Discussion
A wound rotor machine has a rotor resistance giving maximum torque at 20 per cent slip. To place maximum torque at standstill, the rotor resistance must be
Answer: D
Correct answer: multiplied by five
The slip of maximum torque is \( s_{max} = R_2/X_2 \). At present \( R_2/X_2 = 0.2 \); to place the peak at standstill we need \( s_{max} = 1 \), so the rotor resistance must be raised by the factor
\[ \frac{1}{0.2} = 5 \]The reactance is unchanged by the external resistor, so the total rotor resistance is five times the original. The height of the peak is unaffected, only its position.
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