Maintenance and Troubleshooting
of Electric Motors
Motor Maintenance -
SCHEDULED ROUTINE CARE
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
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
- 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
Feel for air being discharged from the cooling air ports.
If the flow is weak or unsteady, internal air passages
clogged. Remove the motor from service and clean.
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.
In wet or corrosive environments, open the conduit box
and check for deteriorating insulation or corroded terminals.
Repair as needed.
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
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
details. Such testing is especially important for motors operated
in wet or corrosive atmospheres or in high ambient temperatures.
Brushes and Commutators (DC Motors)
- 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).
- Stop the motor. Be certain that:
brushes move freely in the holder and the spring
tension on each brush is
brush has a polished surface over the entire
working face indicating good
- 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
- There is no grooving of the commutator (small grooves
around the circumference of the commutator). If there
remove the motor from service immediately as
this is a symptomatic indication of a very serious problem.
- Replace the brushes if there is any chance they will not
last until the next inspection date.
- If accumulating, clean foreign material from the grooves
between the commutator bars and from the brush holders and
- Brush sparking, chatter, excessive wear or chipping, and
a dirty or rough commutator indicate motor problems requiring
prompt service. See "Brush and Commutator
Figure 1. Typical DC Motor Brushes And Commutator
Brushes and Collector Rings (Synchronous Motors)
- Black spots on the collector rings must be removed by rubbing
lightly with fine sandpaper. If not removed, these spots cause
pitting that requires regrinding the rings.
Figure 2. Rotary Converter Armature Showing Commutator
And Slip Rings.
- An imprint of the brush, signs of arcing or uneven wear
indicate the need to remove the motor from service and
repair or replace
Check the collector ring brushes as described under "Brushes
and Commutators". They do not, however, wear as rapidly
as commutator brushes.
Modern motor designs usually
provide a generous supply of lubricant in tight bearing housings.
Lubrication on a scheduled
basis, in conformance with the manufacturer's recommendations,
provides optimum bearing life.
Thoroughly clean the lubrication equipment and fittings before
lubricating. Dirt introduced into the bearings during lubrication
probably causes more bearing failures than the lack of lubrication.
Too much grease can overpack bearings and cause them to run
hot, shortening their life.
lubricant can find its way inside the motor where it collects
dirt and causes insulation deterioration.
Many small motors are built with permanently lubricated bearings.
They cannot and should not be lubricated.
Oiling Sleeve Bearings
As a general rule, fractional horsepower
motors with a wick lubrication system should be oiled every
2000 hours of operation
or at least annually. Dirty, wet or corrosive locations or
heavy loading may require oiling at three-month intervals or
more often. Roughly 30 drops of oil for a 3-inch diameter frame
to 100 drops for a 9-inch diameter frame is sufficient. Use
a 150 SUS viscosity turbine oil or SAE 10 automotive oil.
Some larger motors are equipped with oil reservoirs and usually
a sight gage to check proper level.
(Figure 3) As long as the oil is clean and light in color,
the only requirement is to fill the cavity to the proper level
with the oil recommended by the manufacturer. Do not overfill
the cavity. If the oil is discolored, dirty or contains water,
remove the drain plug. Flush the bearing with fresh oil until
it comes out clean. Coat the plug threads with a sealing compound,
replace the plug and fill the cavity to the proper level.
When motors are disassembled, wash the housing with a solvent.
Discard used felt packing. Replace badly worn bearings. Coat
the shaft and bearing surfaces with oil and reassemble.
Figure 3. Cross Section of the Bearing System of a Large Motor
Greasing Ball and Roller Bearings
Practically all Reliance
ball bearing motors in current production are equipped with
the exclusive PLS/Positive Lubrication System.
