Induction Motor Power Factor and Mechanical Load
Induction Motor Power Factor and Mechanical Load
(OP)
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.





RE: Induction Motor Power Factor and Mechanical Load
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.
RE: Induction Motor Power Factor and Mechanical Load
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:
The best way is to test the motor as lionelhutz said, but this can give you a good idea
RE: Induction Motor Power Factor and Mechanical Load
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.
RE: Induction Motor Power Factor and Mechanical Load
RE: Induction Motor Power Factor and Mechanical Load
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
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"Why not the best?"
Jimmy Carter
RE: Induction Motor Power Factor and Mechanical Load
You can get all information (except insulation resistance, perhaps) from such a diagram.
Gunnar Englund
www.gke.org
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