GATE 2014 EE (EE1) – Question 58
DC current is measured by series-connected PMMC and electrodynamometer ammeters. The PMMC has 100 turns, air-gap flux density 0.2 Wb/m² and coil area 80 mm². Electrodynamometer mutual inductance changes by 0.5 mH/degree of deflection. Spring constants are equal. Find current for equal deflections (A).
Practise this question in The GATE Grind →
Show answer and explanation
Correct answer: 3.0 to 3.4
Explanation
For the PMMC,NBA=100×0.2×80×10⁻⁶=0.0016N·m/A. Equating NBAIwith I²dM/dθand treating the supplied0.5mH/deg as the intended0.5mH/rad gives **I=0.0016/0.0005=3.2A**, the official key. With the literal per-degree unit, dM/dθper radian must be multiplied by180/π, giving I≈0.05585Ainstead. The key omits this angular-unit conversion; retain official grading while disclosing the inconsistency.