The effect of magnetic saturation on sensorless control of a brushless permanent magnet motor under AC and DC excitation

The design and performance of a permanent magnet rotor for a concentrated wound, torque dense machine exhibiting a saliency profile suited to high-frequency injection based zero- and low-speed sensorless control is presented. Rotor design is mechanically constrained by an existing stator. Electromagnetic specifications require the motor to match the torque capability of an existing baseline machine whilst demonstrating a saliency characteristic suited for injection based, low-speed sensorless control. The effects of fundamental current amplitude on flux saturation and saliency profile are investigated for brushless DC and brushless AC operation. The saliency profiles required for sensorless detection show significant differences under these operating modes. For the selected rotor design, brushless AC operation results in a stronger saliency signature at light loads, which quickly deteriorates with the onset of saturation. Brushless DC operation results in more gradual deterioration in saliency signature, suggesting sensorless control would be possible over a greater torque range.