Specifications

167
6) Measure insulation resistance. If low dry out the windings, until correct values are
obtained. If necessary re-enamel or re-varnish all the winding and internal parts except
the stator bore and rotor iron. Dry rotor and stator winding thoroughly.
7) Reassemble motor without using any excessive force. Make sure that machine leads
are on the correct terminals and everything is well tightened.
8) Check the concentricity of the air gap through the air gap holes. Ensure that rotor can
rotate freely. Any difficulty in rotating the rotor or unusual noise should be taken as sign
of interference between stationary and moving parts. Investigate this and eliminate the
cause of the trouble.
9) Check insulation resistance again.
10) Recommission the motor.
15.10 Maintenance of A.C. Motors
A.C. Motors are run for extremely long periods without repairs. If the bearings are properly
lubricated and air passages are kept clear, then most of the A.C. Motors do not have failure
problems, as the motors have no commutators which cause motor failure. There are few motors
which have commutators, such as universal motor, repulsion motors in single phase system.
Squirrel Cage Induction Motor : Overheating and shock may damage the squirrel cage rotor,
which may cause fractures in bars in the slots and end ring connections and joint in the rotor
cage. Satisfactory operation of motor is difficult with fractured rotor cage and end rings. In large
motors the bars are bolted or wedged in slots and can be tightened readily if these become
loose. Loose coils can be detected with the help of growler. If it is not possible to disassemble
the motor, connect an ammeter in in series with one phase and apply 25% of full voltage to one
of the stator phase winding and turn the rotor slowly by hand. If the ammeter reading varies in
excess of 3%, it can be assumed that there is loose bar in rotor.
Wound Rotor Motors : Working principle is same as 3 phase sq. cage rotor induction motor,
having better torque and speed. Wound rotor may have low speed with starter resistance cut off
from rotor circuit. If there are no openings in control and starter circuit, the rotor coils should be
tested for continuity. Growler test may reveal open are short circuit in the coils.There is also
possibility of brushes sticking in the holder or brushes may not have sufficient tension (sparking
and over heating at contact area).
Growler is an electromagnet having 110 V /240 V supply voltage and suitably shaped for
testing armature/ rotor, by placing the growler core (or rotary growler for testing stator winding)
and slowly moving on/in it shorted or broken coils can be detected by noting the change in the
humming noise of the growler.
Synchronous Motors : Construction is similar to d.c. generator(except commutator) or auto
synchronous motor. Centrifugal force has considerable effect on the leads, which should be
securely fastened to the shaft. Change in air gap caused by misalignment of frame or wear in
the bearings causes big troubles in the synchronous motors. Other points of care and
maintenance are similar to other ac motors.