How is speed control of a slip-ring motor effected?

By introducing resistance into the rotor circuit similar to a starting resistance except that the heat losses in the resistance must be dissipated continuously. Unless the duty is intermittent, all except small sizes require some means of cooling the resistors.

Grid resistances with a motor-driven fan may be used in conjunction with a drum controller. Alternative methods are oil-immersed resistances or a liquid resistance cooled by circulating water through cooling tubes.