Electric Motors Maintenance Guide

An electric motor is an electromechanical device that converts electrical energy into mechanical energy.

Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force. The reverse process, producing electrical energy from mechanical energy, is done by generators such as an alternator or a dynamo; some electric motors can also be used as generators, for example, a traction motor on a vehicle may perform both tasks. Electric motors and generators are commonly referred to as electric machines.

Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives. They may be powered by direct current, e.g., a battery powered portable device or motor vehicle, or by alternating current from a central electrical distribution grid or inverter.

The smallest motors may be found in electric wristwatches. Medium-size motors of highly standardized dimensions and characteristics provide convenient mechanical power for industrial uses. The very largest electric motors are used for propulsion of ships, pipeline compressors, and water pumps with ratings in the millions of watts. Electric motors may be classified by the source of electric power, by their internal construction, by their application, or by the type of motion they give.


The key to minimizing motor problems is scheduled routine inspection and service. The frequency of routine service varies widely between applications.

Including the motors in the maintenance schedule for the driven machine or general plant equipment is usually sufficient. A motor may require additional or more frequent attention if a breakdown would cause health or safety problems, severe loss of production, damage to expensive equipment or other serious losses.

Written records indicating date, items inspected, service performed and motor condition are important to an effective routine maintenance program. From such records, specific problems in each application can be identified and solved routinely to avoid breakdowns and production losses.

The routine inspection and servicing can generally be done without disconnecting or disassembling the motor. It involves the following factors:

Dirt and Corrosion

  1. Wipe, brush, vacuum or blow accumulated dirt from the frame and air passages of the motor. Dirty motors run hot when thick dirt insulates the frame and clogged passages reduce cooling air flow. Heat reduces insulation life and eventually causes motor failure.
  2. Feel for air being discharged from the cooling air ports. If the flow is weak or unsteady, internal air passages are probably clogged. Remove the motor from service and clean.
  3. Check for signs of corrosion. Serious corrosion may indicate internal deterioration and/or a need for external repainting. Schedule the removal of the motor from service for complete inspection and possible rebuilding.
  4. In wet or corrosive environments, open the conduit box and check for deteriorating insulation or corroded terminals. Repair as needed.


Lubricate the bearings only when scheduled or if they are noisy or running hot. Do NOT over-lubricate. Excessive grease and oil creates dirt and can damage bearings. See "Bearing Lubrication" for more details.

Heat, Noise and Vibration

Feel the motor frame and bearings for excessive heat or vibration. Listen for abnormal noise. All indicate a possible system failure. Promptly identify and eliminate the source of the heat, noise or vibration. See "Heat, Noise and Vibration" for details.

Winding Insulation

When records indicate a tendency toward periodic winding failures in the application, check the condition of the insulation with an insulation resistance test. See "Testing Windings" for details. Such testing is especially important for motors operated in wet or corrosive atmospheres or in high ambient temperatures.

Brushes and Commutators (DC Motors)

  1. Observe the brushes while the motor is running. The brushes must ride on the commutator smoothly with little or no sparking and no brush noise (chatter).
  2. Stop the motor. Be certain that:
    • The brushes move freely in the holder and the spring tension on each brush is about equal.
    • Every brush has a polished surface over the entire working face indicating good seating.
    • The commutator is clean, smooth and has a polished brown surface where the brushes ride.
    • NOTE: Always put each brush back into its original holder. Interchanging brushes decreases commutation ability.
    • There is no grooving of the commutator (small grooves around the circumference of the commutator). If there is grooving, remove the motor from service immediately as this is a symptomatic indication of a very serious problem.
  3. Replace the brushes if there is any chance they will not last until the next inspection date.
  4. If accumulating, clean foreign material from the grooves between the commutator bars and from the brush holders and posts.
  5. Brush sparking, chatter, excessive wear or chipping, and a dirty or rough commutator indicate motor problems requiring prompt service. See "Brush and Commutator Care" for details.