| Abstract |
The switched or variable reluctance machine offers many advantages for variable speed drive applications. However, the requirement of switched reluctance drives for a shaft position sensor can be a significant disadvantage relative to inverter-fed induction motor drive systems. One solution which has been investigated is to utilise the angle-dependent phase inductance profile of the reluctance machine to derive position information and thereby achieve self-synchronous, or position-sensorless, operation of the drive. Difficulties arise however due to the influence of ohmic losses, magnetic losses, converter voltage ripple, mutual inductance effects and switching noise. This paper presents a new solution to the problem based upon the combined techniques of pulsewidth modulated (PWM) control and synchronous demodulation. In a constant-frequency PWM system, the phase current ripple is amplitude modulated by the inductance profile of the machine. The new technique employs synchronous demodulation to extract the reactive component of current at the PWM frequency from the total motor phase current. The amplitude of this component is a direct measure of the angle dependent phase inductance. The strategy which is described results in an accurate, on-line continuous reproduction of the machine inductance profile which is essentially independent of magnetic losses, mutuaI inductance, inverter link voltage and motor speed. The demodulated signal is ultimately processed by a multi-mode microcontroller-based system to give the position and velocity information required to implement torque and closed-loop speed control of the self-synchronous drive. |
