Jawaharlal Nehru Technological University Hyderabad B.Tech II ELECTRICAL AND ELECTRONIC MEASUREMENTS Previous Question Paper (October - 2020 - R16)

An indicating instrument requires three distinct torques for its proper operation: a deflecting torque, a controlling torque, and a damping torque.

1. Deflecting Torque (T_d): This torque is produced by the electrical energy of the quantity to be measured. It causes the moving system to deflect from its zero position. Common methods include magnetic effects (moving coil/iron), thermal effects, or electrostatic effects.

2. Controlling Torque (T_c): This torque opposes the deflecting torque and ensures that the deflection is proportional to the magnitude of the measured quantity. It also brings the pointer back to zero when the supply is removed. It is typically provided by either spring control (using two phosphor-bronze hairsprings) or gravity control (using a small adjustable weight attached to the spindle).

3. Damping Torque (T_damp): This torque is necessary to bring the pointer to its final steady position quickly without oscillations. It acts only when the moving system is in motion. Common methods include air friction damping (using a light aluminum vane moving in a closed chamber), fluid friction damping (using oil), or eddy current damping (using a metallic disc moving in a magnetic field).

Electrostatic voltmeters are primarily used for high-voltage measurements. Because their inherent capacitance is very small, their range cannot be extended using standard series resistors as in moving-coil instruments. Instead, the following methods are employed:

1. Capacitive Potential Divider: This is the most common method for extending the range. A high-voltage capacitor is connected in series with the electrostatic voltmeter. The total voltage is divided between the series capacitor and the internal capacitance of the voltmeter. The effective voltage across the meter is given by:

where V is the total voltage, C_s is the series capacitor, and C_m is the meter capacitance.

2. Use of an Amplifier: For lower voltage ranges, an electronic amplifier can be used to step up the input voltage before it is applied to the voltmeter, though this is less common for high-voltage applications.

3. Precautions: When using capacitive dividers, the ratio must be independent of frequency. This requires the use of high-quality, low-loss capacitors, and the voltmeter must be shielded to prevent the influence of stray capacitances, which could otherwise introduce significant measurement errors.