What is Slip ring induction motor
In this type of induction motor’s, the rotor comes with the separate winding section as similar to Stator of the motor. Where the rotor and stator slots exactly equal. The rotor can be wound a three-phase star or delta winding, distributed winding, wound for the same as that of the stator of the motor. The construction is laminated and slotted. The slots contain rotor winding as shown in the figure.
The three-phase is winding after connecting the winding in star or delta, are permanently connected in slip rings. The slip rings are mounted on the rotor shaft with fully insulation backup. The slip rings are nothing but a connector, which connects external resistance circuit with motor winding permanently. The slip rings rotate with the same rotor shaft. Actually, A set of carbon brush assembly connects the external resistance with slip rings. Slip ring contains polished surface to avoid friction and rubbing of the carbon brush and to get extended lifetime
Also see: Why Carbon brushes are selected, How to Select Good carbon brushes
Also See: How to calculate current carrying capacity of Carbon Brushes.
At starting time, we add full resistance after some delay 60 sec the resistance will cut off. Finally, to close the rotor winding loop we put some resistance to the rotor. It means, during Running condition the slip ring are shorted. The possibility of the addition of an external resistance in series with the rotor with the help of slip rings. In this, you can add external resistance as per your starting torque.
Note: Make sure the permission of the motor manufacturer. They will help you to add external resistance.