Discontinuous modulation of multiphase inverter-fed AC motors

Different types of discontinuous PWMs of multiphase inverter-fed ac motors are investigated from point of view of ac motor harmonic losses. The derived expressions for computation of the generalized loss-factors and its components are given for different phase number and for all the investigated PWM methods. It is shown that the discontinuous PWM of multiphase system cannot be as successful as that for the case of three-phase system. Therefore in comparison with continuous PWMs the discontinuous ones have no advantages in case of phase number more than 9. However, the stator harmonic current losses are slightly higher while the rotor harmonic current losses are considerably lower than those in case of three-phase system. At the same time for the phase number over 7 a sensible broadening of the linear voltage control region is impossible for any PWM method. The theoretical derived expressions for stator and rotor losses (derived for infinite value of switching frequency, MATCAD program) are checked by results of direct computation for a finite value of switching frequency (MATLAB program) as well by simulations.