Starting resistors are used to start large three-phase asynchronous machines with slip-ring rotor. The starting resistor is started in the rotor circuit and switched off stepwise. It limits the starting current of the motor and increases its effective component (rating factor) and thus the drive torque because only the effective component is decisive for the motor torque.

The resistor and its stepping determine the starting performance of the drive. Stepping is designed either such that the same peak starting current I2 flows from step to step or (ideally) that the motor has a defined starting performance. The ratio 1 / I2 (I1 = current at which the next step is switched) indicates by how much the next step is smaller than the previous one and determines the number of starting steps.

Formerly, the starting steps were switched by way of manually or motor-actuated drum starters or cam-operated switches with a specified switching sequence. Today, mainly contactors are used for this task, which are switched as a function of time, current or rotor speed.

While smaller drives mostly require resistors with a defined stepping, larger motors are operated with dually stepped resistors to limit the number of rotor contactors. 5 or 6 rotor contactors give 2⁵ - 1 bzw. 2⁶ –1 steps, respectively, that are selected according to the desired starting performance and controlled with adequate software.