Air pressure used for cleaning should not
exceed 25 psi nozzle pressure. Excessive
pressure can damage the insulation on the
Wipe all excess dirt, grease, and oil from the
surfaces of a motor with a cloth moistened with an
Do not use flammable or toxic solvents when
cleaning motors. Solvents may cause injury to
personnel or damage to equipment.
Lubrication should be done according to the
manufacturers instructions. Improper lubrication
causes motor bearings to overheat and eventually
causes bearing failure. Check a motor for signs of
grease and oil-seal failure. If an inside seal fails, the
lubricant can get into the motor windings and
deteriorate the insulation. This condition also allows
dust to adhere to the windings and restricts air
circulation, then the motor windings heat and burn out.
Inadequate lubrication causes the bearings to wear
excessively and, eventually, to seize. When lubricating
a motor, refer to the manufacturer's manual to determine
the correct type of lubricant to use. Some motors have
bearings lubricated with oil, while others require
grease. Many motor bearings are lubricated and sealed
at the factory and usually last the life of the bearing.
Check the running temperature of all motors. If the
motor temperature is hotter than specified on the
nameplate, you must find the problem. The normal
procedure for diagnosing motor overheats is to check
the motor for restricted ventilation. Inspect the area
around the motor for any obstructions which could
hamper free air circulation. If air circulation is not
hampered in any way and the motor continues to run
hot, reduce the load on the motor or use a motor with
more power capability.
The proper testing of a motor should be done in a
Proper testing can prevent
unnecessary labor and parts. Testing motors is
generally classed under two major methods: visual tests
and operational tests.
VISUAL TESTS.A visual test can discover a
great deal about the condition of a motor and the
possible causes of trouble. Read the nameplate data and
be sure that the motor connections are correct for the
Look at the windings to see if the insulation has
overheated (or has been overheating). You can tell
when the insulation is burned by the odor within the
motor. If you arent sure of the condition of the
windings, test them with a megger to determine if the
windings have been damaged beyond use. Connect the
leads of the megger to each set of windings.
Disconnect the motor leads from each other to
ensure reading only one winding at a time.
If the winding is good, you will get a reading of
continuity. If the winding indicates a large amount of
resistance, it is damaged and must be replaced.
Now connect one lead from the megger to the frame
of the motor. Connect the other lead of the megger to
each lead of the motor, one at a time. A low-resistance
reading means insulation breakdown or a short to the
motor frame, and replacement of the winding is
Inspect the commutator for solder thrown from the
risers, and for loose, burned, high, and flat bars. Also
test for high mica. Notice the surface film on both the
commutators and slip rings and the general condition of
the brushes. Check the air gap on large motors for any
indication of bearing wear or misalignment. For large
motors, take an air gap measurement at one reference
point on the rotor or armature; then rotate the rotor or
armature and measure four points on the stator or field
frame to the same reference point. The air gap
measurement should be within plus or minus 5 percent
at any of these points.
Check the condition and operation of the starting
rheostat in dc motors and the starting and control
equipment used with ac motors. Also check the
terminal connections on all of the control equipment to
ensure they are correct and secure. Make sure the
proper voltage is at the terminal lead of the motor.
If the visual tests have not revealed the trouble, you
should perform some operational tests on the motor.