A pulse frequency modulated drive for a wide speed range application

This paper briefly describes pulse frequency modulation (PFM) and its properties when applied to variable voltage variable frequency inverters. Various types of PFM are considered, for half-ware symmetric and quarter-wave symmetric synchronous PFM total harmonic distortion (THD) is calculated and presented against the modulation frequency. The harmonic spectrum of PFM is shown to be spread in the f/sub o/-2f/sub o/ range, where f/sub o/ is the base frequency. The dispersion of harmonics over a wide range promises motors driven by PFM modulated inverters to be less noisy. The simulated and measured THD of single chip implementation of QW-symmetric synchronous PFM is compared with a commercial inverter and is found to generate an acceptable waveform. Finally, acoustic noise level of a motor designed for washing machine application is measured at several modulating frequencies when the motor is driven by PFM and PWM waveforms using the same hardware. It is found that when PFM pulses are accurately generated acoustic noise levels can be considerably reduced.