Investigation of Force Generation in a Permanent Magnet Synchronous Machine

Traditional analysis of permanent magnet synchronous machines has focused upon establishing a relationship between the quadrature and direct axis stator current (or voltage) and the electromagnetic force created to establish rotation (torque). In this paper, an alternative analysis of electromagnetic force production is considered. Specifically, the influences of - and -axis stator current on both the radial and tangential components of the airgap flux densities are first evaluated. Using a Maxwell stress tensor approach, the fields are then used to evaluate both the radial and tangential component of force density created in the airgap of the machine. From this perspective several interesting observations are made. First, it is shown that the -axis current has zero influence on the average tangential force (torque), as predicted using traditional analysis, but it has a significant influence on the average radial component of force. Second, it is shown that the -axis current contributes to both the average radial and average tangential components of force. Interestingly, it is also shown that under standard operating conditions, the average radial force far exceeds that of the average tangential component of force. Therefore, one can conclude that the magnetic fields established create a significant component of force in a direction that cannot produce torque.