Sensorless control of AC motor

So called "sensorless control" of AC machines means having no mechanical (speed and/or position) sensors. However, usually this term only includes vector controlled drives, i.e. it does not comprise V/f open-loop control of Induction Motors (which is the simplest technique for driving an IM motor without a mechanical sensor).

For the induction mold, sensorless vector control is a mature technology, it has been around for a few decades. Almost all reputable drive manufacturers have at least one product which implements sensorless vector control of induction motors.

"DTC" is a proprietary ABB technique, while most other manufacturers adopt classical indirect Field Oriented Control, usually performing flux, position and speed estimation by means of observers, which are based on measured current and imposed voltage. Usually, from the user point of view, the drive is expected operate an auto-tuning of the control parameters, based on nameplate values and self-commissioning measurements. AFAIK, the challenges that still remain are mainly in the speed and torque steady-state accuracy, control dynamics and stability in the low-speed operation.

Sensorless control of Permanent Magnet and Synchronous Reluctance motors has been introduced only recently in general-purpose drives, while it has been applied for more than 10 years in application-specific drives. The main drive manufacturers already propose this in their "high-end" products, but being still not a "standard" feature there may be more difference among them about performances. One important difference with respect to the case of induction motors is the very high speed accuracy that can be achieved with these machines (since they are synchronous) and the possibility (implemented only by relatively few companies at the moment) to fully control some of these machines (e.g. Interior Permanent Magnet and Synchronous Reluctance ones) also at zero-speed. This enables position control, although only one or two companies advertise this possibility.