Minimum torque ripple, maximum efficiency excitation of brushless permanent magnet motors

Permanent magnet motors are usually driven in one of two ways. Sinusoidal currents are applied when the motor has a sinusoidal back EMF, and rectangular currents are applied when the back EMF has a trapezoidal shape. If implemented perfectly, each of these drive schemes is capable of producing ripple-free torque, which is desirable in many applications. However, in reality, permanent magnet motors never exhibit perfectly sinusoidal or trapezoidal back EMFs. Moreover, the power electronics used to drive the motor often has limitations that keep it from producing the required current waveform, especially as speed or load torque increases. In addition to these limitations, a permanent magnet motor often exhibits parasitic cogging torque that directly contributes to torque ripple. This work explores the relationships between motor current and back EMF, and identifies minimum torque ripple, maximum efficiency current excitations that can be implemented with finite bandwidth power electronics (current controlled VSI).<>