Sensorless Control of Permanent-Magnet Synchronous Motor Drives

Abstract

The reduced energy consumption is highly demand in motor drives for heating, ventilating, and air conditioning (HVAC) applications. The efficiency advantage makes the permanent-magnet (PM) synchronous machine an attractive alternative to the induction machine in drives for those applications. In order to use PM synchronous machines in HVAC applications, simple, low-cost control methods are also important for them. The particular requirement in control of PM synchronous machines is the synchronization of the AC excitation frequency with rotational speed. A shaft-mounted position sensor is required for achieving this. This shaft-mounted position sensor increases the cost and reduces the reliability in the drive system. This makes it undesirable for HVAC applications. The objective of this research project is to investigate sensorless control methods for PM synchronous machines with particular attention to HVAC application requirements. The understanding of the machine model is a key requirement for machine control. The mathematical models for PM synchronous machines are first derived in this thesis. The control properties of PM synchronous machines are discussed and they are compared. Since high dynamic performance is not a demand for HVAC applications, a suitable control approach for PM synchronous machines is V/f control approach. A substantial part of this thesis is devoted to investigate V/f control approach for PM synchronous machines. In order to provide basics for designing a V/f controlled drive, the stability characteristics of PM synchronous machines under open-loop V/f control, i.e. without having any feedback for V/f control, is analyzed in this thesis. The linearized machine model is the key to analyze the stability characteristics. The stability analysis show that the PM synchronous machine becomes unstable after exceeding a certain applied frequency under open-loop V/f control. In order to show how to stabilize the V/f controlled PM synchronous machines for a wide frequency range, a simplified small signal dynamics model for PM synchronous machines is derived in this thesis. With the help of this model it is shown that by modulating the applied frequency proportional to the perturbations in the power the stable operation of the machine can be achieved for a wide frequency range. In voltage source inverter driven drives the DC-link current perturbations can also be used for