Page 193 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS ELECTRICIAN - CITS

the machine is measured by a 500 volts Megger and the readings so obtained shall not be less than 1 Megohm
in the case of 3-phase and single phase motors. For additional safety 2 megohms are necessary in the case of
ceiling and table fans.

Fig 3 Fig 4

Polarity Test: Correct coil group connection in the winding ensures correct polarity. If there is any confusion in
the coil group connections then the polarity test is necessary to be carried out to check proper polarity.
There are three methods recommended as explained below.
a Magnetic compass method
b Two screwdrivers method

c Search coil method
Magnetic compass method : In this method, the stator is placed in a horizontal position and a low DC voltage is
applied to the winding. The compass needle is then held inside the stator and moved slowly from one pole area to
another pole area as shown in Fig 5. The compass needle will reverse itself on each pole if the winding is correctly
connected. If there is same direction of indication between two adjacent poles, a reverse pole is indicated.

Fig 5

No-load test : After impregnation and assembly of motor, check the rotor for free rotation. Connect the motor
to the rated supply voltage. Run the motor at no load and record the no-load voltage, current and speed of the
motor. In no case these readings increase beyond name-plate values. Inspect the bearing sound and vibration.
Normal sound without vibration is an indication of a good job. However, the perfection of the winding job could be
ascertained only through a load test.

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