PLS is a patented open-bearing system that provides long, reliable
bearing and motor life regardless of mounting position. Its
special internal passages uniformly distribute new grease pumped
into the housing during regreasing through the open bearings
and forces old grease out through the drain hole. The close
running tolerance between shaft and inner bearing cap minimizes
entry of contaminants into the housing and grease migration
into the motor. The unique V-groove outer slinger seals the
opening between the shaft and end bracket while the motor is
running or is at rest yet allows relief of grease along the
shaft if the drain hole is plugged. (Figure 4)
The frequency of routine greasing increases with motor size
and severity of the application as indicated in Table 1. Actual
schedules must be selected by the user for the specific conditions.
During scheduled greasing, remove both the inlet and drain
plugs. Pump grease into the housing using a standard grease
gun and light pressure until clean grease comes out of the
If the bearings are hot or noisy even after correction of
bearing overloads (see "Troubleshooting") remove
the motor from service. Wash the housing and bearings with
a good solvent. Replace bearings that show signs of damage
or wear. Repack the bearings, assemble the motor and fill the
Whenever motors are disassembled for service, check the bearing
housing. Wipe out any old grease. If there are any signs of
grease contamination or breakdown, clean and repack the bearing
system as described in the preceding paragraph.
Figure 4. Cross Section of PLS Bearing System (Positive Lubrication
HEAT, NOISE AND VIBRATION
Excessive heat is both a cause of
motor failure and a sign of other motor problems.
The primary damage caused by excess heat is to increase the
aging rate of the insulation. Heat beyond the insulation's
rating shortens winding life. After overheating, a motor may
run satisfactorily but its useful life will be shorter. For
maximum motor life, the cause of overheating should be identified
As indicated in the Troubleshooting Sections, overheating
results from a variety of different motor problems. They can
be grouped as follows:
- WRONG 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 in
- 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
sections of the motor. Dirt on the frame
prevent transfer of internal
heat to the cooler ambient air.
- OVERLOADED DRIVEN MACHINE: Excess loads or jams in the
driven machine force the motor to supply
higher torque, draw more
current and overheat.
Table 1. Motor Operating Conditions
10 to 40
50 to 150
- Light Duty: Motors operate infrequently (1 hour/day
or less) as in portable floor sanders,
valves, door openers.
- Standard Duty: Motors operate in normal applications
(1 or 2 work shifts). Examples include
air conditioning units,
conveyors, refrigeration apparatus,
laundry machinery, woodworking and
textile machines, water pumps, machine
tools, garage compressors.
Heavy Duty: Motors subjected to above normal operation
and vibration (running 24 hours/day,
365 days/year). Such operations
as in steel mill service, coal
and mining machinery, motor-generator sets, fans,
- Severe Duty: Extremely harsh, dirty motor applications.
Severe vibration and high ambient
conditions often exist.
- EXCESSIVE FRICTION: Misalignment, poor 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
indicates motor problems but ordinarily does not
cause damage. Noise, however, is usually accompanied
Vibration can cause damage in several ways. It tends to shake
windings loose and mechanically damages insulation by cracking,
flaking or abrading the material. Embrittlement of lead wires
from excessive movement and brush sparking at commutators or
current collector rings also results from vibration. Finally,
vibration can speed bearing failure by causing balls to "brinnell," sleeve
bearings to be pounded out of shape or the housings to loosen
in the shells.
Whenever noise or vibration are found in an operating motor,
the source should be quickly isolated and corrected. What seems
to be an obvious source of the noise or vibration may be a
symptom of a hidden problem. Therefore, a thorough investigation
is often required.
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. (See Troubleshooting
Problems D and L.)
Brush chatter is a motor noise that can be caused by vibration
or other problems unrelated to vibration. See Troubleshooting
Problem M for details.
Care of Windings and Insulation
for expensive, high horsepower motors, routine inspections
generally do not involve opening the motor to inspect the windings.
Therefore, long motor life requires selection of the proper
enclosure to protect the windings from excessive dirt, abrasives,
moisture, oil and chemicals.
