# Reflected waves in PWM IGBT drives



## PStechPaul (May 1, 2012)

I just happened upon an informative article about high voltage reflectred waves resulting from PWM drives and the interaction of the high frequency waveforms with the cables and the motor insulation and inductance:
http://www.ab.com/support/abdrives/documentation/techpapers/ieee/pcic.pdf

This paper is somewhat dated (1996) but it addresses some important issues that might affect DIY controller designs. It may explain why there are special cables designed for inverter controls, and motors which are "inverter rated". 

Awhile ago I did some simulations where I added passive LC filters to the output of a PWM drive to smooth the waveform. Perhaps that may help:


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## Tesseract (Sep 27, 2008)

This is only a problem when the cables between a drive and a motor exceed 10's of meters in length; hardly a problem for EVs.

More specifically, this effect occurs when the cable length approaches a significant fraction (typically 1/6th) of the voltage rise time wavelength. E.g. - if the voltage rise time is 0.2us then the wavelength in meters is (300 / (1 / 0.2)) = 60 meters. Cables exceeding 10m in length will start to exhibit transmission line effects. LC networks can be added to the output(s) of the drive to slow down the rate of rise of the voltage waveform, thereby extending the distance the cable will still behave as a lumped circuit, rather than as a transmission line.


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## kennybobby (Aug 10, 2012)

And sometimes those LC filters are referred to as "Reactors", and can be quite heavy in the three-phase variety.


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