JNTU B.Tech III ELECTRONIC MEASUREMENT AND INSTRUMENTATION Previous Question Paper (September - 2021 - R16)

A quadrant electrometer consists of four metallic quadrants, where the two diagonal pairs are connected together. A light metal vane (needle) is suspended horizontally within these quadrants. Let V1 and V2 be the potentials of the two pairs of quadrants, and v be the potential of the needle. The total energy W stored in the system is given by the sum of the energies associated with the capacitances between the needle and the quadrants.

The deflecting torque T is derived from the rate of change of energy with respect to the angular displacement θ:

Assuming the rate of change of capacitance is linear, dC1/dθ = -dC2/dθ = k, the equation simplifies to:

Important features:

• It is primarily used for measuring very small potential differences or charges.
• It can be used as an electrostatic voltmeter when the needle is connected to one pair of quadrants.
• It exhibits extremely high input impedance, drawing negligible current from the source.
• It is capable of measuring both AC and DC voltages.
• The instrument is sensitive to external electrostatic fields, requiring effective shielding.

To extend the current range of a moving coil instrument, a low-resistance resistor known as a shunt is connected in parallel with the meter movement. The shunt provides a bypass path for the majority of the current, allowing only a small, safe fraction to pass through the meter coil.

If I is the total current to be measured, Im is the full-scale deflection current of the meter, Rm is the meter resistance, and Rs is the shunt resistance, the required shunt value is calculated using the current division principle:

where n is the multiplying factor defined as n = I / Im. By selecting an appropriate value of Rs, the instrument's range can be increased to measure currents many times higher than its basic full-scale capacity.