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Energy measurements

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#1 joze


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Posted 11 August 2006 - 02:39 PM

Hello all!

My question is about measuring consumed energy with VSDs. We have a situation where there was a DOL starting of motor (high power) and using flaps for flow regulation in verntilation system. Now a frequency inverter will be installed (plus line chokes).

Of course, measurements will be taken to estimate the energy saving due to frequency regulation vs. mechanical regulation (on 100% power). Is there any special consideration regarding measurement? Is it possible that the measurements are not correct because of the inverter operation?

Thank you for your answers,


#2 jraef


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Posted 11 August 2006 - 05:44 PM

It is difficult to measure power in a VFD circuit using external means. You can do it, but only with the best and most expensive meters that filter and adjust for harmonics. For a true comparison though, be sure to measure the input power to the VFD, not the output power, because that will take into account the losses in the VFD, however small they may be.

The easiest way is if your VFD has a kW output or display, because it is internally doing those complex calculations. You can assume a 3-5% efficiency loss in most cases, with the 5% being at lower speeds and 3% at full speed (in a typical VAV fan system).
"He's not dead, he's just pinin' for the fjords!"

#3 marke


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Posted 11 August 2006 - 10:06 PM

Hello joze

A simple rotating disk KWHr meter on the input of the drive will tell you what you are paying for and because they use a magnetic multiplier, they give an accurate result for the drive input.

Best regards,

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