How to Calculate HP on motor driven through VFD

[exteach]

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?

 

[davidbeach]

Where and how did you measure your volts, amps, and power factor?

 

[exteach]

David - thanks for the response. For now, I'm just doing some rough estimating. For volts, I used the voltage available from the MCC. For Amps, I used the read-out on the VFD. For efficiency and pf, I estimated 88% and 90% respectively, which were the lowest values I felt might be in the ball park. If any of these numbers are too low, that just makes the calculated motor BHP even worse, compared to my expected BHP (as calculated by completely independent methods).

However, my question is,... is the formula valid for a motor being controlled by a VFD.

 

[starkopete]

Hi,
Voltage should be motor terminal voltage and should me in true RMS.

Thanks

 

[davidbeach]

Formula is valid if: voltage and current are both measured at the motor terminals; are both true rms (good luck with that) and if the complex voltages and currents are used to calculate the power factor. Voltage on the MCC bus and current in the motor circuit = garbage in garbage out. You multiplied apples by oranges, expected to get pears but wound up with kumquats instead.

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.

 

[jraef]

You multiplied apples by oranges, expected to get pears but wound up with kumquats instead.

I like that.


"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

 

[itsmoked]

Brings up a question: If you want power and the VFD shows only current and voltage can you just multiply those? Is the current displayed the real current, or complex current? And can you assume the voltage displayed is the V_RMS fed to the motor?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

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?
I'm not sure how this would work out. Just throwing it out for discussion.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Most VFDs are displaying a fairly accurate depiction of output current and voltage. I'd say probably +-2%; not revenue accurate, but fairly accurate. The current is measured, usually with Hall Effect transducers, and the voltage is controlled by the VFD algorithm itself so they know what it is with a high degree of certainty.


"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

 

[exteach]

waross (Electrical) 23 Apr 09 21:38
"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?

 

[ScottyUK]

To quote a previous comment use the syntax (quote ScottyUK) More of Scotty's usual drivel (/quote) but substitute square brackets [ ] for those shown. There's a list of the functions available if you click the "Process TGML" link below the editing box.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Like this:
[ignore]

Scotty wears [/ignore][COLOR=red yellow]red[/color][ignore] ballerina tutus!

[/ignore]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DickDV]

Or, if you have a strobe light or hand-held tach, measure shaft speed and then calculate the motor slip in %. Slip in % times the nameplate torque will give you torque.

Multiply torque times rpm and divide by 5250 to get hp.

 

[edison123]

jeff

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).

 

[exteach]

There's a list of the functions available if you click the "Process TGML" link below the editing box

Thanks Scotty. BTW, do you really wear red ballerina tutus?

measure shaft speed and then calculate the motor slip in %. Slip in % times the nameplate torque will give you torque. Multiply torque times rpm and divide by 5250 to get hp.

That looks like a cool formula, except that I've never seen a motor nameplate that gives me torque.

 

[waross]

Derive the torque from rated speed and rated HP.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[starkopete]

Hi,
Wouldn't "nameplate" torque be: nameplate HP times 5250 and then divided by nameplate rpm?

Thanks

 

[dpc]

I'd think a vector drive would be smart enough to calculate the horsepower and display it. At least the input power to the motor.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[ScottyUK]

Keith!

You promised that was our little secret.


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If we learn from our mistakes I'm getting a great education!