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Juan Pérez-Lanzac
09-24-2013 03:38 AM

"European Efficiency" for variable speed drives

Global efficiency equation similar to solar inverters "European Efficiency" for variable speed drives? I would like to know if there is any standard defining a "global" efficiency equation such a the so called "European Efficiency" one used in solar industry for inverters but for VSD.
09-24-2013 05:41 AM
Top #2
Mario Maggi
09-24-2013 05:41 AM
I never heard about the existence of some efficiency standards for VSD. Please consider that PV inverters works usually in conditions more stable than VSD, and therefore it is more easy to fix some rules. In a VSD you can have frequency continuously variable, and in these conditions is very difficult to state how to measure efficiency. Please consider that the efficiency alone of a VF inverter should be high, but motor/generator and main interface shoud be lower, in a certain configuration of parameters. In another setting, inverter efficiency should decrease but other related efficiency can increase.
09-24-2013 08:16 AM
Top #3
Lee Teschler
09-24-2013 08:16 AM
I don't know about standards, but this article might shed some light on the subject of VSD efficiency:
09-24-2013 10:37 AM
Top #4
Andrew Cooper
09-24-2013 10:37 AM
I am almost certain that there is no internationally accepted standard that defines this, unfortunately.
09-24-2013 01:21 PM
Top #5
Martin Brown
09-24-2013 01:21 PM
One confusing factor is that inverters are classed by motor power. That is, a 7.5kW inverter is designed to work with a 7.5kW motor (10hp for you guys across the pond). Now this is the mechanical power output from the motor, which has electrical and mechanical losses, so there is maybe 8.0kW or more of electrical power going in at a power factor of 0.8 say. So the inverter is feeding maybe 10kVA. If is 95% efficient, does this mean its losses are 500W, or 375W?
Another problem is that measuring efficiency can be difficult, as it is the difference of two large numbers, one of which (the input) is a non sinusoidal current, and the output is worse, a PWM voltage waveform and current with ripple on it. You need expensive 3ph kit to make this measurement properly.
All that said, if the technology is much the same (IGBTs, PWM) efficiencies don't vary too much as manufacturers all want the highest efficiencies in their drives if only to keep the heatsinks small and allow high ambient operation.
09-24-2013 03:49 PM
Top #6
Juan Pérez-Lanzac
09-24-2013 03:49 PM
Thanks very much to all of you!
Mario: I agree on your coments. Efficiency in VSD is by itself difficult to measure and even to be defined!
Lee: thank you for the article. It is indeed a very good one on the subject.
Andrew: I also have the same feeling, there are no accepted standards yet which define this.
Martin: Maybe the Inverter efficiency could be defined based on a set of standard tests for the Inverter operating alone in a kind of "close loop"? Yes, 3ph equipment to measure PWM voltage an current is very expensive!!

I believe that in a very standarized application market it shouldn't be very difficult (though challenging) to reach an agreement on this. In the piece of market I deal with that is, DFIG converters for wind power applications, I think we are still far from this.
09-24-2013 06:37 PM
Top #7
Michael Björkman
09-24-2013 06:37 PM
Gents, at the moment there is work going on within CENELEC in order to be able to define the conditions under which either the motor efficiency or the drive efficiency can be determined. On one hand you need to define a 'standard motor' that is used in drive efficiency determination - on the other hand a standard drive must be used to determine motor efficiency when fed by a drive. There are a great number of parameters than need to be defined, in addition to the needs of the working machine - which complicates things again.. A first draft is expected next year.
09-24-2013 09:25 PM
Top #8
Claude Lambert
09-24-2013 09:25 PM
Insights into this issue is available from International Energy Agency's IEA 4E^ EMSA^^ website including especially Motor Policy Guide link at very recently published, which offers an assessment of existing policies for various countries and regions and which points to following recent and on-going standardization efforts in terms of:
* minimum energy performance standards for motors covering many countries covering EU's 27 countries and several other countries including Canada & USA

* international efficiency classifications for motors IEC 60034-30

* motor test standard IEC 60034-2-1

* motor and VFD standard IEC 60034-2-3

* motor application guide IEC 60034-31

* and so on.

Refer also to EMSA website's free efficiency tool, Motor Systems Tool, download at link

^ 4E ~ Efficient Electrical End-User Equipment
^^ EMSA ~ Electric Motor System Annex
09-24-2013 11:44 PM
Top #9
George Shurina
09-24-2013 11:44 PM
VFD efficiency is measured by using meeting such as a Multilin PQM on the input and output of the VFD. you use the KW hours record so the longer you let the meters record the more accurate your measurements. then simply divide power out by power in.

calculating VFD losses you need to consider the motor efficiency since the VFD rating is actually the power it can produce on the motor shaft.

Th determine the actual VFD efficiency you need to know the heat generated by the switching and conducting losses. This is a complex thermal model. you should be able to get the efficiency curves from the VFD manufacturer.
09-25-2013 02:38 AM
Top #10
Mario Maggi
09-25-2013 02:38 AM
George, in PV inverters there is a good reason to have the maximum inverter efficiency. Instead, in the combination VFD (inverter+motor) it is not interesting to know exactly the inverter efficiency. In certain conditions it is possible that inverter efficiency is lower but motor efficiency is better. The same VFD in another working point could have a better inverter efficiency but a bad motor efficiency. Therefore, selecting a VFD, I will look to total efficiency and never to the inverter efficiency alone.
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