Page 289 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Electro static voltmeter
Force of attraction between electric charges on neighbouring plates between which a p.d is maintained
These force gives a deflecting torque

Used for the measurement of high very high voltages
Used as laboratory type rather than industrial type
Types of electro static voltmeter
Quadrant type –used up to 20 kv

The attracted disc type –used up to 500 kv
1 Attracted disc type voltmeter
It consists of 2 parallel plates C & D
D is fixed and earthed
C is connected to positive end of supply and suspended by coach spring which carries a micro meter head for
adjustment
When pd is applied between plates C is attracted towards D and returned to its original shape by micro meter
head
The movement of these head indicate the force F with which C is pulled downwards
Movement of plate C is balanced by control device which actuates a pointer attached to it that sweeps over a
calibrated scale
Theory
Before the movement capacitance of capacitor is C farad then initial energy = (1/2) Q 2/C
By the movement of plates capacitance changes to c+dc.so final energy stored = (1/2) Q2/C+ dc =(Q2/2C)
(1-dc/C)

Change in stored energy=Q2dC/2 CC

F x dx =Q2dC/2 C C
F= V2 Dc /2 dx
Force is directly proportional to squire of the applied voltage to be measured
2 Quadrant type voltmeter

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