23 Jul 2024


Which motor control method should you use?

Intro: Modern variable-speed drives (VSDs) allow users to implement a choice of different control philosophies. Liam Blackshaw, ABB’s UK product manager for LV drives, explores some of the advantages and disadvantages of each.

Traditionally there are three main ways of using an AC drive to control a motor. The most basic of these is scalar control – also known as U/f or V/f control. Both the voltage and frequency references are controlled and fed into a modulator, which simulates an AC sine wave and modulates to control the speed of the motor. It typically has a speed accuracy of ±2–3%.

While effective and relatively low cost to implement in simpler applications, the main drawback of scalar control is that the voltage and frequency references are essentially the only variables used. Scalar control is an open-loop form of control with no feedback in terms of the speed or position of the motor’s shaft at a given time.

Scalar control is best suited to applications where extreme precision is less of a priority –some pumps and fans, for example.

Higher levels of precision are possible by using other control methods. But this may require a separate encoder attached to the motor and cabled back to the drive.

Vector control provides more field-orientated control, while also controlling frequency and voltage. Unlike scalar control, vector control uses control loops, allowing operators to set a desired actual motor speed, rather than a frequency. Vector control provides a closed control loop and, as a result, can adjust for errors by taking continuous current measurements and calculated speed values from the drive, and feeding this information back into the control system to ascertain speed indirectly. Vector control without an encoder will typically provide a speed accuracy of ±1–2%, and is best suited to constant-torque applications such as conveyors.

Direct torque control (DTC) achieves an even higher level of precision by calculating the motor’s torque algorithmically using inputs of motor phase currents and DC bus voltage measurements, in addition to the states of the power-switching transistors within the drive. This method controls motor flux and torque directly, as opposed to inferring it from current measurements, and no modulation or external feedback devices are needed. The result is a drive with a speed accuracy of ±0.1–0.5% – close to the theoretical maximum. Equipping this system with a standard encoder can improve the speed accuracy to ±0.01%.

More information about DTC is available in a White Paper than can be downloaded from https://drivesncontrols.news/abx0rd