Current Sensing Issues on Saliency-Based Position Sensorless Drive for Permanent Magnet Machine with PWM Voltage Injection

Abstract

This paper analyzes current sampling issues on the saliency-based position sensorless drive for permanent magnet (PM) machines using the pulse width modulation (PWM) voltage injection. On the basis, PWM voltage with the switching frequency is persistently injected into the machine. The rotor position is estimated through the spatial signal from the PWM voltage reflected current ripples. Because the rotor position is estimated every PWM cycle, multiple transient currents instead of single average current should be sampled per PWM. It leads to the issue on the selection of current sampling points with respect to different PWM patterns. In this paper, an asymmetric current sample is proposed for the position estimation using PWM voltage injection. By directly sampling phase currents during the injection period, the better signal-to-noise ratio (SNR) of position signal is concluded comparing to conventional symmetric current sample. In addition, shunt resistors are typically installed for the current sensing in inverters at low cost. A modified PWM control is also proposed for the position estimation compatible with low-side shunt current sensing. According to preliminary experimental results, the position estimation error is decreased using the proposed asymmetric current sample for the inverter with Hall-effect current sensors. More importantly, the inverter with low-side shunt sensors can be used for the saliency-based drive using the proposed PWM voltage pattern.