Generation Method using a Software Technique for PWM Signals with Superaudible Frequencies

Abstract

A software technique generation method using the space vector theory for PWM signals with superaudible frequencies was studied. This method is characterized by being able to obtain PWM signals of superaudible frequency without shortening the sampling time of the PWM control. This is achieved by dividing the duration times of voltage vectors which are obtained when the magnetic flux deviation vector of the primary magnetic flux references, generated between the n and (n+l) sampling times, is decomposed in the direction of the two voltage vectors which are closest to the direction of the magnetic flux deviation vector. In this PWM control, control is also excuted which suppresses the distortion of the primary currents due to the dead time of the PWM signals and the pulse dropping phenomena. The current distortion in the former is suppressed by correcting a rising or falling timing of the PWM signal on the basis of the primary current polarity got using the phases of their fundamental components. The current distortion in the latter is suppressed by controlling the limiting value of the amplitude of the magnetic flux deviation vector so as to ensure a minimum duration time of the zero vector, even if the phase of the primary magnetic flux is varied.
With the PWM control, a switching frequency of 16kHz was obtained with a sampling time of 120μs. The switching frequency could be kept constant at 16kHz, even if the inverter frequency was varied. Moreover, the current distortion factor was controlled to less than 6%, and the magnetic noise of a motor was also below the noise level of a motor driven by a commercial power source.