Permanent-magnet alternating current (PMAC) motors have become popular in many applications due to its high efficiency and power density. Although PMAC drive system with employment of rotor position sensor(s) may be easily implemented, the rotor position sensor(s) may increase cost of the drive system. Furthermore, the rotor position sensor(s) may occupy space in the motor thus making it hard to fit in a compact motor.
Many PMAC motors, especially compact motors, have sinusoidal back-electromotive force (EMF) when they are running smoothly. These PMAC motors may be driven by sinusoidal current in time domain to reduce torque ripple and improve motor efficiency. However, it may be hard to achieve sinusoidal current in a sensorless drive system. Brushless direct current motor (BLDC) drive is widely used due to its simplicity and high reliability. But electromagnetic torque ripple in BLDC motor may cause acoustic noise and vibration when in motor operation. In addition, current commutation may induce sudden jumps in current waveform, thus causing additional noise. In many applications, fluid dynamic bearings may be used to reduce noise generated by vibration. However, with the use of fluid dynamic bearings motor may be more sensitive to noise generated by electromagnetic toque and current commutation.