When the two slip-ring motors are required to run at the same speed, it is possible to do this by connecting their rotors together through the slip rings in conjunction with a single slip resistance. The starter for such a scheme includes a single rotor resistance, the last step of which is the continuously-rated slip resistance, and two-stator contactors, one for each motor.
In order to limit the circulating current in the event of the motors being out of phase when started, a reactance is usually inserted in the interconnecting tie. The reactor is wound in two sections, and connected so that it is non-inductive to currents flowing through each half into the slip resistance but inductive to circulating currents between rotors. This reactance also assists load sharing when the two motors are driving a common load, as for example travel motors at opposite ends of an overhead crane.
The controller gives speed control by varying resistance in series with the rotor windings and also breaks the three-stator phase in the ‘off’ position. The moving-copper-contact rings are shown as thick horizontal lines, while the forward and reverse steps are indicated by the numbered vertical lines. The diagram below shows the connections of series limit switches when used.