Random pulse width modulation techniques are known to reduce acoustical noise and vibrations in inverter fed a.c. drive systems. The randomization can easily be done by varying widths of individual switching intervals. A random modulation strategy can be optimized with respect to a given objective. Four such strategies are described and compared. Harmonic torques in an induction motor, switching losses in the inverter, and the harmonic power in the spectrum of the inverter output voltage are targeted for optimization. Results of experimental investigation are presented for four random strategies and compared with fixed switching frequency operation. The test results show that the random strategies flatten out the power spectra and acoustical spectra. Vibrational tests show a reduction in vibration using random modulated strategies.
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