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VFD partial load efficiencies
4

VFD partial load efficiencies

VFD partial load efficiencies

(OP)
I have this table for VFD efficiencies I extracted from an NREL publication and I'd like to calcualte line power on the VFD.

            VFD Efficiency at
VFD   |               % Full Load    
HP    |    25    33    50    66    75    100
------|----------------------------------------------
1     |    0.12    0.28    0.52    0.68    0.75    0.92
2.5   |    0.21    0.36    0.58    0.73    0.79    0.93
5     |    0.27    0.42    0.63    0.76    0.82    0.94
10    |    0.33    0.48    0.67    0.80    0.85    0.96
25    |    0.42    0.55    0.74    0.84    0.89    0.97
50    |    0.48    0.61    0.78    0.88    0.92    0.98
100   |    0.55    0.67    0.83    0.91    0.95    0.99
250   |    0.63    0.75    0.89    0.96    0.98    1.01

Is the % Load of the VFD calculated as,
VFD%Load = Motor Load / Motor Nameplate Rated Power?

Like this,

Motor Rated Power = 100 HP
Load = 50 HP
Motor Reduced Load = 50%
Motor Reduced Load Eff = 0.89
Motor Brake Power = 50/0.89 = 56.2 HP (41.9 kW)

VFD Rated Power = 75 kW
VFD Load% = 41.9/75 = 55.8 %
100 HP VFD eff = (50%,0.83)    (66%, 0.91)
giving a VFD Eff = 83 + (55.8-50)*(0.91-0.83)/(66-55)*100
VFDreduced eff = 87.2%

So,

Line Power = 41.9 kW / 0.872 = 48 kW

Right?

Thanks


 

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying."  Tony Hayward CEO, BP

**********************
"Being GREEN isn't easy" ..Kermit
http://www.youtube.com/watch?v=hpiIWMWWVco

http://vir

RE: VFD partial load efficiencies

That chart makes no sense to me, you would have to ask the person who developed it what they mean. Given the .92 value at 100% speed, I would venture to guess that they are saying the 1HP VFD is only 92% efficient at 100% speed. That would make it a very old chart.

But then they imply that same VFD drops down to being only 12% efficient at 25% speed? What kind of VFD did they use to come up with that kind of horrific value?

That leads me to believe that their use of the term "VFD Efficiency" is not what I think of it as being.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: VFD partial load efficiencies

(OP)
That what it means.  It's probably is an old chart.  Its the only one I've ever been able to find.  I've been looking for an updated one.  I sure would like a new one. (Hint Hint)

Uh, and the question?

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying."  Tony Hayward CEO, BP

**********************
"Being GREEN isn't easy" ..Kermit
http://www.youtube.com/watch?v=hpiIWMWWVco

http://vir

RE: VFD partial load efficiencies

(OP)
You inspired me to do another search.
This one does have better values.
http://www1.cetim.fr/eemods09/pages/programme/038-Brunner-final.pdf

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying."  Tony Hayward CEO, BP

**********************
"Being GREEN isn't easy" ..Kermit
http://www.youtube.com/watch?v=hpiIWMWWVco

http://vir

RE: VFD partial load efficiencies

101% efficiency is pretty good.  I agree with Jeff - the chart doesn't appear, on the surface, to make much sense.  

But yes, if you know the VFD efficiency, the input kW would be computed as you suggest.  

 

David Castor
www.cvoes.com

RE: VFD partial load efficiencies

(OP)
Thanks.  I'll go with it.

Yes I thought the 101 was a bit strange.
The new chart looks more reasonable in all aspects.
Thanks to both for the help.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying."  Tony Hayward CEO, BP

**********************
"Being GREEN isn't easy" ..Kermit
http://www.youtube.com/watch?v=hpiIWMWWVco

http://vir

RE: VFD partial load efficiencies

I want qty 1000 of those 101% efficiency drives. We have solved the world's energy crisis!

I like the information in your 2nd link a lot better, that's more of what I would expect to see. It looks to be a well thought out study. Thanks for that by the way, it may come in useful to me too.

