Why is the efficiency of an induction motor (wound rotor) so poor at high at slips?

The efficiency of a wound rotor induction motor decreases at high slip because, at higher slips, the rotor experiences greater relative motion between the rotating magnetic field and the rotor, leading to increased rotor currents and losses. This results in higher copper losses due to increased current flowing through the rotor windings. Additionally, the increased slip causes a reduction in the motor's output torque and power factor, further contributing to overall inefficiency. Consequently, as slip increases, more energy is lost as heat, reducing the motor's overall efficiency.