Question

Current in a pressure coil of the Dynamometer type wattmeter ________

(a) lags the applied voltage

(b) leads the applied voltage

(c) is in phase with the applied voltage

(d) there is a phase difference of 90 degrees

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Question is taken from Errors in Dynamometer Wattmeter in portion Measurement of Power and Related Parameters of Electrical Measurements

Answer 1

Correct choice is (a) lags the applied voltage

To elaborate: The current through the pressure coil is in phase with the applied voltage in an ideal wattmeter. The pressure coil is assumed to be purely resistive. Practically, the pressure coil has a small inductance L. As a result the current lags the applied voltage by a certain angle.

where, R2 = r2 + R = total resistance of the pressure coil

r2 = resistance of pressure coil

R = series resistance with the pressure coil

I1 = load current.