Encoderless control of synchronous machines with permanent magnets - impact of magnetic design

High resolution optical encoders are used in industrial servo drives with position and/or speed control for providing the respective feedback signals. In applications with lower accuracy requirements resolvers are used as feedback sensors with respect to lower cost. As a resolver is nothing else than an electrical AC machine of special design, there is the basic idea to operate the servo motor itself like a resolver - in parallel to its original drive operation, of course. High frequency resolver signals are injected to the servo motor without disturbing its original purpose (i. e. the production of torque). The reaction of the servo motor on the high frequency signals can be detected by the current sensors, which are available with respect to the current control loop anyway. A lot of schemes and concepts for encoderless AC drive control providing good operation during standstill are already published. Acceptance by industry, however, is not very extended. The main reasons for hesitations in industry are (1) enhanced processing performance being necessary for the controller or signal processor (2) additional sensors or hardware (3) parameters to be adjusted with respect to encoderless control. A high frequency component is superposed to the current control output signal resulting in a high frequency current response, which can be used for detecting the orientation of the rotor. The position dependent high frequency currents can be measured with the standard current sensors available in industrial drives anyway. There is no need for any additional hardware being not available in standard industrial drives. The estimated rotor position is adjusted by a tracking controller, which works without impact of any machine or drive parameter. Therefore it is possible to detect even small anisotropies as typical for synchronous machines with surface mounted permanent magnets. The software effort is comparable to the software for a rotor model of a standard field oriented control for induction machines. Standard microcontrollers used in servo drives with field oriented control are sufficient to perform the sensorless control scheme. The experimental results presented in this contribution show the encoderless control scheme having no limitations with respect to a minimal speed; the drive is able to provide full torque in encoderless operation even at standstill. The control behaviour of position and speed control is equal to a servo drive with resolver feedback. Recent developments show that synchronous machines with conservative magnetic design show better performance in encoderless control than "modern" motor designs. This contribution shows some interesting results obtained from synchronous machines with permanent magnet excitation as well as from synchronous reluctance machines. Future developments and possibilities will be discussed.