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As to adding slip rings to a squirrel cage 1) you would end up having to put winding onto teh rotor and this goes against using squirrel cage also the induced field in the cage will interact with the field from the rotor winding and at work not turn at best very bad torque ripple.
Yep all slip-ring Induction machine have coils on the rotor so the the electrical power fed to the rotor can produce the rotor field needed for operation[/quote]
Yet, the main reason for slip rings in induction motors has nothing to do with the ability to supply an external exitation.
With a squirrel cage motor, the start up current and torque is fixed by the size of the squirrel cage conductors.
With large Hp motors, that current becomes unmanageable at startup. Remember, the rotor is initially at standstill and there is maximum current induction in the squirrel cage. It is very much like a large transformer working into a short circuited secondary winding. This current reduces as the rotor speeds up and hence the frequency of the magnetic flux changes.
So, the slip rings simply provide an easy means to connect an external resitor bank for a single step or multi step starting sequence. The resistors limit the maximum rotor current and hence the stator startup current.
Once the rotor is up at full speed the slip rings are shorted out. The rotor then works similarily than a squirrel cage one, with the circulating currents being dependent on the load and, hence, the speed of the magnetic flux changes due to 'slip' ( the difference in rotor RPM and stator field frequency).
The slip rings are only in use during start up and there is no energy being fed into these for that purpose.
Klaus