Like the squirrel cage motor, the slip ring motor resembles a three-phase transformer with rotatable winding. Unlike the former, the ends of the windings are brought out to a set of slip rings with current collectors = brushes. These external rotor connections allow introducing resistors into the rotor circuit. The resistors limit the starting current and increase the motor torque. Thanks to starting resistors in the rotor circuit the motor torque is more or less proportionate to the rotor current – within certain limits. This way, the starting phase can be precisely controlled through the stepwise switching off of the starter resistors. After starting, the slip rings can be short circuited by way of a so-called brush lifting gear on the motor so that no additional losses occur in the rotor circuit. While the slip ring motor used to be the most frequently used drive in crane systems and all other applications where a speed control was necessary or desired, nowadays only large motors from about 630 kW and mainly with operating voltages from 6 kV are manufactured and used. In all other applications the slip ring motor has been superseded by a combination of frequency converter + squirrel cage motor.