GATE EE 2015 Set 2 — Question 15

MCQ+1 / -0.33EasyPMMC & Moving IronElectrical Quantity MeasurementElectrical & Electronic MeasurementsDynamometer Instruments

Electrical & Electronic Measurements → Electrical Quantity Measurement → Dynamometer Instruments

Last updated 5 September 2026

Question

Match the following. Instrument Type Used for P. Permanent magnet moving coil 1. DC Only Q. Moving iron connected through current transformer 2. AC Only R. Rectifier 3. AC and DC S. Electrodynamometer

Instrument Type	Used for
P. Permanent magnet moving coil	1. DC Only
Q. Moving iron connected through current transformer	2. AC Only
R. Rectifier	3. AC and DC
S. Electrodynamometer

Correct answer (C) P-1, Q-2, R-3, S-3 Solution The matching is based on the operating principles of the instruments: P. Permanent magnet moving coil (PMMC): the deflecting torque is proportional to the instantaneous current, so the pointer follows the average value, which is zero for AC. It is used for DC only (P-1). Q. Moving iron connected through a current transformer: a moving-iron meter by itself responds to both AC and DC, but a current transformer works by electromagnetic induction and needs a time-varying flux, so it cannot pass DC. The combination is therefore usable on AC only (Q-2). R. Rectifier instrument: a rectifier feeds a PMMC movement. An AC input is rectified into a unidirectional current, and a DC input passes straight through (in the forward sense), so the meter deflects for both AC and DC . Its scale is normally calibrated in rms for sinusoidal AC, so on DC the reading has to be corrected by the form factor (1.11) (R-3). S. Electrodynamometer: the torque is proportional to the product of the fixed-coil and moving-coil currents. On AC both reverse together, so the torque stays unidirectional. It is used for AC and DC (S-3). Matching: P-1, Q-2, R-3, S-3, which is option (C). Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer (C) P-1, Q-2, R-3, S-3 Solution The matching is based on the operating principles of the instruments: P. Permanent magnet moving coil (PMMC): the deflecting torque is proportional to the instantaneous current, so the pointer follows the average value, which is zero for AC. It is used for DC only (P-1). Q. Moving iron connected through a current transformer: a moving-iron meter by itself responds to both AC and DC, but a current transformer works by electromagnetic induction and needs a time-varying flux, so it cannot pass DC. The combination is therefore usable on AC only (Q-2). R. Rectifier instrument: a rectifier feeds a PMMC movement. An AC input is rectified into a unidirectional current, and a DC input passes straight through (in the forward sense), so the meter deflects for both AC and DC . Its scale is normally calibrated in rms for sinusoidal AC, so on DC the reading has to be corrected by the form factor (1.11) (R-3). S. Electrodynamometer: the torque is proportional to the product of the fixed-coil and moving-coil currents. On AC both reverse together, so the torque stays unidirectional. It is used for AC and DC (S-3). Matching: P-1, Q-2, R-3, S-3, which is option (C).

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