How to Calculate HP on motor driven through VFD
How to Calculate HP on motor driven through VFD
(OP)
In the past, I've always used the following formula to estimate the BHP being produced by a three-phase motor:
Watts = volts x amps x 1.73 x motor eff x power factor
However, in running this calculation on a particular motor that's operating at 30Hz (half speed), the answer I'm getting appears to be too high. So, my question is, does the above formula work for motors being driven through a VFD?
Watts = volts x amps x 1.73 x motor eff x power factor
However, in running this calculation on a particular motor that's operating at 30Hz (half speed), the answer I'm getting appears to be too high. So, my question is, does the above formula work for motors being driven through a VFD?





RE: How to Calculate HP on motor driven through VFD
RE: How to Calculate HP on motor driven through VFD
However, my question is,... is the formula valid for a motor being controlled by a VFD.
RE: How to Calculate HP on motor driven through VFD
Voltage should be motor terminal voltage and should me in true RMS.
Thanks
RE: How to Calculate HP on motor driven through VFD
Best would just be to use the display on the VFD to give you the power out of the VFD. The voltage and currents will be very difficult to measure accurately due to the high harmonic content.
RE: How to Calculate HP on motor driven through VFD
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RE: How to Calculate HP on motor driven through VFD
Keith Cress
kcress - http://www.flaminsystems.com
RE: How to Calculate HP on motor driven through VFD
Use the ratio of the output frequency over the rated frequency times the rated voltage to determine the voltage to use in the power formula.
In the example given the voltage would be multiplied by 30Hz/60Hz or 0.5
Does this make your result more reasonable?
I'm not sure how this would work out. Just throwing it out for discussion.
Bill
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"Why not the best?"
Jimmy Carter
RE: How to Calculate HP on motor driven through VFD
"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: How to Calculate HP on motor driven through VFD
"How about using the V/H ratio. Use the amps on the VFD display.
Use the ratio of the output frequency over the rated frequency times the rated voltage to determine the voltage to use in the power formula.
In the example given the voltage would be multiplied by 30Hz/60Hz or 0.5
Does this make your result more reasonable?"
Bill - Yes, that would make the result more reasonable, but I don't think that's a good criteria for deciding that it's correct. BTW, what's the trick to correctly quoting a previous reply?
RE: How to Calculate HP on motor driven through VFD
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If we learn from our mistakes I'm getting a great education!
RE: How to Calculate HP on motor driven through VFD
[quote ScottyUK] Scotty wears red ballerina tutus![/quote]
Keith Cress
kcress - http://www.flaminsystems.com
RE: How to Calculate HP on motor driven through VFD
Multiply torque times rpm and divide by 5250 to get hp.
RE: How to Calculate HP on motor driven through VFD
I am surprised the VFD "measures" the voltage by calculation. I thought they did some serious fourier analysis of the output voltage and put otu the true rms value on the display.
But then I saw one german vfd oem "calculate" the motor speed based on its "algorithm" when actually the motor was turning much slower (due to multiple paths of winding broken).
RE: How to Calculate HP on motor driven through VFD
Thanks Scotty. BTW, do you really wear red ballerina tutus?
That looks like a cool formula, except that I've never seen a motor nameplate that gives me torque.
RE: How to Calculate HP on motor driven through VFD
Actually I believe the slip formula is a little oversimplified.
Rated slip is synchronous speed minus rated speed.
Example: 1800 RPM - 1750 RPM = 50 RPM slip.
(1800 RPM - Measured RPM)/Slip RPM = % HP loading.
Caveat. This depends on the nameplate value for motor speed being accurate. I have not used this method in the field and have no idea of the precision of the nameplate speed as reported by the manufacturer. A small error in reported speed will result in a large error in calculated HP.
Bill
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"Why not the best?"
Jimmy Carter
RE: How to Calculate HP on motor driven through VFD
Wouldn't "nameplate" torque be: nameplate HP times 5250 and then divided by nameplate rpm?
Thanks
RE: How to Calculate HP on motor driven through VFD
"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
RE: How to Calculate HP on motor driven through VFD
You promised that was our little secret.
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If we learn from our mistakes I'm getting a great education!