Page 294 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

• One is fixed and other is movable
• Movable strip is attached to the spindle, which also carries the pointer
Working
• When the meter connected to the supply, coil produces a magnetic field

• Both vanes to become magnetized and same poles are created
• They repel each other
• Repulsion between the similarly magnetized vanes produces a proportional rotation

Parts and essential torques of moving iron
• Control torque is provided by spring or weight (gravity).
• Damping torque is normally pneumatic, the damping device consisting of an air chamber and a moving vane
attached to the instrument spindle.
• Deflecting torque produces a movement on an aluminum pointer over a graduated scale.
• The deflecting torque is proportional to the square of the current in the coil
• Moving iron instruments having scales that are nonlinear
• It is crowded in the beginning and open at the end

Dynamometer type instruments
• These instruments are similar to the permanent magnet type instruments, except that the permanent magnet
is replaced by a fixed coil.
• The coil is divided into two halves, connected in series or parallel with the moving coil.

• Moving coil mechanism to be placed between the fixed coil
• The two halves of the coil are placed close together and parallel to each other to provide uniform field within
the range of the movement of moving coil.

• Moving coil is mounted on spindle and is free to move in air
• Controlling torque provided by two spring and it also allows the flow of current to the moving coil

• It can work in both AC and DC
• Because when the current is reversed in AC, the direction of flux produced in fixed and moving coil is also
reversed
• So the direction of torque is same
• It can be used as Voltmeter or Ammeter
• Best suits as a power meter
Working
• When the current is pass through the two coils,the magnetic field is produced.

• The field strength is proportional to the field current

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