VOLTAGE MEASUREMENTS ON VFD OUTPUT

[GusD]

GusD (Electrical) Mar 22, 2003
Voltage measurements on a VFDrive with IGBTs in the inverter.
We have just installed a new VFD (IGBT) to replace an older VFD (GTOs)that gave us a lot of problems over the years.After commissioning the unit we experienced some problems with blown power fuses as well as converter power Diodes .
When we measure the output voltage to motor we see 660 volts instead of 575/600 voltsAC.These tets meters are all digital and quite reliable.The 3Ph/575V input voltage to the system increases to 615v at the output of a RC Harmonic filter and to the input of the converters.
I don't believe that we applying 660 volts RMS to the this motor.My question is why don't these meters read a True RMS even if this voltage is a pulse generated voltage?
I also have measured spikes when on PK/Hold mode of approx 1450 volts.This is a fairly normal spike for an Inverter operation.I would like to know how close to RMS value is this spike?
Thank you for the help
GusD

GusD

 

[jbartos]

Suggestion: Suitably designed filters usually reduce peaks. True RMS meters have been dealt with in this Forum, e.g. old Simpson, etc.

 

[jbartos]

Suggestion: Visit

http://www.abmtmi.com.tw/specification/2010spec.pdf

http://www.nostek.com/kproduct_clamp_info.htm

for more info.
Incidentally, this side explains the 1.1 multiple between the RMS and average values. I have posted this item several times in this Forum. It is hard to understand since the Standard Handbook for Electrical Engineers and IEEE Stds removed or downplayed the Form factor that the 1.1 multiplier is.

 

[dydt]

hi...

try to get the old reliable Simpson meter...or any d'Arsonval movement meters...it pays to have then in the shop esp. in this situation..

dydt

 

[GusD]

Thank you for your replies.
Your Form Factor (1.1) fits perfectly on this problem.
As for the input Reactor/Cap Harmonic filter ,it does show an increase in the measured voltage (same RMS meter) Input to filter a typical sinuzoidal sinewave voltage of 575 volts .The output of the RC filter, approx 615 volts (same RMS meter used).On the Inverter side,we are measuring aapprox 1.1 of the true RMS value, which should be about right.

Thank you JBartos GusD

 

[electricuwe]

Sorry, jbartos,

your explanation is to simple. What happens if the speed of the motor is reduced. Does the form factor theory still hold true under these conditions?

See the following thread:
thread237-49039

 

[jOmega]

The problem really is one of "crest factor" ... which most digital meters get hung up on.

That is why the old analog type meters work so well.

Visit the FLUKE site and do some digging amongst the app. notes... this has been around since digital meters first hit the marketplace and were applied to pulse type waveforms...

 

[jOmega]

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

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