Influence of three-phase symmetry on pulsating torque in induction motor drives

In sine-triangle (ST) pulse width modulation (PWM), the carrier frequency is chosen to be an odd-triplen multiple (e.g., 3, 9, 15... etc.) of the modulating signal frequency in order to maintain the three-phase symmetry among the three pole voltages. However, for ST PWM with non-triplen pulse numbers (e.g., 5, 7, 11... etc.), the three-phase symmetry is not maintained. This paper studies the influence of three-phase symmetry on the line current and torque harmonics in a motor drive, considering pulse numbers of 5, 7 and 9. Absence of three-phase symmetry is shown to result in balanced third harmonic/triplen frequency components in the three-phase line currents. These triplen frequency components interact with the fundamental flux to produce even torque harmonics of order other than multiples of 6. It is predicted that a pulse number of 5 would lead to significant torque harmonics of order 2 and 8, and that a pulse number of 7 would lead to 4
^th
and 10
^th
harmonics torques. Those theoretical predictions are confirmed by simulations and experimental results on a 3.7 kW induction motor drives.