The construction of a stator is the same for both the squirrel cage and slip ring induction motor. The main difference in a slip ring induction motor is on the rotor construction and usage. Some changes in the stator may be encountered when a slip ring motor is used in a cascaded system, as the supply for the slave motor is controlled by the supply from the rotor of the other slip ring motor with external resistance mounted on its rotor.
The slip ring induction motors usually have a “Phase-Wound" rotor. This type of rotor is provided with a 3-phase, double-layer, distributed winding consisting of coils used in alternators. The rotor core is made up of steel laminations which have slots to accommodate formed 3-single phase windings. These windings are placed 120 degrees electrically apart.
The rotor is wound for as many poles as the number in the stator and is always 3-phase, even though the stator is wound for 2-phase. These three windings are “starred" internally and the other end of these three windings are brought out and connected to three insulated slip-rings mounted on the rotor shaft itself. The three terminal ends touch these three slip rings with the help of carbon brushes which are held against the rings with the help of a spring assembly.
These three carbon brushes are further connected externally to a 3-phase start connected rheostat. The slip ring and external rheostat makes it possible to add external resistance to the rotor circuit, enabling them to have a higher resistance during starting and thus higher starting torque.
When running under normal conditions, the slip rings are automatically short-circuited by means of a metal collar, which is pushed along the shaft, thus making the three rings touch each other. Also, the brushes are automatically lifted from the slip-rings to avoid frictional losses, wear and tear. Under normal running conditions, the wound rotor acts the same as the squirrel cage rotor.