Just out of nosy curiosity, what is the purpose behind you gathering this info? Are you evaluating the efficacy of an energy saving VFD installation?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: VFD partial load efficiencies

(OP)
Right now I'm doing a study of VFD-pump installations in a generic sense that will classify and rank systems that might save energy costs by installation of VS controls.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying."  Tony Hayward CEO, BP

**********************
"Being GREEN isn't easy" ..Kermit
http://www.youtube.com/watch?v=hpiIWMWWVco

http://vir

RE: VFD partial load efficiencies

Biginch (and Jeff)
There is a new standard trying to get out into the big wide world that covers, amongst other things, VFD efficiency. It is owned by the organisation AHRI (Air-Conditioning, Heating, and Refrigeration Institute) and will be known as "Standard 1210, Performance Rating of Variable Frequency Drives". The intention is that this standard will be used by consultants and institutes who are looking to advise their clients and contractors alike regarding performance of the VFD's used on Green Buildings to ensure that complying with higher level of performance in the building is in line with the products being supplied.
It is taking forever to be released but when it is, I feel it will be suitable for many other industries looking at the products and their performance. Hopefully it will provide clarity and demystify certain thoughts such as "all VFD's are the same...".
It will also include factors such as recommended limits of harmonic (current) distortion levels from the product, stresses on the motor insulation as a result of PWM waveform etc.
At the moment it is still in draft form and not released. When it's released I will try and remember to advise the forum

RE: VFD partial load efficiencies

I think it would be interesting to see any generic results showing the difference between a valve and a VFD.

Looking at the paper for a VFD, it appears that there is about 3-4% extra losses at full speed and load and the losses increase to about 6% to 7% as the speed reduces to 75% speed. The relative efficiencies track with load fairly well so the load doesn't really matter. So, a system with a valve with less than about 4% to 5% losses would compete very well with a VFD.

RE: VFD partial load efficiencies

(OP)
Yes, except I think you mean beat a VFD.  And efficiencies only track with load when there is no static head, which is shall we say, "unusual" in petroleum and water pipelines.  And it's also not all that often that you really want to run these facilities to deliver less product than what you can at 100% speed anyway. Product delivered = Revenue! Nobody wants 75%a revenue, so we don't want to run there often.  Multi-product lines usually maximize revenue when you just turn them on and don't use any pressure or flow control of any kind.  But I thought I'd classify them in regards to percentage of static head in relation to BEP head, to see where the inflection point really is.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying."  Tony Hayward CEO, BP

**********************
"Being GREEN isn't easy" ..Kermit
http://www.youtube.com/watch?v=hpiIWMWWVco

http://vir

RE: VFD partial load efficiencies

I'm talking efficiency of the VFD and motor combo which basically follows the same curve as the motor by itself.

The VFD adds about 3% extra losses (motor + VFD) best case and it increases from there. In the common range where a pump or fan would operate the extra losses appear to increase to around 6% or 7%. This is assuming the typical range would be around 70% speed to 100% speed.

One other thing to consider is that paper did not include any type of harmonic mitigation or vfd to motor filtering. Typically, you'll see at least one line reactor in the package. As a general rule of thumb you could assume 2 reactors (input and output) will double the VFD package losses. More complex filters will add even more losses.

Sure, you don't want to lower the speed of a motor if the process allows for more revenue producing output at full speed but you may have to lower the speed of the motor if the process itself requires less output.

RE: VFD partial load efficiencies

(OP)
Thanks Lionel. Yes I got that.  There are some cases where its useful to flow at lower speeds, they're just a bit more unusual for my applications.  It would most likely be an unforecast product demand reduction that would force a slowdown in delivery until a distribution tank empties, providing a pipeline shutdown wasn't convenient.  They're usually not.

That filter effect on power is quite interesting.  I wasn't aware of that effect.  By doubling the "VFD package" losses, do you mean multiply the VFD partial load efficiency values in the table by 2?  And how would addtional filters affect that factor?

BTW, I'm trying to develop an equation to duplicate the VFD partial load efficiencies in that table.  Its hard to come up with something that is within 1 or 2 percent of all those values, even if you ignore the lowest power ratios and the 1 HP motor.  If anyone comes up with one, its good for a virtual beer.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying."  Tony Hayward CEO, BP

**********************
"Being GREEN isn't easy" ..Kermit
http://www.youtube.com/watch?v=hpiIWMWWVco

http://vir

RE: VFD partial load efficiencies

I'd think that line reactor losses would be about proportional to current. I'd also think the extra loss to account for the reactors could just be added when the motor torque is above about 50%.
 

RE: VFD partial load efficiencies

(OP)
That's going to be always for my apps ... almost.  

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying."  Tony Hayward CEO, BP

**********************
"Being GREEN isn't easy" ..Kermit
http://www.youtube.com/watch?v=hpiIWMWWVco

http://vir

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