Just guessing here of course, no way of knowing from pics, but it could be resistance-start. A wound-rotor induction motor has a stator like the squirrel cage induction motor, but a wound rotor, with insulated windings, brought out to slip rings and brushes. No power is applied to the slip rings. Their sole purpose is to allow a resistance to be placed in series with the rotor windings during startup. This resistance is shorted out once the motor is started; the rotor is then electrically similar to a squirrel cage type.
Why resistance? Squirrel cage induction motors draw 500% to over 1000% of full load current during starting. While this is not a severe problem for small motors, it is for large ones. Placing resistance in series with the rotor windings not only decreases starting current, but also increases starting torque.
Breakdown torque peak is shifted toward zero speed by increasing rotor resistance.
The resistance decreases the torque available at full running speed. But that resistance is shorted out by the time the rotor is at speed. A shorted rotor operates like a squirrel cage rotor. Heat generated during starting is mostly dissipated external to the motor in the starting resistance. The complication and maintenance associated with brushes and slip rings is a disadvantage of the wound rotor as compared to the simple squirrel cage rotor.
This motor is suited for starting high inertial loads. A high starting resistance makes the high pull out torque available at zero speed. For comparison, a squirrel cage rotor only exhibits pull out (peak) torque at 80% of its synchronous speed.
