Torque pulsations of multiphase inverter-fed ac motors

Torque pulsations of different types of PWMs with multiphase inverter-fed ac motors are investigated. For different phase numbers expressions for the torque ripples and computation of their components are derived for all the investigated PWM methods. It is shown that current harmonics can be decomposed - depending on the phase number - to several groups, and only current harmonics belonging to the first of these groups can produce fundamental MMF space harmonics and adequate torque pulsations. Therefore different PWM methods with multiphase systems produce considerably lower torque pulsations than with three-phase systems, especially in case of high modulation index value. Stator current harmonics belonging to groups other that the first are only able to produce MMF space harmonics of higher orders. The theoretical results were verified by simulations.