Question 44

The Correct Answer is A. ### Explanation of Why Option A is Correct Option A ("relatively high starting torque and low stator current") correctly describes the performance of a wound-rotor induction motor (WRIM) with high resistance inserted in series with the rotor winding at startup. **1. High Starting Torque:** The starting torque ($T_{st}$) of an induction motor is maximized when the rotor resistance ($R_2$) is equal to the standstill reactance ($X_{20}$), i.e., $R_2 = X_{20}$. By inserting external resistance into the rotor circuit (which is the primary function of the wound rotor), the total rotor circuit resistance can be intentionally increased to match or approach $X_{20}$. Operating the motor near this condition ensures a **relatively high starting torque**. This control over starting torque is the main advantage of the WRIM. **2. Low Stator Current (Low Starting Current):** The magnitude of the starting current ($I_s$) in an induction motor is inversely proportional to the total impedance of the motor circuit (referred to the stator side). The insertion of external resistance ($R_{ext}$) into the rotor circuit significantly increases the overall rotor impedance ($Z_2$). This increased impedance effectively limits the magnitude of the reflected current drawn from the stator supply (the starting current). Therefore, the motor draws a **relatively low stator current** at startup, which is desirable for limiting voltage sag on the power system. --- ### Explanation of Why Other Options Are Incorrect **B) relatively high starting torque and high stator current:** While the starting torque is high (as explained above), the inserted rotor resistance fundamentally serves to limit the starting current. Therefore, having a high resistance and a high starting current are contradictory. **C) relatively low starting torque and low stator current:** The WRIM is designed specifically to achieve high starting torque by adjusting rotor resistance. If the inserted resistance results in low torque, it means the resistance value is too high or too low compared to $X_{20}$, but typically, when resistance is added for starting, it aims for high torque. More importantly, using a high-resistance addition to achieve lower torque would defeat the motor's purpose. **D) relatively low starting torque and high stator current:** This combination is generally characteristic of a standard squirrel-cage induction motor started directly across the line (DOL). A squirrel-cage motor (without specialized rotor design) often has low rotor resistance ($R_2 \ll X_{20}$), resulting in poor starting torque and excessively high starting currents. The WRIM with added resistance achieves the exact opposite characteristics: high torque and limited current.