Question

For a slip ring induction motor if the rotor resistance is increased then

Answer 1

This has to do with 2 things in my opinion:

1. As the rotor resistance is increased, the current in the rotor decreases, which decreases the opposing armature reaction. As a consequence, the net flux present in the air gap has a higher magnitude. Higher the flux, higher the torque because of direct proportion.

2. More the resistance, higher the power factor. Higher the power factor, more is the tapping of electrical energy (in terms of active power). More the active power, more is the mechanical output.