Motor Power factor
Motor Power factor
(OP)
I have a question concerning motor nameplate data, specifically power factor. The motor I have does not list the power factor and I would like to calculate the actual HP required for the pump that the motor drives.
I thought I would use the following formula:
FLA = ((HP*746)/(1.73*V*PF*EFF))
I could use the actual current and voltage and solve for the actual HP. This would be ok if I knew the PF.
What I think I can do is to use the nameplate current, voltage & EFF and solve for nameplate PF, then use that PF and the actual voltage and current to solve for actual HP.
The only problem is that I am not sure if the PF will be constant. (I think that this value will change as the motor load changes)
Can someone tell me if my values will be correct even if the pump is oversized?
Motor data:
20 HP
460V
23 Amp
EFF 91.7%
Across the line starting
Driving a pump that handles varying fluid densities
I thought I would use the following formula:
FLA = ((HP*746)/(1.73*V*PF*EFF))
I could use the actual current and voltage and solve for the actual HP. This would be ok if I knew the PF.
What I think I can do is to use the nameplate current, voltage & EFF and solve for nameplate PF, then use that PF and the actual voltage and current to solve for actual HP.
The only problem is that I am not sure if the PF will be constant. (I think that this value will change as the motor load changes)
Can someone tell me if my values will be correct even if the pump is oversized?
Motor data:
20 HP
460V
23 Amp
EFF 91.7%
Across the line starting
Driving a pump that handles varying fluid densities





RE: Motor Power factor
What also works occasionally is to turn off EVERYTHING sharing the power meter and just run your motor for 10 minutes and do the math.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor Power factor
The Kh is the number of Watt hours per revolution of the disk. You can time a couple of revolutions and do the math. This is much more accurate than using the kWHr register.
The Kh factor will be included on the information on the face of the meter.
Note 1; You will be working in Watt-hours, NOT kiloWatt-hours.
Note 2; Be sure to multiply by the proper multiplier if metering transformers are in use.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Motor Power factor
RE: Motor Power factor
If you transpose the inductive reactance to KVARs, it is quite easy to use KVARs and KVA to determine kW.
kW2 = KVA2 - KVAR2
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Motor Power factor
The easiest way is to just use a wattmeter. Either borrow one or just ask a meter supplier to demo a meter for you. One unit we have is a Fluke 43B - simple and quick to use. A simple single CT unit will be accurate enough - measure voltage L1 - L2 and current on line 3.