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# Power factor formula

#1
04-17-02, 04:14 PM
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Join Date: Jan 2002
Location: Arizona
Posts: 170
Power factor formula

Hey,
Need to try and find a formula used with electrical motors.I know the power formula,IxE=Pwhich is watts and I know ohm's law but this one formula has to do with a percentage.The question goes something like this.If you have a single phase motor and it's pulling 1 amp the power factor is(for example) 86%.I would appreciate any and all help,I'm actually trying to help a friend out who like me has alot of books but maybe not the right one.We want to know how they end up with the % value.

#2
04-17-02, 05:30 PM
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Join Date: Dec 2001
Location: New York
Posts: 1,287
Power factor is related to the voltage leading or lagging the current waveform. Once you determine the angular difference in the waveform (on an ossiliscope), you take the cos(angle).
The applicable formulae are power factor pf = cos ( angle ),
I = Current, V=Voltage
Apparent Power ( P ) = I · V (this is in units of VA)
True Power = P · pf (this is in units of watts)

#3
04-17-02, 10:25 PM
FREDDYG_001
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Posts: n/a
A Brewer, Here is one more example I dug up out of one of my old school books.

Cosine=VR/V From the voltage triangle

Cosine=R/Z From the impedance triangle

Cosine=W/VA From the power triangle

Example:

Power factor= W/VA = 500/625 = 0.8 or 80% lag

Power factor= R/Z = 20/25 = 0.8 lag

Power factor= VR/V = 100/125 = 0.8

Of the three forms given for the power factor, the most commonly used is:

Power factor= W/VA

W = V x A x cosine
or
W = V x I x cosine

Where cosine = power factor

From: Alternating Current Fundamentals(Third Edition)
By: John R. Duff/Stephen L. Herman

#4
04-18-02, 06:17 AM
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Join Date: Dec 2001
Location: New York
Posts: 1,287
I also meant to mention, refering to your % thoughts, that power factor is sometimes refered to as either a decimal or %. For example, a pure resistive load will result in a power factor of 1.0 or 100%.

#5
04-18-02, 10:09 AM
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Join Date: Jan 2002
Location: Arizona
Posts: 170
Hey,
I really appreciate the help,I thank you all for the quick responses,it's exactly what I was looking for.