Starting method for sensorless operation of slotless permanent magnet synchronous machines

Slotless permanent magnet synchronous motors (PMSM) are receiving much attention for drive applications because of their high efficiency, high power-to-volume ratio, and their elimination of cogging torque. Furthermore, motor control without a rotational transducer is a subject that is receiving significant attention. In order to eliminate the need for high-resolution rotor position sensors, many sensorless techniques have been developed. It is well known, however, that sensorless operation is problematic at standstill-especially for a round rotor machine such as the slotless PMSM. Unless the initial rotor position is known, it is not possible to start the machine at full torque or to guarantee dither free startup. This paper presents a sensorless drive for a slotless PMSM with emphasis placed on stable startup by estimating the initial rotor position before starting. To estimate initial position, voltage pulses are applied to the stator windings. The resultant current measurements are then used to estimate the initial rotor position before starting. The proposed starting method for standstill rotor position identification is verified by experiment on a slotless PMSM.