Question

A 50 Hz voltage is measured with a moving iron voltmeter and a rectifier type AC voltmeter connected in parallel. If the meter readings are Va and Vb respectively. Then the form factor may be estimated as?

(a) \(\frac{V_a}{V_b}\)

(b) \(\frac{1.11 V_a}{V_b}\)

(c) \(\frac{\sqrt{2} V_a}{V_b}\)

(d) \(\frac{π V_a}{V_b}\)

I had been asked this question in an online quiz.

I'd like to ask this question from Advanced Problems on Indicating Instruments topic in section Extension of Instrument Ranges of Electrical Measurements

Answer 1

The correct answer is (b) \(\frac{1.11 V_a}{V_b}\)

For explanation: Form factor of the wave = \(\frac{RMS value}{Mean value}\)

Moving iron instrument will show rms value. Rectifier voltmeter is calibrated to read rms value of the sinusoidal voltage that is, with form factor of 1.11.

∴ Mean value of the applied voltage = \(\frac{V_b}{1.11}\)

∴ Form factor = \(\frac{V_a}{V_b/1.11} = \frac{1.11 V_a}{V_b}\)