Motor Horsepower
Motor Horsepower
(OP)
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.
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.





RE: Motor Horsepower
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
www.cvoes.com
RE: Motor Horsepower
And the power it's using is found using the above equation?
RE: Motor Horsepower
(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...
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RE: Motor Horsepower
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
www.cvoes.com
RE: Motor Horsepower
RE: Motor Horsepower
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
RE: Motor Horsepower
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.