Electrical Engineering ⇒ Topic : Multicellular Electrostatic Voltmeter

Multicellular Electrostatic Voltmeter

The major drawback of quadrant type voltmeter is that deflecting torque is very **small for low voltages. Therefore such an instrument cannot measure accurately voltages below 250 V. This difficulty has been overcome in a multicellular electrostatic voltmeter which can read as low as 30 volts. Fig. (a). shows the constructional details of multicellular voltmeter. It is essentially a quadrant type voltmeter with the difference that it has ten moving vanes instead of one and eleven fixed plates forming "cells" in and out of which the vanes move. The moving vanes are fixed to a vertical spindle and suspended by a phosphor-bronze wire so that the vanes are free to move, each between a pair oi fixed plates. At the lower end of the spindle, an aluminium disc hangs horizontally in an oil bath and provides damping torque due to fluid friction. The controlling torque is provided by the torsion of the suspension wire as the moving system rotates. The upper end of the suspension wire is attached through a coach spring S to a torsion head H. The torsion head is provided with a tangent screw for zero adjustment. The function of the coach spring is to prevent the suspension wire from breaking when accidentally jerked. Should the moving vanes be jerked downward, then the coach spring yields sufficiently to allow the safety sleeve E to come into contact with the guide stop G before the suspension wire is over strained. The scale is horizontal if the pointer is straight but the indications can be given on a vertical scale by bending the pointer at right angles.

                                                        figure (a)

   The working principle of multicellular voltmeter is exactly similar to the quadrant type. By using a number of inter-leaved stationary and moving plates, we are able to increase the capacitance and hence the deflecting torque. Consequently, the multicellular voltmeter is much more sensitive than the quadrant type and can accurately measure low voltages