How to measure/estimate motor power output ?

[Discreet544]

Hi Folks;

I am measuring the output power of an electric motor (10 HP, 0.85 P.F., 1765 RPM, 60 Hz, 91.7 Nom FL. Efficieny). Using a watt transducer, I can measure the input power, and while operating in full load efficiency, I can get the full load output power at rated RPM. However, I also need to know the output power at lower RPMs, as I am using a VFD. I know that the efficiency drops down at lower RPMs, so I can't assume the efficiency remains constant at 91.7%. I wonder if there is any developed formula to estimate efficiency loss, or if you know any other theoretical or experimental approaches to measure the power output of the motor at lower RPMs.

Thanks;

 

[ScottyUK]

Speed is obviously easy to measure, either directly or using the VFD's in-built model: the model used in a vector drive is usually very accurate if the drive is confugured correctly. It is possible to fit shaft-mounted torque transducers which give you the other paramteter need to calculate output power. Your motor is probably large enough to allow such a transducer to be fitted, although at the small end of the range I think.

 

[ScottyUK]

Ok, they are smaller than I thought!

http://www.datum-electronics.co.uk/torque-transducers-torque-sensors-and.aspx

 

[Discreet544]

Thanks ScottyUK;

I know about (Torque * RPM = PowerOut). However it s not financially possible to go for it right now. That s why I wonder if there is any approach to estimate the efficiency variation as RPM changes.

 

[mikekilroy]

what is the motor running? Perhaps this info could help decide how to determine power output....

and how did you determine the exact full load output power into this load to start with?

 

[Discreet544]

Thanks mikekilroy;

I assume you mean the type of liquid, then it is pure water now. But the main objective is measuring power consumption while pumping pulp slurry.
You are right ! I think I got mixed Full RPM with Full Load ! Then I correct my self: "I can measure the input power at full RPM." Then here arises another question too: is the efficiency at full RPM equal by the efficiency at full load ?

Sorry, this is not my field of expertise, so I do appreciate any help.

 

[waross]

Read power consumption with a Wattmeter on the VFD input. Compare motor efficiencies with pump efficiencies and you may decide not to worry too much about the motor efficiency.
This may be a good time to review the Cowern papers.

http://www.baldor.com/pdf/manuals/PR2525.pdf

Edward Cowern, P.E. has the ability to present good information in an easy to understand manner. An hour or so with this information will give you a lot of confidence in the matter of electric motors. Well worth the time.
A comparison of standard and high efficiency motors starts on page 65 (PDF page 72). You are probably safe using the nameplate efficiency from 50% load to 100% load. As the motor slows down the pump efficiency falls off so fast that it will probably swamp out the changes in motor efficiency.

Bill
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"Why not the best?"
Jimmy Carter