Modified Variable Time Delay Control for Dual Three-Phase Permanent-Magnet Synchronous Machines With Constraint on Random Number

In order to suppress the switching frequency harmonics more effectively, this paper presents modified variable time delay pulse-width modulation control method for dual three-phase permanent-magnet synchronous machines with constraint on random number. Variable time delay control is an effective approach to spread the switching frequency harmonics with the fixed sampling periods. However, overly short randomized switching periods result in the clamping to the predefined lower switching period which accounts for the increase of average switching frequency and poor effect on switching frequency harmonics dispersion. The cause of the clamping to the predefined lower switching period in the conventional variable time delay method is theoretically detailed. Based on the analysis, the constraint on random number is proposed. The proposed method can avoid overly short switching periods. Therefore, the average switching frequency can be reduced and the switching frequency harmonics can be spread more effectively. A series of experimental results are given to verify the effectiveness of the proposed method.