Induction Motor Power Factor and Mechanical Load

[KnThrak1982]

Can someone please explain to me (quantitively prefered) how to determine the power factor of a 3-phase induction motor at a load less than the rated load?


I've been able to find methods of improving power factor through voltage control etc, but haven't been able to prove the actual relationship.

 

[LionelHutz]

It is not a linear relationship. If you get the motor data the manufacturer will typically include the power factor for a number of different loads. It's typically 25%, 50%, 75% and 100% load. You could draw a graph and fit a curve to these points.

The only other way I know is to test the motor. This test could be in the field with a power meter or it could be a controlled test in a lab. You have to record the kW and power factor to get the relationship.

Actually, I guess it is possible to get the electrical model of the motor and from that you should be able to determine the power factor vs load.

 

[tem1234]

Hi,

If you go to the link below, there's a little diagram of the power factor VS load for a 15 HP motor

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

The best way is to test the motor as lionelhutz said, but this can give you a good idea

 

[KnThrak1982]

Thanks for the help.

From a generic design point of view, how is this relationship explained? I can see that current does not vary significantly with load, hence high VA in with low Watts out = low power factor. I just don't understand why this is the case.

LionelHutz mentions the electrical model of the motor. What aspects of this model are key (rotor resistance/ inductance etc)? How would a motor with a high PF at lower loads differ from a motor with a low PF at these loads?

I know I'm asking a lot here, I just don't know where to begin working this out.

 

[davidbeach]

A good electric machines text would be a good start. With enough information on the motor and the load you can calculate motor slip for any load and the use the Steinmetz model of the motor to calculate most anything else as a function of slip.

 

[waross]

Draw a power triangle.
This is a right triangle.
The base represents the load and the losses in Watts.
The altitude represents the reactive current or VARs.
The hypotenuse represents the VA.
The ratio of the base over the hypotenuse represents the power factor.
The altitude or VARs are relatively constant.
The Watts include the work done and the losses. The work is linear but the losses are not. Some losses such as windage and bearing friction are constant. Winding resistance losses vary as the square of the motor current. As a result the power factor versus load is a curve rather than a straight line.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

The Steinmetz model (aka transformer model) is available as a diagram. Google Ossanna diagram. There's a very good tutorial at

http://www.ew.e-technik.tu-darmstadt.de/publics/EMD/OSSANNA-circle.pdf

You can get all information (except insulation resistance, perhaps) from such a diagram.

Gunnar Englund

http://www.gke.org

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