When the need is indicated by severe operating conditions
or a history of winding failures, routine testing can identify
deteriorating insulation. Such motors can be removed from service
and repaired before unexpected failures stop production. See "Testing
Whenever a motor is opened for repair, service the windings
- Accumulated dirt prevents proper cooling and may absorb moisture
and other contaminants that damage the insulation. Vacuum the
dirt from the windings and internal air passages. Do not use
high pressure air because this can damage windings by driving
the dirt into the insulation.
- Abrasive dust drawn through the motor can abrade coil noses,
removing insulation. If such abrasion is found, the
winding should be revarnished or replaced.
- Moisture reduces the dielectric strength of insulation
which results in shorts. If the inside of the motor
is damp, dry
the motor per information in "Cleaning and Drying Windings".
- Wipe any oil and grease from inside the motor. Use care
with solvents that can attack the insulation.
- If the insulation appears brittle, overheated or cracked,
the motor should be revarnished or, with severe conditions,
- Loose coils and leads can move with changing magnetic fields
or vibration, causing the insulation to wear, crack
or fray. Revarnishing and retying leads may correct minor
If the loose coil situation is severe, the motor
- Check the lead-to-coil connections for signs of overheating
or corrosion. These connections are often exposed
on large motors but taped on small motors. Repair as
- Check wound rotor windings as described for stator windings.
Because rotor windings must withstand centrifugal
forces, tightness is even more important. In addition,
for loose pole pieces or other loose parts that create
- The cast rotor rods and end rings of squirrel cage motors
rarely need attention. However, open or broken rods create
unbalance that increases with the number of rods broken.
An open end ring causes severe vibration and noise.
Routine field testing of windings can identify
deteriorating insulation permitting scheduled repair or replacement
motor before its failure disrupts operations. Such testing
is good practice especially for applications with severe operating
conditions or a history of winding failures and for expensive,
high horsepower motors and locations where failures can cause
health and safety problems or high economic loss.
The easiest field test that prevents the most failures is
the ground-insulation, or &127megger," test. It applies
DC voltage, usually 500 or 1000 volts, to the motor and measures
the resistance of the insulation.
NEMA standards require a minimum resistance to ground at 40
degrees C ambient of 1 megohm per kv of rating plus 1 megohm.
Medium size motors in good condition will generally have megohmmeter
readings in excess of 50 megohms. Low readings may indicate
a seriously reduced insulation condition caused by contamination
from moisture, oil or conductive dirt or deterioration from
age or excessive heat.
One megger reading for a motor means little. A curve recording
resistance, with the motor cold and hot, and date indicates
the rate of deterioration. This curve provides the information
needed to decide if the motor can be safely left in service
until the next scheduled inspection time.
The megger test indicates ground insulation condition. It
does not, however, measure turn-to-turn insulation condition
and may not pick up localized weaknesses. Moreover, operating
voltage peaks may stress the insulation more severely than
megger voltage. For example, the DC output of a 500-volt megger
is below the normal 625-volt peak each half cycle of an AC
motor operating on a 440-volt system. Experience and conditions
may indicate the need for additional routine testing.
A test used to prove existence of a safety margin above operating
voltage is the AC high potential ground test. It applies a
high AC voltage (typically, 65% of a voltage times twice the
operating voltage plus 1000 volts) between windings and frame.
Although this test does detect poor insulation condition,
the high voltage can arc to ground, burning insulation and
frame, and can also actually cause failure during the test.
It should never be applied to a motor with a low megger reading.
DC rather than AC high potential tests are becoming popular
because the test equipment is smaller and the low test current
is less dangerous to people and does not create damage of its
Cleaning and Drying Windings
Motors which have been flooded
or which have low megger readings because of contamination
by moisture, oil or conductive dust
should be thoroughly cleaned and dried. The methods depend
upon available equipment.
A hot water hose and detergents are commonly used to remove
dirt, oil, dust or salt concentrations from rotors, stators
and connection boxes. After cleaning, the windings must be
dried, commonly in a forced-draft oven. Time to obtain acceptable
megger readings varies from a couple hours to a few days.
