Interference reduction by harmonic phase shift in multiple PWM rectifier operation

摘要

A method and apparatus for reduction of radio frequency interference caused by pulse width modulation of rectifier switching devices (56A, 56B, 58A, 58B) in multiple rectifiers 50 connected to a common power source. In general, the invention reduces radio frequency signals introduced into the power source by phase shifting operation of the rectifiers such that one of the rectifiers has switching devices which are gated into conduction during a first portion of a pulse width modulation cycle and switching devices in another rectifier are gated into conduction during another portion of the pulse width modulation cycle. In this manner, switching of devices in one of the rectifiers produces harmonic currents during one portion of a PWM cycle while the switching devices in the second rectifier produce harmonic currents during another portion of the cycle. The phase shifting of the harmonic currents prevents the harmonics from adding together into larger amplitudes and thereby reduces the amplitude of the harmonics. The system is particularly useful in transit car operation in which multiple cars may be train lined together and respond to the same train commands to apply power to electric motors on each of the cars. In this instance, the switching devices in each of the rectifiers in each of the cars may be phase shifted to prevent summation of harmonic currents; or the rectifiers within each car, if multiple rectifiers are used in each car, may be phase shifted.