Rockwell Automation 1329R Std. Industrial AC Induction Motors - Manual (Page 12)

12 Standard Industrial AC Induction Motors
Operation
Due to the inherent characteristics of insulating materials, abnormally
high temperatures shorten the operating life of electrical apparatus.
The total temperature, not the temperature rise, should be the measure
of safe operation. The class of insulation determines the maximum
safe operating temperature. Aging of insulation occurs at an
accelerated rate at abnormally high temperatures. A general rule for
gauging the effect of excessive heat is that for each 10 degrees C (50
degrees F) rise in temperature above the maximum limit for the
insulation, the life of the insulation is halved.
Unbalanced voltage or single-phase operation of polyphase machines
may cause excessive heating and ultimate failure. It requires only a
slight unbalance of voltage applied to a polyphase motor to cause
large unbalance currents and resultant overheating.
Periodic checks of phase voltage, frequency and power consumption
of a motor while in operation are recommended. Such checks assure
the correctness of frequency and voltage applied to the motor and
yield an indication of the load offered by the apparatus which the
motor drives.
Comparisons of this data with previous no-load and full-load power
demands will give an indication of the performance of the complete
machine. Any serious deviations should be investigated and
corrected.
Stator problems can usually be traced to one of the following causes:
• Worn bearings
• Operating single-phase
• Moisture
• Poor insulation
• Overloading
• Oil and dirt
Dust and dirt are usually contributing factors. Some forms of dust are
highly conductive and contribute materially to insulation breakdown.
The effect of dust on the motor temperature through restriction of
ventilation is a principal reason for keeping the windings clean.
Squirrel-cage rotors are rugged and in general give little trouble. The
first symptom of a defective rotor is lack of torque. This may cause a
slowing down in speed accompanied by a growling noise or perhaps
failure to start the load. This is caused by an open or high resistance
joint in the rotor bar circuit. Such a condition can generally be
detected by looking for evidence of localized heating.
!
ATTENTION: Surface temperatures of the motor
enclosure may reach temperatures which can cause
discomfort or injury if contact is made with hot surfaces.
When installing, protection must be provided by the user to
protect against accidental contact with hot surfaces. Failure
to observe this precaution could result in personal injury.