Motor Horsepower

[impeller1]

Good day,

I have a pump and motor configuration. I'm trying to find the equation to calculate the horsepower that the motor is producing. I have the motor current and voltage. it's a 3 phase motor, so I'm assuming I have to use this;

P = (3)^1/2 x V x I x cos(power factor) ??

Can someone confirm this or correct me if I'm wrong?

Also, if the motor is controlled by a VFD, do I then have to multiply this value by the VFD's efficiency to obtain a value for the motor horsepower?

Thanks a lot.

 

[dpc]

Your power formula would give the power that motor is USING not producing, assuming you know the actual power factor at this voltage and current.

Regarding the VFD question, this depends on where you are measuring the voltage and current. With a VFD it is generally best to get the kW information directly from the VFD.



David Castor

http://www.cvoes.com

 

[impeller1]

Ok, so is the power its producing equivalent to when you multiply the power its using by its efficiency?? Am I correct??

And the power it's using is found using the above equation?

 

[jraef]

You didn't tell us why you want this information, so it depends on what the meaning of "it" is.
(OMG, I'm becoming a politician!)

If by "it" you mean the motor itself, then as dpc said, better to use the data from the VFD because measurement of the output of a VFD is tricky at best, inaccurate in most cases, unless you have very sophisticated equipment specifically designed for that purpose.

If by "it" you mean the power consumption of entire motor circuit, including the VFD, then yes you must factor in the efficiency of the VFD itself. Also tricky, unless you choose to believe all the marketing info from the VFD manufacturer that claims 95% efficiency...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[dpc]

impeller,

Yes, that's correct in theory. But bear in mind that both the power factor and the efficiency are functions of the motor loading, so without measurements, you are only making an approximation.

As jraef said, trying to measure volts and amps on the VFD input to determine motor kW will non-productive.

David Castor

http://www.cvoes.com

 

[impeller1]

Ok ok..let me explain exactly what I want to do..(Sorry, should have done this before) , I'm trying to determine the pump's BHP from operation. I dont have any flowmeters so I cant tell using the flow. So I was trying to determine how to get the motor output horsepower, then use the motor's efficiency to get the power being delivered to the pump. 2 of the 5 motors have VFDs, the others dont. How do I proceed???

 

[rbulsara]

output=(input*efficiency)

or output=input-losses

Input power is easy to measure, for losses, which vary with load and actual health of the equipment, you may have to rely on some empirical data, estimate or contact pump and motor manufacturers.

Rafiq Bulsara

http://www.srengineersct.com

 

[LionelHutz]

VFD - Read the output kW from the VFD and multiply by the motor efficiency. If you don't know the efficiency then use something like 93%.

No VFD - use your formula and multiply by the motor efficiency. If you don't know the efficiency then use around 94% in this case.