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How Do You Workout The Power Factor For A Motor?


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#1 bookworm

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Posted 12 December 2007 - 03:14 AM

What bits of info do you need to work backwards (reverse enggineer) and work out the power factor for a motor?

(so you don't have to install a meter which would be expensive) to work out the power factor.

Thanks in advance. laugh.gif

#2 marke

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Posted 12 December 2007 - 05:40 PM

Hello Bookworm

Power Factor is the ratio of KW to KVA. If you know the voltage, current and power that the motor is drawing, then you can calculate the power factor.
Alternatively, if you measure the angle between the voltage waveform and the current waveform with an oscilloscope.

If you are really in trouble and just want an estimate, then measure the open shaft current of the motor to get the magnetizing current.
The magnetizing current is constant and load independent.

An approximation of the power factor will be the cosine of the angle phi where phi equal arcsine((magnetizing current)/(loaded current))
Note this is very much an approximation as it does not take into account the leakage reactance of the motor.

Best regards,

#3 bookworm

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Posted 12 December 2007 - 09:47 PM

Thanks for replying

ok but here's the thing these motors are 150kw 250kw and 400kw (rated power) no way i can just stick an oscilloscope and measure them...

what's the best way to get the pf of each motor? an estimate would be good enough. i know their rated power, rated v and current. and rpm. i have their brand, frame, made.

This mag current (that is independent of the load) you speak of can i get that from the data sheet? (dont have the measure?)

so after i get that, I go COS(ASIN(mag current/loaded current))?

#4 jraef

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Posted 14 December 2007 - 06:23 PM

Most motor nameplates will show the power factor. It might be listed as "cos." or "cos phi" or cos Φ (the Greek letter phi). If not, what is your purpose for knowing or how accurate do you need to be? A safe bet is to assume no worse than a .7 pf for the purposes of correcting with capacitors, many people use a .8 pf.
"He's not dead, he's just pinin' for the fjords!"

#5 marke

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Posted 14 December 2007 - 08:33 PM

Hello bookworm

You can connect an oscilloscope to a motor of that size. It becomes more difficult if it is an MV motor though.

If you have data on the motor, the power factor will be quoted.

The power factor of the motor is not constant and is load dependent. If you are looking for the full load, 3/4 load and/or 1/2 load power factor of the motor, it will be on the motor data sheets that you can get from the supplier. Otherwise, check the name plate for the full load power factor.

The magnetizing current is rarely quoted on the data sheet, but the power factor usually is.

You do need to have some specific information on the particular motor. Guessing the power factor can be very erroneous as the magnetizing current can vary from 20% of rated current to greater than 60% of rated current, and if you are looking to correct the motor, then you will almost definitely be seriously out.

Best regards,

#6 bookworm

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Posted 17 December 2007 - 02:39 AM

thanks marke. i have the 3 motors and only just got 1 datasheet for one of them but it had the info i was looking for

load %, power factor, efficiency graph

sad thing is for the other 2 motors... the manufacs won't send me the data sheet. they say they will but never sent... T__T so lame.




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