BRUSH AND COMMUTATOR CARE
Some maintenance people with many relatively trouble-free AC
squirrel cage motors forget that brushes and commutators
require more frequent routine inspection and service. The
result can be unnecessary failures between scheduled maintenance.
As indicated in Troubleshooting Problem M on Page 27, many
factors are involved in brush and commutator problems. All
generally involve brush sparking usually accompanied by chatter
and often excessive wear or chipping. Sparking may result from
poor commutator conditions or it may cause them.
The degree of sparking should be determined by careful visual
inspection. The illustrations shown in
Figure 5 are a useful guide. It is very important that you
gauge the degree number as accurately as possible. The solution
to the problem may well depend upon the accuracy of your answer
since many motor, load, environmental and application conditions
can cause sparking.
It is also imperative that a remedy be determined as quickly
as possible. Sparking generally feeds upon itself and becomes
worse with time until serious damage results.
Some of the causes are obvious and some are not. Some are
constant and others intermittent. Therefore, eliminating brush
sparking, especially when it is a chronic or recurring problem,
requires a thorough review of the motor and operating conditions.
Always recheck for sparking after correcting one problem to
see that it solved the total problem. Also remember that, after
grinding the commutator and properly reseating the brushes,
sparking will occur until the polished, brown surface reforms
on the commutator.
NOTE: Small sparks are yellow in color, and the large sparks
are white in color. The white sparks, or blue-white sparks,
are most detrimental to commutation (both brush and commutator).
Figure 5. Degrees of Generator and Motor Sparking
First consider external conditions that affect commutation.
Frequent motor overloads, vibration and high humidity cause
sparking. Extremely low humidity allows brushes to wear through
the needed polished brown commutator surface film. Oil, paint,
acid and other chemical vapors in the atmosphere contaminate
brushes and the commutator surface.
Look for obvious brush and brush holder deficiencies:
- Be sure brushes are properly seated, move freely in the holders
and are not too short.
- The brush spring pressure must be equal on all brushes.
- Be sure spring pressure is not too light or too high. Large
motors with adjustable springs should be set at about
3 to 4 pounds per square inch of brush surface in contact
- Remove dust that can cause a short between brush holders
- Check lead connections to the brush holders. Loose connections
Look for obvious commutator problems:
- Any condition other than a polished, brown surface under the
brushes indicates a problem. Severe sparking causes a rough
blackened surface. An oil film, paint spray, chemical contamination
and other abnormal conditions can cause a blackened or discolored
surface and sparking. Streaking or grooving under only some
brushes or flat and burned spots can result from a load mismatch
and cause motor electrical problems. Grooved commutators should
be removed from service. A brassy appearance shows excessive
wear on the surface resulting from low humidity or wrong brush
- High mica or high or low commutator bars make the brushes
jump, causing sparking.
- Carbon dust, copper foil or other conductive dust in the
slots between commutator bars causes shorting and sometimes
If correcting any obvious deficiencies does not eliminate
sparking or noise, look to the less obvious possibilities:
- If brushes were changed before the problem became apparent,
check the grade of brushes. Weak brushes may chip. Soft, low
abrasive brushes may allow a thick film to form. High friction
or high abrasion brushes wear away the brown film, producing
a brassy surface. If the problem appears only under one or
more of the brushes, two different grades of brushes may have
been installed. Generally, use only the brushes recommended
by the motor manufacturer or a qualified brush expert.
- The brush holder may have been reset improperly. If the
boxes are more than 1/8" from the commutator, the
brushes can jump or chip. Setting the brush holder off
sparking. Normally the brushes must be equally spaced
around the commutator and must be parallel to the bars
so all make
contact with each bar at the same time.
- An eccentric commutator causes sparking and may cause vibration.
Normally, concentricity should be within .001" on high
speed, .002" on medium speed and .004" on slow
- Various electrical failures in the motor windings or connections
manifest themselves in sparking and poor commutation.
Look for shorts or opens in the armature circuit and
shorts or opens in the field winding circuits. A weak
interpole circuit or large air gap also generate brush