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
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
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