Question

The speed of a d.c. series motor at an armature current of 10 A is 800 rpm. If load torque is proportional to square of speed, then at 5 A the speed will be

• A. 4000 rpm
• B. 200 rpm
• C. 1600 rpm
• D. 3200 rpm

Hint:

In DC series motors, speed varies inversely with load current if torque is constant.

Answer 1

The Correct Option is D

Since \( T \propto I^2 \) and \( T \propto N^2 \) ⇒ \( I^2 \propto N^2 \) ⇒ \( I \propto N \)
So, \( \frac{N_2}{N_1} = \frac{I_1}{I_2} = \frac{10}{5} = 2 \Rightarrow N_2 = 2 \times 1600 = 3200 \, \text{rpm} \)