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How do you control torque on an AC motor?

The torque on an AC motor can be controlled by adjusting the frequency of the AC power supplied to the motor using a variable frequency drive (VFD) or by adjusting the amount of current supplied to the motor using a motor controller.

Torque is a critical parameter to control in AC motor applications. Torque determines the torsional force generated by the motor, which in turn determines the motor's ability to do useful work. Accurate torque control results in optimum machine performance and efficiency. In this paper, we will use induction motors as an example of how to regulate motor torque in AC motors.
How do you control torque on an AC motor

Controlling the torque of induction motors

The most common type of AC motor is an induction motor. In an induction motor, torque is caused by the relative motion between the stator and rotor magnetic fields. There are several ways to control the torque of an induction motor:

  • Varying the voltage: Lower voltages reduce flux and therefore torque. Higher voltages increase flux and torque. Continuous torque regulation can be achieved by controlling the voltage using a variable frequency drive (VFD).
  • Changing Frequency: Torque varies directly with slip (the difference between synchronous speed and rotor speed). Increasing the frequency increases the synchronous speed, which increases the slip and torque for a given load. VFDs make it easy to change frequency.
  • Changing Poles: Some motors have multiple windings. Switching between pole pairs changes the synchronization speed and slip, which change the torque.
  • Rotor Resistor: Introducing an external rotor resistor increases slip and increases torque. Simple and low cost, but only provides step torque adjustment.


Torque control requires AC motors to be matched to their driving loads for maximum efficiency and control. Although the technology varies from motor type to motor type, the torque of an AC motor can be effectively controlled by adjusting the current, slip, frequency, resistance, and phase angle.

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