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efficiency of VFD and asynchronous motor as function of frequency ??
2

efficiency of VFD and asynchronous motor as function of frequency ??

efficiency of VFD and asynchronous motor as function of frequency ??

(OP)
Hello,
I try to get a grip on the advantages of applying a VFD to save energy while driving a pump. Suppliers tend to give the impression, that the efficiency of a VFD and an asynchronous electrical motor remains constant whatever frequency is generated by the VFD and put into the motor. I am not an electrician, but I assume that using a motor with another frequency than the usual 50 or 60Hz is loading the motor in a more inefficient way. In general the nominal power of the motor should be chosen 15% higher, when working in combination with a VFD. Are there any typical characteristics that indicate the efficiency of the VFD and/or the elektrical motor as a function of the relative speed
(compared to the nominal value?). With these graphs I might be able to calculate realistic energy savings instead of the very optimistic tools the VFD suppliers put at our disposal.
I would therefore be grateful if somebody could help me out here!
 

Karel Postulart, The Netherlands
Nuon Power Generation

RE: efficiency of VFD and asynchronous motor as function of frequency ??

There was a difference with older VFDs. Modern VFDs can be programmed to run the motor as optimal as possible. Especially at part loads. I would say that a modern motor and a modern VFD always is more efficient than any other means of adjusting flow or pressure.

But a well designed pump running at the right speed for a constant flow will always be better than a badly chosen pump and motor that have been 'corrected' with a VFD.

The motor runs at its optimum flux at all speeds if the VFD is correctly programmed. Optimum flux usually means optimum efficiency.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: efficiency of VFD and asynchronous motor as function of frequency ??

This has been discussed here many times. See this link;

http://www.eng-tips.com/viewthread.cfm?qid=260203

What I have found is that anytime you vary the pump speed, more energy is used per gallon produced, not less.  You are right to suspect the "energy savings" tools supplied by manufacturers.  It is more a marketing tool and not realistic data.

RE: efficiency of VFD and asynchronous motor as function of frequency ??

Not being an ME, all I can add to this discussion is that the argument that VFDs don't save energy is flying in the face of in excess of 20+ years of empirical data showing the opposite. I totally agree with Gunnar's assessment that an application with a constant flow rate that has been properly designed is not a good candidate for a VFD. I know for a fact that the VFD concept is sometimes over sold by zealots, but a small percentage of misapplications does not warrant a condemnation of the entire concept. VFDs match the motor HP to the load requirements, eliminating wasted energy elsewhere in the system. The small amount of energy wasted in the VFD is often over blown to make an argument against it, sometimes based on very old data. This accounts for the "sweet spot" of energy savings, that 50-85% flow mentioned in the earlier thread, but the fact is, there are a LOT of systems that can benefit from operating in that flow range.

Again to Gunnar's point, if the application is not one that can benefit from variable flow operation, then by all means don't apply a VFD to try to fix other inefficiencies. That is the first step in evaluating the idea.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
  
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RE: efficiency of VFD and asynchronous motor as function of frequency ??

20 years of empirical data from VFD manufacturers flies in my face.  Just look at your pump curve.  Anytime you vary the flow with a VFD while maintaining the head required, you are using more energy per gallon, not less.  And I am afraid it is more than a small percentage of misapplications, because of the same confusion as here.  I fell for it myself once but, never again.

RE: efficiency of VFD and asynchronous motor as function of frequency ??

jonr12;  It would be more helpful if you didn't state blanket facts that are incorrect when applied across a very complex subject.  You may have been screwed in a past event - that was a gross misapplication - but that doesn't mean VFD controlled pumps are all a waste.

I suggest you go back and read BigInch's excellent dissertation on the subject.  It's about the 6th post down in this thread: thread407-260203: VSD energy savings

Keith Cress
kcress - http://www.flaminsystems.com

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