Measuring output of a VFD 4

[underling99]

Greetings, I'm trying to measure the output voltage of a vfd (yaksawa pc606) thats programmed for a max voltage of 200 v.
Using 2 different t-rms dmm's i am measuring appx. 250v.
Are the dmm's leading me "astray" because of the high switching freq??
Thanks in advance

 

[jraef]

Yes, most likely.
Unless your DMM specifically says it reads true RMS voltage, it does not. If so, you are reading peak voltages of the pulses. Either get a DMM with true RMS sensing, or believe it or not, a cheap little analog VOM works well for that. I carry one around specifically for that purpose. It's not good for much else, but the old iron vane meter has no choice but to show you RMS voltage on a PWM output. Quando Omni Flunkus Moritati

 

[underling99]

Jaref, Thanks for the input. The DMM's I used are : Fluke 87
and an Amprobe (can't remember the model). Both are true-rms
DMM's. I'll try using my simpson 860.

 

[electricuwe]

underling99,

your meters are showing the true rms value correctly, which is always approximately the DC-link voltage.

The reason is that they take into consideration all frequencies involved up to several kHz.

If you want to measure the RMS-Value of the fundamental you can use a low pass filter consisting of a suitable RC-combination to remove the switching frequency.

 

[underling99]

Greetings again, Thanks to all for the help.
Using a Simpson and a 1940's vintage Weston I measured 200-202 vac.
I understand now,Question answered.
Till next time,
Underling99

 

[d23]

underling99:

Several million years ago when transistor PWM drives first started coming out most manufactures recommended that you do not use a true RMS DMM to check output voltage. The originally recommended was that you use an analog meter (Simpson, Triplet, etc.) to check output voltage. In house we built a capacitor filter that gives us a sinusoidal wave shape for DMM's. You may want to contact yaksawa or a company that packages yaksawa drivers and get their recommendation.

Good Luck

 

[jbartos]

Suggestion: It may also be a good idea to check the output voltage waveform, e.g. by an oscilloscope. This may reveal any waveform irregularity. Depending on the oscilloscope, the 200VAC could also be measured.

 

[buzzp]

Your 87 will measure it fine. You may want to look at the 'crest factor' for the given accuracy. Then ask the drive mfg or look in their specification for crest factors. It will change some with the installation (load, wire, etc) but likely not significant. The same will apply with your clamp-on meter.

 

[jOmega]

underling99

http://www.ece.uwaterloo.ca/~ece100/lab/il1notes.pdf

See section 1.3 ... on page 5 ...for discussion on Crest Factor influence on digital meters.