PWM methods of a multiple inverter for an adjustable frequency drive have been investigated. In a multiple inverter, of which configuration inevitably includes power transformers connecting output terminals of unit inverters, the voltage-time product should be reduced especially in the lower frequency region, otherwise magnetic saturation of the transformer cores can occur. Suppression of harmonic components is another important consideration to prevent torque ripple and electromagnetic interference. It is, however, very difficult for a single PWM method to cover the whole operational frequency region, keeping both the voltage-time product and the harmonics reduced. Here, a series of PWM methods are proposed; low, medium and high speed region PWMs. They are changed depending on the output frequency of the inverter, and reduces the voltage-time product and harmonics in each operational frequency region. Simulations and experimental results using small capacity test circuit show the effectiveness of the methods.
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