# DMM with Low Pass Filter



## dronai (Apr 11, 2011)

Is it true a standard DMM will not give accurite readings on the load side of a VFD without a Low Pass Filter ?

I was told this is BS


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## Big John (May 23, 2010)

I'm not a VFD guy, but I know that I've gotten different readings with the low-pass meter than without. 

It seems like some outputs have a whole lot of high frequency junk coupled into them, and that can cause you to read higher than what your actual power frequency is.

But I'm pretty sure that the last I saw this was a VFD with no load reactors. So I wonder if you have proper reactors if it would attenuate the high-end frequencies, anyway? Maybe someone a bit more experienced can answer that.

-John


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## dronai (Apr 11, 2011)

I went to a Fluke training. I don't know if this just a sales pitch. My new manager said this is BS !

The Fluke Rep told me, and demonstrated that without the pass filter the readings would be inaccurite.


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## ilikepez (Mar 24, 2011)

yeah its true. I attached my Fluke 87 V to a Powerflex just a few weeks ago to see what would happen. Without the low pass filter it was 20-30 volts high all of the time.


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## dronai (Apr 11, 2011)

ilikepez said:


> yeah its true. I attached my Fluke 87 V to a Powerflex just a few weeks ago to see what would happen. Without the low pass filter it was 20-30 volts high all of the time.


Load side right ?


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## ilikepez (Mar 24, 2011)

load side.


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