Page 192 - Electrician - TT (Volume 2)

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If there is any open in one coil, that coil can be bypassed and left out in the chain of windings. Then
the motor can run, but if the open is in more than one coil, bypassing of the coil is not possible.
This type of repairing is possible for small capacity motors where the winding has a large number
of coils. Ex: Ceiling fans. But this procedure should be avoided as far as possible.
If the polarity of one or two coils in a multiple pole fan motor is changed the fan will run slowly and
produces more heat.
Short circuit test/growler test : Two or more turns that contact each other electrically will cause a short circuit
in the winding. This short circuit will cause excessive heat to be developed during the operation of the machine.

Short circuit can be detected by any one of the following methods.
a Internal growler method
b Voltage drop test
c Ohmmeter method.
Internal growler method : The internal growler consists of a coil of wire wound on a laminated iron core and
connected to 240V AC supply. After the stator is removed the growler is placed on the core of the stator and
moved from slot to slot as shown in Fig 2. A shorted coil will be indicated by rapid vibration of a metal blade
provided with the growler and in some types of internal growlers, glow of the neon lamp provided with the growler
indicates short in winding.

Fig 1 Fig 2

Voltage drop method : In this method the winding is connected to a low voltage DC supply as shown in Fig 3 and
the voltage drop is measured across each coil by a milli voltmeter. The voltage drop across good coils will be the
same whereas voltage drop across shorted coils will be low.
Ohmmeter method : For this method, measure the resistance of the each coil by a low range ohmmeter or
Kelvin bridge or Post Office Box. All the coils should read the same value of resistance. The coil which reads
lower resistance than the other coils or that which reads zero resistance is assumed to be shorted and needs
replacement. On the other hand the coil which reads high resistance when compared to similar coils or which
reads infinite value of resistance indicates open in that particular coil.
Ground test and insulation/resistance test : Grounded winding may cause a fuse to blow up or it may cause
the winding to smoke, depending on the extent of the ground. It may give shock to persons when they come in
contact with the frame which is not properly earthed.
The aim of this test is to check any direct connection between windings and earth(ground). For this, the neutral of
the supply is connected to the body of the machine and the phase wire is connected through a series test lamp.
The open end of the test lamp is touched to each end of the winding in sequence. If the lamp remains dark it
means winding is not grounded and if it glows, the winding is earthed. This is a fast, rough practical method.
If a Megger is used for testing the grounded winding, one terminal of the Megger is connected to the body and
other to the windings as shown in Fig 4. If the pointer of the Megger shows infinity, the winding is correct and there
is no connection between the windings and the body. Insulation resistance between windings and the body of

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CITS : Power - Electrician & Wireman - Lesson 86-92 CITS : Power - Electrician & Wireman - Lesson 86-92