Torque Squirrel Cage Induction Motor

In induction motor, torque produced is proportional to the rotor copper loss. Since the rotor winding is not approachable, its torque cannot be changed. However, at design stage it is taken care of. The rotor cage (shorting ring and the axial bars) are designed with suitable number and size to develop desired starting and full load torque.

Double cage rotor (with two layers of axial bars at different diameter of rotor) motors are designed to give higher torque at starting as well as at full load running conditions. At starting the slip is 1 p.u. and therefore induced current are of supply frequency. The flux inducing does not penetrate deep enough (skin effect) and is mostly linked by bigger diameter cage and produces desired torque. At full load condition slip is small and therefore flux penetrates deeper and links with inner cage designed with better conducting material than that of outer cage material and gives designed torque. That is how torque of squirrel cage rotor motor is controlled at design stage itself.

You got answers that dealt with explanaing the Voltage /Torque relation in such motors and the fact that this is the only way to control the torque (but with some caution to the damages etc...)

In fact, there are electrovic boards available that allow to control the torque of induction, squirel cage motors, by varying the voltage. These are used in many equipment that need to wind materials of any king and maintain a constant tension during winding...maybe this is your inerest?? Check supplier on internet.

Torque is what the motor experiences at its output shaft. So the answer is 'vary the mechanical load'.