the motor's vent openings. Vacuum the motor's vents (if any). Dirty filters and coils can also prevent the motor from cooling. Clean them all.

Check for loose or faulty connections at the motor. This can be a safety issue; high-resistance connections at the terminal board can cause severe overheating. Make sure that all connections, particularly quick connects, are snug.

Is the rotor misaligned? This is usually the result of end-play system failure, where there is too much end play. If there is over 1/16 inch of end play, replace the motor.

Bearings

Bad bearings can cause a number of symptoms and overheating problems, depending on the type of bearing in the motor. Let's take a look at the most common symptoms.

Worn bearings can cause the following symptoms:

  • Rotor rub.

  • Motor bark or growl on startup.

  • Side-to-side play.

  • Front-to-back play.

    If any of these symptoms are present, replace the motor.

    Worn bearings can be caused by excessive side loading of the motor's shaft. Correct this by replacing the motor with one designed to handle excessive side loading. Ball bearings can handle side loading better than sleeve bearings.

    Wear can also be caused by an overhung load. If you find this is the case, try to put the load closer to the bearings, or replace it with a motor designed to handle heavier side loads.

    Worn bearings can be caused by excessive thrust loading of the motor shaft. If this is the case, replace the motor with one designed to handle heavier thrust loads. Ball bearings can handle more thrust loading than sleeve bearings.

    In some cases on large, integral hp pump motors, not enough thrust loading on the motor shaft can also cause bearing wear. Replace the motor with one that is matched to handle the thrust load.

    Bearings may also wear when there is insufficient lubrication; this can be due to high ambient temperatures. You can replace the motor with one that uses a higher temperature lubricant, or replace it with a motor that can be relubricated. This means you need to make sure it is relubricated regularly.

    Sometimes lubrication breaks down due to contamination. Replace the motor with one that has sealed bearings, or with a motor that can be relubricated and relubricate it regularly.

    If the motor has a sleeve bearing, has it been loaded into the wick window? Orientate the replacement motor so that the load is not into the wick window or use ball bearings.

    Is the motor a sleeve bearing motor running at below 500 rpm? If the motor is running below 500 rpm, replace it with a ball bearing motor.

    Likewise if the motor is a sleeve bearing motor and is in a rapid start-stop cycle, it should be replaced by a ball bearing motor.

    A final note: Tight belts can also cause excessive bearing wear. When you install a new motor, make sure the belt is tightened properly.

    For more information, click on the Emerson Climate Technologies logo above.

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