VFD Efficiency Measurement Problems
VFD Efficiency Measurement Problems
(OP)
Our client's VFD specification for some 125 HP, 600V cooling tower fan drives requires a factory load test to demonstrate the VFD full load efficiency is 90% or better. (Motor not included.) The vendor's fabricator has set up a test system with a 125HP AC motor driving a DC motor as a variable load. True RMS multi-function meters on the input and output of the VFD drive measure power in and out.
But the KW in and out readings don't make sense. (KW greater than KVA, KW in less than KW out.) I am guessing harmonics are the problem. I have had them check and recheck the CT and voltage connections.
Now, the vendor is saying it is impossible to measure KW in and out on a VFD in this fashion.
Is there a recommended method of measuring full load efficiency of a variable frequency drive? Will true RMS meters work on the high harmonic content waveforms?
But the KW in and out readings don't make sense. (KW greater than KVA, KW in less than KW out.) I am guessing harmonics are the problem. I have had them check and recheck the CT and voltage connections.
Now, the vendor is saying it is impossible to measure KW in and out on a VFD in this fashion.
Is there a recommended method of measuring full load efficiency of a variable frequency drive? Will true RMS meters work on the high harmonic content waveforms?






RE: VFD Efficiency Measurement Problems
RE: VFD Efficiency Measurement Problems
The Yokogawa meters are used for real high accuracy.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD Efficiency Measurement Problems
Personally I think this endeavor is whack. You will not get any meaningful results EVER if you don not spend a small fortune on special equipment and someone who understands it well.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: VFD Efficiency Measurement Problems
RE: VFD Efficiency Measurement Problems
BTW, the test set up was designed primarily for extended burn-in load tests and thermal cycling. The efficiency test was supposed to be a simple meter reading during the burn-in.
RE: VFD Efficiency Measurement Problems
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD Efficiency Measurement Problems
RE: VFD Efficiency Measurement Problems
I believe we will settle for manufacturer's watts loss data and efficiency curves. My vendor's fabricator has spent too much money trying to prove the efficiency using expensive meters.
RE: VFD Efficiency Measurement Problems
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: VFD Efficiency Measurement Problems
It all depends on what you already have, I think.
I have done fairly accurate comparative efficiency measurements for fan installations by first running a DOL fan and checking air pressure delta and motor speed. I have then put the inverter in the system and adjusted it to same pressure delta and speed. The increase in input power, which is easy to measure with moderately expensive watt meters, represents losses in inverter (plus PWM losses in motor).
The same technique can be used for other load types.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD Efficiency Measurement Problems
What value do you expect from loss calculation and thermal design ? If this result is in that range as indicated above it may be sufficient to do some temperature measurements on main components and compare it to design data to prove that you have achieved at least 90%.
RE: VFD Efficiency Measurement Problems
My test with the meters was going to prove the complete assembly’s efficiency, two units at a time.
As suggested, we will use manufacturer's data to prove >90%. If that is insufficient for our client, I can run the fans in bypass mode and measure power input at the 600V substation, then run the same fans on the VFD's at 60Hz. The difference would be the VFD's losses plus any losses due to harmonics.
Thanks again for the help.