Design Consideration on the Square-Wave Voltage Injection for Sensorless Drive of Interior Permanent-Magnet Machines

Although it is widely known that the saliency-based position-sensorless drive is able to achieve the closed-loop control at zero and low speed, there is little literature addressing the consideration on the selection of injection voltage frequency. This paper evaluates the square-wave injection voltage at different frequencies for the design of an interior permanent-magnet (PM) machine sensorless drive. It is shown that more flux saturation on high-frequency (HF) d-axis inductance occurs than the saturation on q-axis inductance due to the magnetic relaxation. The HF saliency ratio (Lqhf/Ldhf) is increased by increasing the injection frequency. The performance of a saliency-based sensorless drive can be enhanced by properly designing the frequency of injection voltage. This paper also includes the experimental comparison between closed-loop encoder-based and closed-loop saliency-based sensorless operation of an interior PM machine drive.

I. Introduction

Elimination of position sensors and their associated cables can be achieved by using the rotor position information provided by the “machine itself” in ac drives. At zero and low speed, position estimation relies on the tracking of machine spatial saliencies. With the high-frequency (HF) voltage persistent excitation, these saliencies are measurable through the current sensing from the inverter terminal [1] –[3]. For permanent-magnet (PM) machines startup, the magnet polarity detection is required to identify the location of the north pole or south pole for the field-oriented control [4].