AC motor controls are generally more complex and expensive. AC motors themselves can be more affordable, though, and connect more easily to ubiquitous AC mains power. If you need precise, on-demand braking and direction reversal, AC motor controllers are the superior choice.
Since these motor controllers offer more adjustability and are more complex, they need significant tuning when setting them up in an installation. AC controllers also often come with the features needed to readily connect to broader control systems, making them well-suited to fast-paced, highly connected environments.
Variable speed drives are the most common types of AC motor controls, and they fall into two main categories.
Variable frequency drives (VFDs) modify the frequency of the supply power, and this adjusts the motor speed. VFDs convert the supplied AC power to DC through a rectifier and then convert it back to AC at the desired frequency through an inverter. VFDs often maintain a voltage-to-frequency ratio throughout operation to give consistent torque. This allows robust motor performance across a wide range of speeds and applications, though it is not well-suited for low speeds or holding-torque (stopped) applications.
Vector control drives have more complete control ranges than VFDs. They have algorithms for separately controlling the motor’s magnetising and torque-producing currents to efficiently convert supply power to torque with minimal heat build-up, including at low speeds and holding torque.