Page 290 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

A light aluminium vane C is mounted on spindle S is situated partially with in a hollow metal quadrant B

Alternatively, vane may be suspended in the quadrant
Vane and quadrant are oppositely charged by the voltage under measurement the vane is attracted inwards into
the quadrant causing the spindle and hence the pointer to rotate
The deflection torque Is proportional to square of the voltage so an uneven scale damping is by vane immersed
in oil in case of suspended type or by air friction in case of pivoted type instruments
T is proportional to square of the voltage to be measured whether the voltage is alternating or direct. the alternating
circuits the scale will read rms value
Advantages of electrolytic voltmeter
First grade accuracy
Give correct readings both in ac and dc

Ac circuits scale will read rms values whatever the waveforms
No iron is used in these instruments so free from hysteresis and eddy current loss and temperature errors
Their power loss is small
Unaffected by stray magnetic field
Limitations of electrolytic voltmeter
They are expensive and cannot be made robust

Torque is proportional to square of the voltage so scale is not uniform.
Types of measuring instruments
Ammeter and voltmeter
1 Moving coil type

a Permanent magnet(for DC only)
b Electro dynamic or Dynamo meter type(for DC/AC)
2 Moving iron type(both for DC/AC)
a Attraction type
b Repulsion type

3 Hot wire type(for DC/AC)
4 Induction type(for AC only)
a Split phase type
b Shaded pole type
5 Electrostatic type for voltmeter only (for DC/AC)
Watt meters
6 Dynamo meter type(for DC/AC)
7 Induction type(for AC only)
Energy meters

8 Electrolytic type(for DC only)
9 Motor meters
a Mercury motor meter(for DC only)
b Induction type (for AC only)
Permanent magnet moving coil instrument (pmmc)
Principle of operation:
When a current carrying conductor. Is placed in a magnetic field, it experiences a force and tends to move in the
direction as per fleming’s left hand rule.

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