GATE 2015 EE – Question 59
A DC motor has the following specifications: 10 hp, 37.5 A, 230 V; flux/pole = 0.01 Wb, number of poles = 4, number of conductors = 666, number of parallel paths = 2. Armature resistance = $0.267\ \Omega$. The armature reaction is negligible and rotational losses are 600 W. The motor operates from a 230 V DC supply. If the motor runs at 1000 rpm, the output torque produced (in Nm) is ________.
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Correct answer: 57 to 58
Explanation
The back emf is $E = \frac{P\Phi Z N}{60 A} = \frac{4 \times 0.01 \times 666 \times 1000}{60 \times 2} = 222$ V. The armature current is $\frac{230 - 222}{0.267} = 29.96$ A. The electrical power developed is $222 \times 29.96 = 6652$ W, and after the 600 W rotational loss the output is 6052 W. At $\omega = 104.72$ rad/s the torque is $\frac{6052}{104.72} = 57.8$ Nm.