Instant Closing of Permanent Magnet Synchronous Motor Control Systems at Open-Loop Start

Nowadays, position sensorless permanent magnet synchronous motor drives are gaining popularity quite rapidly, and have become almost standard in many applications such as compressors, high speed pumps, etc. All of these drives involve estimators to calculate the speed and the position of the rotor, which are necessary for proper operation of vector control. While these estimators, with the exception of injection-based ones, work well in the middle and high-speed ranges, they cannot operate at low speeds. In order to overcome this problem, sensorless control systems include different starting techniques, with the most popular being open-loop starting. In this approach, the motor is accelerated in open-loop mode until it reaches the speed where estimator operates stably, then the control system is closed. However, the weakest point of this method is the technology of closing the system, which typically creates transients and can even be the cause of loss of stability. This paper proposes a method for instant and seamless transition from open-loop to closed loop which works perfectly under different load conditions. Other starting techniques are considered and compared with the proposed method.