slip ring induction motors

I think this will lead to a sparking effect.

As long as you have rated voltage the machine can operate comfortably at no load or anything between no load and full load. What is your application?

If by underloading, you mean pressure applied to hold the brushes on the slip rings, I would imagine it would allow the motor to slip. My experience with wound-rotor motors included those with resistors and saturable reactors wired to the armature (via slip rings) for speed control. If one of the brushes loses contact with the slip ring, the secondary will single phase, lose torque, and slip.

I mean by underloading the current and power of the motor for example if the rated power is 1.8 MW and the actual load is 1 MW What is the result of this phenomena on brushes ?

Also is there a relation between the type of slip ring and the wear rate of the brushes ?

Thanks