What is the purpose of slip-rings on a wound rotor induction motor?

The purpose of slip rings on a wound rotor induction motor is to connect external resistances to the motor. This design allows for the adjustment of rotor resistance during the starting process, which is critical because it helps in managing the motor's starting torque and overall performance. By using slip rings, operators can add external resistance to the rotor circuit. This increases the slip of the motor during startup, allowing for higher starting torque and smoother acceleration. Once the motor reaches its full speed, the external resistances can often be removed or minimized, enhancing the efficiency of the motor in normal operation.

Other options, while relevant to the operation of a motor, do not accurately represent the role of slip rings. For instance, while cooling of motor windings is necessary for overall motor health, it is not a function performed by slip rings. Similarly, enhancing the starting torque is a result of changing rotor resistance but does not reflect the direct purpose of the slip rings themselves. Improving efficiency is also a result of operational adjustments made possible by slip rings, rather than a primary function of the rings. Therefore, the identification of external resistance connection as the correct purpose aligns directly with the mechanical and operational principles of wound rotor induction motors.