Understanding the Generating Voltmeter

A generating voltmeter is a type of electrostatic voltmeter specifically designed for measuring high DC voltages. Unlike passive electrostatic voltmeters, the generating voltmeter uses a mechanical system to produce an AC signal proportional to the DC voltage being measured.

The fundamental principle behind a generating voltmeter involves varying the capacitance between two electrodes in the presence of the high DC voltage. This variation in capacitance induces an AC current or voltage, the amplitude of which is directly proportional to the DC voltage being measured.

Role of the Motor in a Generating Voltmeter

To achieve this varying capacitance, a rotor driven by a motor is used. This rotor mechanically modulates the capacitance between fixed and rotating electrodes at a specific frequency.

The rate at which the capacitance changes is crucial for the accuracy of the measurement. If the rate of change of capacitance is not constant, the induced AC signal's amplitude will not be linearly proportional to the DC voltage, leading to inaccurate readings.

Therefore, the motor driving the rotor must rotate at a constant speed. A constant speed ensures a constant frequency of capacitance variation, resulting in an induced AC signal whose amplitude is directly proportional to the DC voltage being measured. This stable AC signal can then be easily amplified and measured using standard AC amplification and rectification circuits.

Analyzing the Options

• A constant speed motor: As explained, a constant speed motor is essential for generating a reliable AC signal proportional to the DC voltage by ensuring a constant rate of capacitance variation.
• A variable speed motor: A variable speed motor would cause the frequency and potentially the waveform of the induced AC signal to fluctuate, making accurate measurement of the DC voltage difficult.
• A variable speed motor with a capacitor: While capacitors are part of the measuring circuit, a variable speed motor still poses the problem of inconsistent signal generation due to non-uniform capacitance variation rate.

Based on the working principle, the core component required to drive the capacitance variation reliably is a motor that runs at a steady, constant speed.

Thus, a generating voltmeter uses a constant speed motor.