Electrical Engineering ⇒ Topic : Quadrant Type Voltmeter

Quadrant Type Voltmeter

Fig.(a) shows the simplified diagram of a quadrant electrostatic voltmeter. It consists of a light aluminium vane A suspended by a phosphor bronze string mid-way between two inter-connected quadrant shaped brass plates BB. One terminal is joined to fixed plates BB (positive for direct voltage) and the other to the movable plate A (negative for direct voltage). The controlling torque is provided by the torsion of the suspension string. Damping is provided by air friction due to the motion of another vane in a partially closed box.

Working. When the instrument is connected in the circuit to measure the p.d., an electric force exists between the plates. Consequently, the movable vane A moves in between the fixed plates and causes the deflection of the pointer. The pointer comes to rest at a position where deflecting torque is

                                                               figure (a)

equal to the controlling torque. Since the force of attraction between the movable plate A and the fixed plates BB is directly proportional (p.d.)², the instrument can be used to measure either direct or alternating voltages. When used in an a.c. circuit, it reads the r.m.s. values. More robust but *less accurate voltmeters are made by pivoting the moving system. In pivoted voltmeters, the controlling torque is provided by a spiral spring.

Theory. The capacitance C between the plates depends upon deflection θ i.e., upon the position of the movable plate (or vane)A. Suppose that at any instant, the applied alternating voltage is v.

Hence the instrument has non-uniform scale. The non.linearity in the scale can be corrected by shaping the movable vane A in such a way as to increase dC/dθ for small deflection: and to make the scale nearly uniform for larger ones