P.F.C. and Motor Speed Relation
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gar
Senior Member
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- Ann Arbor, Michigan
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- EE
110118-1227 EST
If an induction motor is fed from a constant frequency and constant voltage, then typically as speed decreases (load increases causing more slip) the power factor increases up to rated load. See fig 13-12, p 213, of Bailey and Gault.
Thus, the reactive component of current as a percent of total current should be diminishing as the motor load increases and speed decreases.
I suspect this is not your question.
.
If an induction motor is fed from a constant frequency and constant voltage, then typically as speed decreases (load increases causing more slip) the power factor increases up to rated load. See fig 13-12, p 213, of Bailey and Gault.
Thus, the reactive component of current as a percent of total current should be diminishing as the motor load increases and speed decreases.
I suspect this is not your question.
.
Maybe he is refering to a motor connected to a VFD, and the motor slows down due to a decrease in the input frequency.
A motor can be represented by a voltage source in series with the stator winding resistance, the stator winding inductive reactance, and the counter EMF. (There might be some other items in this circuit, depending on how exact you want to be).
Anyhow, The counter EMF opposes the applied voltage. It is determined by the rotor properties. Its magnitude is some constant multiplied by the speed of the motor.
Reduce the speed of the motor, and you reduce the counter EMF. With a smaller Counter EMF, you have more voltage across the stator winding. Since the stator winding is mostly inducatance (the resistance is very small), there is more inductive current. So the reactive power goes up.
Anyhow, that's the basic idea as I remember it from the simple motor's equivalent circuit. I'm probably leaving out some details, that may or may not be important.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
110118-2363 EST
electrics:
If you can find a copy of Bailey and Gault, 1951, then look at p 223 and 224 for a discussion of "Changing Number of Poles".
Their effort was to go from a 4 pole to 8 pole and keep the motors as comparable as possible. This involved a number assumptions. It doesn't directly address your question and I do not have to time to try to do an analysis relative to your question.
One important point is that the power output is reduced to about 1/2. Almost self-evident, but they provide a reason.
.
electrics:
If you can find a copy of Bailey and Gault, 1951, then look at p 223 and 224 for a discussion of "Changing Number of Poles".
Their effort was to go from a 4 pole to 8 pole and keep the motors as comparable as possible. This involved a number assumptions. It doesn't directly address your question and I do not have to time to try to do an analysis relative to your question.
One important point is that the power output is reduced to about 1/2. Almost self-evident, but they provide a reason.
.
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