# old VFD DC bus testing paper



## paulengr (Oct 8, 2017)

CAUSA said:


> I ran in to a VFD problem a few days ago and lost my voltage reducing probes for testing the Bus. I forgot what the reduction ratio that I had purchased.
> So out of curiosity sake.
> 
> I dug the old article out of the old portable drive. I though I would share.
> ...


Just measure both with a good true RMS multimeter.

You CAN do it with an oscilloscope but that’s more of a shop tool. All we need in the field is to know DC bus voltage and AC ripple.

There is an exception. Bearing currents are peak DC currents measured with a Rogowski coil. They are random pulses so you need a very fast meter, 100 MHz DSO or faster. Similarly with todays DSOs if you need to look at the inout power a DSO that can do software FFT is generally not as good as a real spectrum analyzer but it gets the job done.


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