What Is Direct Torque Control?

Jordan Weagly

Direct torque control is a method of optimizing and maintaining normal operations, typically within an alternating current (AC) motor. There are several applications for this kind of control, usually in machines that require consistent and reliable torque. Compared to other methods of controlling AC motors, direct torque control has several advantages and several disadvantages, though much of this depends on the application. Certain technological possibilities enable and further enhance this and other variable frequency drives — machines typically responsible for controlling the electric power supplied to a motor.

Direct torque control can help improve performance and energy efficiency within alternating current motors.
Direct torque control can help improve performance and energy efficiency within alternating current motors.

In essence, the process of direct torque control involves monitoring certain variables within the motor and adjusting the amount of power to keep those variables within an optimal range. Specifically, the main variables measured are voltage and current. From these values, the magnetic flux and the torque of the motor can be derived. Once these measurements have been taken, the electric current being fed to the motor is adjusted, if necessary, to maintain the optimal ranges of torque and flux.

Applications for direct torque control are numerous in industrial processes, because many machines often need precise torque over long periods. Most often, direct torque control will be implemented on three-phase AC motors, though other designs can often integrate similar processes. Early experiments with direct torque control placed the systems inside locomotives, and direct torque control can now be used in electric car motors.

Advantages to this kind of control generally stem from the consistent measurements and adjustments that are made to optimize operations. Ideally, any adjustments will be made almost instantly. This can raise the efficiency of the motor overall and help reduce energy loss. Additionally, this type of control can reduce the mechanical resonance of a motor, further increasing efficiency, and even cutting down on audible machine noise at low speeds.

Disadvantages to these systems often start with incorrect measurements. There are often measurement errors at low speeds, for instance, which can lead to improper adjustments and efficiency loss. Incorrect measurements also can occur at high speeds and across the whole spectrum of torques. As a result, high-quality measuring and monitoring equipment is typically required.

High-speed computer technologies play an important role in effective direct torque control. So many fast calculations are required that extremely fast computers and other digital controllers are often essential to making the proper adjustments on time. Additionally, speed and position sensors are often necessary, especially in low-speed applications.

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