Page 291 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Consists of permanent magnet and rectangular coil wound with very fine insulated copper wire of many turns
.Coil is wound on an aluminium bobbin which is free to rotate by about 90◦.Coil and former are attached with
spindle can move on a calibrated scale.

Two springs one at top and other at bottom were attached to the assembly and serves two purposes
One is to provide path for current to the coil and other for providing controlling torque.
Horse shoe shaped Permanent magnet is used.Core is made of soft iron, Magnetic poles& iron core are cylindrical
in shape. This has two advantages
Firstly, the length of the air gap is reduced (flux leakage=0)
Secondly, it ensures uniform magnetic field in the air gap

Working

When a current is passed through a coil in a magnetic field, the coil experiences a force proportional to the current.
The deflecting torque is produced by the electromagnetic action of the current in the coil and the magnetic field.
How the deflection torque is produced
Force F=BILN Newton , B is the flux density in the air gap, I is the current ,L is the length of one conductor in
air gap and N is the number of turns
Net deflecting torque=force x perpendicular distance between the centre of the conductor to the centre of the
spindle
T = Fr Newton meter
T = BILNr

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