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

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

(d) \(\frac{πV_a}{V_b}\)

I have been asked this question in an online interview.

My question comes from Problems of Series Resonance Involving Quality Factor in chapter Resonance & Magnetically Coupled Circuit of Network Theory

Answer 1

Right choice is (b) \(\frac{1.11V_a}{V_b}\)

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 sinusoidal voltage that is, with form factor of 1.11.

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

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