A comparison of low speed sensorless control techniques for low voltage PM machines

Two encoderless control methods for permanent magnet machines are compared for automotive applications. At low and zero speeds, model based methods cannot be utilised, and therefore a saliency tracking method is needed. A ldquod-axis injectionrdquo method and a ldquocurrent derivativerdquo method are evaluated in terms of quality of position signal and additional current distortion introduced. The acoustic noise produced by both methods is also compared. The acoustic noise is important in automotive applications as it can affect driver comfort. Experimental tests for a low voltage 1 kW SMPM (surface mounted permanent magnet) machine for a wide range of loads indicate that the current derivative method performs better than the d-axis injection method for quality of position signal, and it also introduces a lower audible noise. However for both techniques the quality of the position estimate is improved at the expense of additional audible noise and additional current distortion.