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Q. I have a motor that is running very hot. Any ideas why?

Q. I’m getting a lot of noise and vibration from a motor. Looks like it’s mounted and coupled correctly, but can’t uncover the problem.

Q. I have the same motor design that is failing quite often. Any ideas why?

Q. I’ve heard that voltage unbalance can reduce the service life of a motor. Is that true?

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Q. I have a motor that is running very hot. Any ideas why?

The Pro Says: It could be a number of things. But whatever it is, the problem should be fixed quickly because that motor is heading for a breakdown. Excessive heat buildup will shorten the life of the motor’s insulation and hence, winding life. After overheating, a motor may run satisfactorily but its useful life will be shortened.

Here are a few things to look for:

Poor Cooling: Accumulated dirt or poor motor location may prevent the free flow of cooling air around the motor. In other cases, the motor may draw heated air from another source. Internal dirt or damage can prevent proper air flow through all sections of the motor. Dirt on the frame may prevent transfer of internal heat to the cooler ambient air.

Undersized Motor:  It may be too small or have the wrong starting torque characteristics for the load. This may be the result of poor initial selection or changes to the load requirements.

Excess Motor Loading: Excess loads or jams in the driven machine force the motor to supply higher torque, draw more current and overheat.

Too Much Friction: Shaft misalignment, worn bearings and other problems in the driven machine, power transmission system or motor increase the torque required to drive the loads, raising motor operating temperature.

Electrical Overloads: An electrical failure of a winding or connection in the motor can cause other windings or the entire motor to overheat.

Noise and Vibration: Noise indicates motor problems but ordinarily does not cause damage. Noise, however, is usually accompanied by vibration.
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Q. I’m getting a lot of noise and vibration from a motor. Looks like it’s mounted and coupled correctly, but can’t uncover the problem.

The Pro Says: Finding the cause of excessive vibration can be tricky. But the sooner you find it, the better off the motor and the driven equipment will be often required.

Misalignment of the Motor Shaft and Driven Equipment: This the most common cause. Noise and vibrations can be caused by a misaligned motor shaft or can be transmitted to the motor from the driven machine or power transmission system. They can also be the result of either electrical or mechanical unbalance in the motor.

After checking the motor shaft alignment, disconnect the motor from the driven load. If the motor then operates smoothly, look for the source of noise or vibration in the driven equipment.

If the disconnected motor still vibrates, remove power from the motor. If the vibration stops, look for an electrical unbalance. If it continues as the motor coasts without power, look for a mechanical unbalance.

Electrical unbalance occurs when the magnetic attraction between stator and rotor is uneven around the periphery of the motor. This causes the shaft to deflect as it rotates creating a mechanical unbalance. Electrical unbalance usually indicates an electrical failure such as an open stator or rotor winding, an open bar or ring in squirrel cage motors or shorted field coils in synchronous motors. An uneven air gap, usually from badly worn sleeve bearings, also produces electrical unbalance.

The chief causes of mechanical unbalance include a distorted mounting, bent shaft, poorly balanced rotor, loose parts on the rotor or bad bearings. Noise can also come from the fan hitting the frame, shroud, or foreign objects inside the shroud. If the bearings are bad, as indicated by excessive bearing noise, determine why the bearings failed.
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Q. I have the same motor design that is failing quite often. Any ideas why?

The Pro Says: Could be a number of things. First determine the root cause of failure. As a team, evaluate what type of failure is most common. Sometimes the root cause of failure is many different items. After fixing the most common failure, move to the next item that can be evaluated and corrected.

Here are a few things to look for:

Bearing Failures: Probably the most common failure area in a motor. Just a few of the items to check:

• Bearings lubricated with grease- Analyze the bearing’s load zone to see if a wear pattern or signs of thrusting have occurred. If so, one or more of the following is the probable cause: greasing intervals too far apart, wrong grease being used, defective seals, wrong type of bearing for the load, bearing life exceeded, improper seals being used, elevated bearing temperatures, poor unit alignment, etc.

• Babbitt Bearings lubricated with oil- Inspect babbitt in load zone to see if wear pattern or signs of thrusting have occurred. If so, one or more of the following is the probable cause: improper oil or oil level, defective seals, elevated bearing temperatures, oil rings not turning properly, worn oil rings, oil windows in bearing were not properly designed, type of Babbitt used, poor unit alignment, etc.

Thermal Overloading:

• Unit is rewound due to thermal degradation of winding - Winding should be evaluated to determine if: more copper can be added, air flow through machine can be improved, the winding is contaminated and blocking air flow, the duty cycle amps for the application and time rating on machine are correct, the unit can be converted from totally enclosed to blower-cooled or air-over-motor, etc.

Larger Units with Low Megger Readings to Ground:

• Contamination/moisture - If the unit is an open machine and mill contamination/moisture gets into machine, it may pay to convert the open self-ventilated enclosure to totally enclosed air to water heat exchanger.
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Q. I’ve heard that voltage unbalance can reduce the service life of a motor. Is that true?

The Pro Says: Voltage unbalance does degrade the performance and shortens the life of a three-phase motor. Voltage unbalance at the motor stator terminals causes phase current unbalance far out of proportion to the voltage unbalance. Unbalanced currents lead to torque pulsations, increased vibrations and mechanical stresses, increased losses, and motor overheating that results in a shorter winding insulation life.

Voltage unbalance is defined by the National Electrical Manufacturers Association (NEMA) as 100 times the absolute value of the maximum deviation of the line voltage from the average voltage on a three-phase system, divided by the average voltage.

For example, if the measured line voltages are 462, 463, and 455 volts, the average is 460 volts. The voltage unbalance is:

It is recommended that the voltage unbalances at the motor terminals not exceed 1%. Unbalances over 1% require derating of the motor per Figure 20-2 of NEMA MG-1-2003, Revision 1-2004, and will void most manufacturers’ warranties. Common causes of voltage unbalance include:

• Faulty operation of power factor correction equipment.
• Unbalanced or unstable utility supply.
• Unbalanced transformer bank supplying a three-phase load that is too large for the bank.
• Unevenly distributed single-phase loads on the same power system.
• Unidentified single-phase to ground faults.
• An open circuit on the distribution system primary.

The efficiency of a rewound, 1800-RPM, 100 HP motor is given as a function of voltage unbalance and motor load in the table. The general trend of efficiency reduction with increased voltage unbalance is observed for all motors at all load conditions.

* Results vary depending upon motor design, speed, full-load efficiency, and horsepower rating. Typically, electric motors have peak efficiency near 75% load, but the above motor tested in the lab showed otherwise.

Voltage unbalance is probably the leading power quality problem that results in motor overheating and premature motor failure. If unbalanced voltages are detected, a thorough investigation should be undertaken to determine the cause. Energy and dollar savings occur when corrective actions are taken.

Example Assume that the motor tested as shown in the above table was fully loaded and operated for 8,000 hours per year, with an unbalanced voltage of 2.5%. With energy priced at $0.05/kWh, the annual energy and dollar savings, after corrective actions are taken, are: Annual Energy Savings = 100 HP x 0.746 kW/hp x 8,000 hrs/yr x (100/93 – 100/94.4) = 9,517 kWh Annual Dollar Savings = 9,517 kWh x $0.05/kWh = $476 Overall savings may be much larger because an unbalanced supply voltage may power numerous motors.

Further Considerations Voltage unbalance causes extremely high current unbalance. The magnitude of current unbalance may be 6 to 10 times as large as the voltage unbalance. For the 100 HP motor in this example, line currents (at full-load with 2.5% voltage unbalance) were unbalanced by 27.7%.