WEG VFD output readings

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Jonathan.McNutt@syntronmh

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I was working with a WEG VFD today when a stumbled across an unusual situation, to me at least.

The drive was enabled and set to 60Hz, at this point i went to check output voltage with my Fluke 177. My meter was reading around 525VAC phase to phase and about 380vac phase to ground.

I decided to swap to my Klien CL700 meter and was reading 450vac phase to phase and about 250vac phase to ground.

I checked both meters at a KNOWN, NONVARIABLE voltage source. The meters read the same.


In my situation i understand my drive has approximately a 460VAC 3 phase input and based on the drive specs and parameters it should have 480vac 3 phase output at 60 Hz simulated by a switching DC voltage.


So why the odd readings on both my meters but they both read perfectly normal on a standard ac voltage source? Not only did my meters read slightly to vastly different from my expectations but when i bench tested this drive to setup my parameters before installing on the machine i didnt see these types of readings.

thanks for any insight you may have.
 
If you are trying to measure the output voltage using a digital meter, the results may not be accurate. The output is pulse-width modulated square waves, not the sine wave that most meters are expecting. The best option is to use the voltage information provided by the drive. Some higher-end DMMs (Fluke 289) can (per Fluke) provide accurate evaluation of effective output voltage.
 
  1. The output voltage of a standard VFD will never be higher than the input voltage, it can't be. So if your input is 460V, your output cannot be higher than 460V.
  2. The output of a VFD is a VERY complicated waveform that relies on the inductive qualities of the motor in order to make it approximate a sine wave. Most affordable digital multimeters are not going to be good at interpreting that complex waveform. Ironically though, and old Simpson or Triplett analog meter would do a better job. I keep mine around specifically for that very reason. What you experienced with the different meters reading differently is reflective of that complexity, i.e. totally expected results.
  3. Voltage to ground is a pointless exercise. The output of the VFD is not referenced to ground, so it is going to be "floating". You will get different readings depending on the capacitance to ground at the place and time you are reading it.
  4. As Dave Castor said, it's best to just use the display information of the VFD if you want to know what's going on with the output.
 
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