3 phase induction motor

In an industry here we want to find maximum torque obtainable from a 3 ph. Indn motor via rotor resistance method.Can anyone give a method to find it for 5 HP Motor.we give supply of (3PH 415V 50hz 1 MVA). A lot of improvements required here as it uses 1970's technology. Overburdened by electricity bills.They are still using resistance for speed control.