Electrical Engineering ⇒ Topic : Electrostatic Voltmeter
This is basically a variable capacitor with air as dielectric. In a variable air capacitor such as one in radio, the capacitance is minimum when the variable vane is completely out and capacitance increases as the vane moves inside the fixed vane and reaches a maximum when fully in.
The force of attraction or repulsion between the two capacitor plates carrying charges of same polarity or opposite polarity is balanced by a spring. The force F is created by the p.d.between the plates.
Electrostatic voltmeters are also called electrometers and are of two types: (i) Quadrant type and (ii) Attracted disc type.
When the capacitor moves, work is done.
Work done dW = τ dθ ................ (1)
The instantaneous energy stored in the capacitor is given by
From Eq. (1) and Eq. (2), we can write
The average torque over an entire period T of the ac voltage is given by
The average torque is proportional to the square of the input voltage.
Electrostatic voltmeters are essentially voltmeters. They can be used both for ac and dc measurement. They are very accurate because there is no iron loss for hysteresis and eddy current. Waveform and frequency do not have any effect. Their power requirement is very low, being negligible at dc.
The chief disadvantage of these instruments is that a fairly high voltage is required for creation of force and so the meter is not useful at low voltages. The useful range of these instruments is from few hundreds to 100 kV. Thus, for ac, the frequency range is up to 110 kHz.The capacitance range is from 10 pF at 110 kV to 200 pF at 100 V. Accuracy is ±1% to ±0.5%.
These instruments are based on the fact that an electric force (attraction or repulsion) exists between charged plates or objects. An electrostatic voltmeter is essentially an air capacitor; one plate is fixed while the other, which is coupled to the pointer, is free to rotate on jewelled bearings. When p.d. to be measured is applied across the plates, the electric force between the plates gives rise to a deflecting torque. Under the action of deflecting torque, the movable plate moves and causes the deflection of the pointer to indicate the voltage being measured. Such instruments can be used to measure direct as well as alternating voltages\
There are three types of electrostatic voltmeters viz
(1) Attracted disc type - usual range from 500 V to 500 kV
(2) Quadrant type - usual range from 250 V to 10 kV
(3) Multicellular type - usual range from 30 V to 300 V
Two things are worth noting about electrostatic voltmeters. First, the deflecting torque is very small for low voltages. For this reason, they are not very sensitive to measure small voltages.
Secondly, the instrument is only available for the measurement of p.d., that is to say as voltmeter. It cannot be used as an **ammeter
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