squirrel cage motor power factor 7

[don01]

We have been having a discussion on the relationship of load and power factor in squirrel cage motors. Generally it is agreed that pf. improves as the load approaches 100% BUT nobody can find a graph or curve on paper.

Does anyone know of a site or link with this available.
We have done a bit of searching but obviously aint spellin somethin right

thanks
Don

 

[Marke]

Hello Don

There are big differences in the power factor curves of motors. The main contributing current is the magnetising current of the motor. This is independent of load and varies from as low as 20% of the rated current of the motor for large two pole motors, to 60% for small submersible pump motors.
There is a smaller inductive component that is load dependent and this is the leakage reactance.

The power factor of the motor is essentially the ratio of the load current (resistive) to the total current (square root of (load current squared plus magnetising current squared).
The leakage reactance will have the effect of reducing the full load power factor, and the motor losses will have the effect of increasing the unloaded power factor. The shape of the curve is determined by the magnetising current of the motor.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[electricpete]

Look on the right side half-way down here to see typical characteristics vs load for 15 hp motor (mileage may vary).

http://www.motorsanddrives.com/cowern/motorterms16.html

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[UKpete]

There is also a variation in power factor with the size of the motor - generally speaking the higher the full load power, the higher the power factor.

The following links don't give a curve, but do give the power factor at 1.0, 0.75 and 0.5 x full load across a wide range of power ratings.

http://www.brookcrompton.com/pdf-files/2209E_nema_v1.1e.pdf

(thats for NEMA motors)

http://www.brookcrompton.com/pdf-files/2210E_trange_v2.1e.pdf

(for larger IEC motors up to 315kW/420hp)

 

[don01]

thanks all
True to form this gruop of dynamos has the answers.
(that is a really bad pun, hey?)
Marke,that is probably the bit our debate was missing

again, thanks

 

[jraef]

e-pete,
Nice find on that website, thanks for that. If anyone else is interested, click the link back to the main page and peruse the list of papers available. They are excellent.

"Venditori de oleum-vipera non vigere excordis populi"

 

[edison123]

pete,

Good reference. I too vote a star.

 

[edison123]

and one for you too jraef for pointing ou the good thing

 

[jraef]

Aw shucks, I didn't deserve it...

 

[electricpete]

me neither - I just found it on google.

An interesting thing is that if you go in through the "front door" following the links from the home page, you get locked into frames (at least in my browser) and you can't bookmark any of the individual pages.

If you use google you get in through the "backdoor" and you can find more direct access to the articles (bookmarkable) here

http://www.motorsanddrives.com/cowern/contents.html

Probably most people don't care... but it's just one of those curious things.

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[Hewlett]

....and for those of using "Opera" browser, get this view (frame maximized) by right click > "Frame" > "Open"
H