Why can external resistors give a wound-rotor motor high starting torque?

In a wound-rotor induction motor, external resistors connected to the rotor windings via slip rings allow for high starting torque while simultaneously limiting the starting current.

In standard induction motors, starting at full voltage can result in extremely high currents because the relative velocity between the rotating magnetic field and the rotor is at its maximum at zero speed. By inserting external resistance into the rotor circuit, the motor can achieve a higher starting torque without the excessive current draw that would otherwise occur if the rotor resistance were limited solely to the internal rotor bars. As the motor accelerates, these resistors can be adjusted or removed to optimize performance, providing a controlled start that is particularly beneficial for heavy mechanical loads.12