Specifications

168
Single Phase Motors: Centrifugal (CF) switch is one of the main cause of troubles in single
phase motors. If the springs of centrifugal switch become weak, the C.F. switch will operate
before reaching the full speed, which will cause motor to run at sub normal speed and stop. If
the switch sticks closed, the starting winding will remain in the circuit, overheat and damage the
starting winding. Commutator motors need almost same type of care and maintenance as in dc
motors.
15.11 The causes of low insulation resistance of electrical equipment and ways to rectify them
Low insulation is almost invariably the result of absorption of moisture by the insulated
windings, since most of the insulating materials employed are hygroscopic (moisture absorbent).
To maintain high insulation values, the following precautions should be taken.
a) Don’t allow dust to accumulate on the motor windings. Dust and dirt absorb and retain
moisture, leading to leakage of electricity which may finally result in a breakdown. Clean
up the motor windings periodically by blowing compressed air, and wipe the outer surface
of windings clean and bright by a dry cloth.
b) Oil and grease are equally bad; since they are much more difficult to remove once they
reach the windings and soak them, as a result of worn-out bearings, over-oiling, leaky
gaskets of oil level indicators, etc. Oil and grease not only make the equipment messy
but are good places for dust to settle in. This must be avoided.
c) Protect large motors, rotary convertors, etc. against inclement weather when they are
idle, by covering them over with a large tarpaulin and keeping the windings warm by
connecting up a few electric radiators or infra red lamps all round. Change over the
working and stand-by sets regularly to maintain both in good condition.
d) In spite of all precautions, sometimes motors do get submerged under water. Retrieve
them as soon as possible and blow the wet surfaces with compressed air. Dry out by
putting them in a hot chamber.
Dampness in windings can be removed by drying out the equipment thoroughly in a hot
chamber or in an impregnating plant, the inside of which is maintained at a temperature of 80
0
C
to 100
0
C. The heating should be carried out for several hours and in the case of large equipment
for one or two days, if required until all the moisture has been driven out. This can be ascertained
by recording the insulation resistance readings at regular intervals of one or two hours.
20
0
C30
0
C40
0
50
0
C60
0
C
66 KV and above 1200 600 300 150 75
22 KV to 44 KV 1000 500 250 125 65
6.6 KV to 19KV 800 400 200 100 50
Below 6.6 KV 400 200 100 50 25
Winding voltage
Megaohms resistance at
Table 15.1 Winding insulation resistance at different temperatures