Page 284 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

3 Excess voltage across P.C.
4 Vibration

5 Stray magnetic field
Adjustment
In order to prevent error due to creeping, two diametrically opposite holes are drilled in the disc. This cause
sufficient distortion of the field. The result is that the disc tends to remain stationary when one of the holes comes
under one of the shunt magnet.
Phase error
It is necessary that the E/M should give correct reading on all power factors, which is only possible when the field
set up by the shunt magnet lags behind the applied voltage by 90˚.
Adjustment -by changing the position of shading band (shading ring or power factor compensator) placed on the
central limp of shunt magnet we can correct the phase error.
Speed error
Sometimes the speed of the meter is either fast or slow, resulting in wrong recording of energy consumption.
Adjustment -Error can be correctly adjusted by adjusting the position of the break magnet. Moment of break
magnet in the direction of the spindle will reduce the breaking torque and vice-versa.
Insulation tester(megohmmeter)
Insulation resistance is measured in mega ohms. The instrument used to measure insulation resistance is known
as megger.
There are two types of megger
1 Magneto generator type
2 Transistorised type (Electronic insulation tester)
Magneto generator type megger

In this tester the testing voltage is produced by a magneto generator when the handle is cranked at 160 rpm . The
testing voltage is in the order of 500 V or high as 1 Mega volt. V DC is produced by internal circuitry.
It consists of three main parts .

1 Hand driven DC generator
2 A cranking system
3 A meter calibrated to measure high resistance.
The current supplied by the megger is in the order of 5 to10- mA. The meter scale is calibrated in Meg ohms

Transistorised type (electronic insulation tester)
This type of megger consists of
1 Power source
2 An oscillator
3 Step up transformer
4 Converter

5 A moving coil meter
The voltage coils are connected across the generator terminal. The current coil is connected in series with the
resistance to be measured. The unknown resistance is connected in between terminals L and E . The rated
resistance in meg ohms and the rated voltage of the insulation resistance tester having the following ranges are
recommended by IS 2992 of 1980

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CITS : Power - Electrician & Wireman - Lesson 50-53 CITS : Power - Electrician & Wireman - Lesson 50-53