Motor Power factor

[RJC3PO]

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

 

[itsmoked]

The horsepower will vary due to your fluid densities and that will vary the PF. So try to google the motor and find a PF curve or bite the bullet and use a wattmeter. Or assume PF = 0.8

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

 

[waross]

If you use keith's method it is a lot easier to use the K_h factor.
The K_h 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 K_h 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

 

[DickDV]

Why not determine motor shaft speed and motor slip to get torque. Multiply torque times rpm divided by 5250 to get hp.

 

[waross]

The power factor is determined by the inductive reactance of the motor. It is relatively constant. If you can determine the inductive reactance at any loading, you can use this figure to calculate the motor power consumption at any loading. Once you determine the power consumption of the motor, you will still have to subtract the losses to arrive at the HP.
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

 

[LionelHutz]

The power factor will change with the load and that will throw off your calculations.

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.