Electrical Engineering ⇒ Topic : Range Extension of Electrostatic Voltmeters
Range Extension of Electrostatic Voltmeters
The range of electrostatic voltmeters can be increased by the use of multipliers. Two types of multipliers are employed for this purpose viz
Resistance potential divider - for ranges upto 40 kV
Capacitance potential divider - for ranges upto 1000 kV
The first method can be used for both direct and alternating voltages whereas the second method is suitable only for alternating voltages.
(1) Resistance potential divider. This divider consists of a high resistance with tappings taken off at intermediate points. The voltage r 'to be measured is applied across the whole of the potential divider and the electrostatic voltmeter connected across part of it (resistance r in this case) as shown in Fig. 16.48. Since voltmeter *practically carries no current, the p.d. v across it is the same fraction of the applied voltage V as the resistance across it (i.e., r) is of the whole resistance (i.e., R) i.e.,
Thus if the voltmeter is connected across 1/5 of the whole resistance (i.e.,R/r= 5), then voltage V to be measured is 5 times the reading of the voltmeter. The advantage of this method is that there is no shunting effect of the voltmeter.The drawback is that there is no power loss in the resistance divider.
(2) Capacitance potential divider. In this method, a single capacitor of capacitance C is connected in series with the voltmeter and the whole circuit is connected across the voltage V to be measured as shown in Fig. (b). Let v volts be the reading of the voltmeter. Since the voltage across a capacitor is inversely proportional to its capacitance,
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