Page 292 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Here B,L,N,r are constant and denoted K
Torque = kI
The deflection is proportional to the current and hence the scale is uniformly divided
Uniform scale

The controlling torque
It is provided by two phosphor bronze springs. These springs serve as a flexible connection to the coil conductors.
Springs are spiralled in the opposite direction in order to neutralize the effect of temperature changes

Damping Torque
Itis caused by the eddy current set up in the aluminum former which prevents the oscillation of the pointer.
Advantages
1 The PMMC consumes less power and has great accuracy.
2 It has uniformly divided scale

3 The PMMC has a high torque to weight ratio.
4 It can be modified as ammeter or voltmeter with suitable resistance.
5 It has efficient damping characteristics and is not affected by stray magnetic field.
6 It produces no losses due to hysteresis

Disadvantage
1 The moving coil instrument can only be used on D.C supply as the reversal of current produces reversal of
torque on the coil.
2 It is costly as compared to moving coil iron instruments.

3 It may show error due to loss of magnetism of permanent magnet.

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