Variable speed, robust synchronous reluctance motor drive systems

A variable speed SynRM drive system has been studied and tested extensively in this project, including position sensorless control, small dc-link active damping control, and their possible interactions.

Position estimation via flux linkage is first investigated, since the focused applications are e.g. HVAC applications where high dynamic control performance during standstill to low speed opera-ion range is not required. A commonly used closed-loop flux observer is adopted and it is found that it cannot always work properly as expected. System oscillation phenomenon can be observed. Two solutions are proposed and verified. One solution is to enhance the closed-loop observer with a PR controller, whose cutoff frequency should be adjusted according to the estimated speed. The other is to damp the oscillation by varying the PI parameters of the speed and current regulators according to the speed estimation error. The former solution can work very well during the steady-state conditions. While the latter solution is a more simple and robust solution, especially at the conditions with sudden speed changes such as step load on or off. Regarding this project, the adapive PI solution is preferred since it can meet the project topic and scope better, i.e. a simple but robust solution.

Small dc-link drive system, in which the electrolytic dc-link capacitors are replaced with film capacitors, is obtaining more and more interests from the industrial side, since the film capacitor has much longer expected service lifetime and also has the potential to achieve a compact design of the dc-link capacitor bank at high power ratings. However, due to the well-known negative impedance characteristic of the PWM inverter with constant power load, system instability may occur and active damping control may be required.

The active damping methods based on voltage/current injection are summarized. Furthermore, a virtual positive impedance active damping method is introduced and verified. Compared with the voltage/current injection solutions, which require online calculation of the compensating gain factors according to the system parameters and operating conditions, this method can ensure the system stability without the knowledge of the system parameters and operating conditions. The compensating gain factor introduced in the proposed method is used to control the level of the damping effect, such as without damping, with damping, twice damping effect, etc. Thus, the compensating gain factor is a fixed user input, rather than a varying value calculated online which may have stability problem. Therefore, this method is much simple and robust.

Finally, the preferred flux linkage based position estimation method with adaptive PI controllers is used to cooperate with different active damping methods, so that the whole variable speed SynRM drive system can be built and tested. It is found that the whole system can perform well.

Based on the above findings, it can be concluded that the whole variable speed SynRM drive system can perform well. The solution based on the flux linkage based position estimation method with adaptive PI controllers and the virtual positive impedance active damping method is preferred and recommended so far.