The Electrical Engr.

What causes change in rotor resistance externally in a three phase slip-ring induction motor to change its Torque? Slip-Ring Induction Motor (Credit: http://yourelectricalguide.com) While studying about induction motor types you must have come across this machine whereit has three conductors for rotor connected externally to a rheostat. But how exactly they help? Why do we need those rotor external resistance? Let's find out. The one line answer to this question most of the time people suggest is, "To increase the Motor Torque". Well I willn't say it's incorrect but it's vague or incomplete.  Torque in an induction motor depends on two things, air gap flux and rotor magnetic flux generated due to induced emf . When rotor cuts air gap flux, a current flows through rotor conductors. As a result, the rotor generates its own flux and hence under the effect of this stator and rotor flux, there is a resultant flux which us responsible for torque.