Power Factor Correction

[maguire.david.c]

I'm an instructor myself and we teach to correct power factor with capacitive reactance by hooking the appropriate size capacitor in parallel with the inductive load. One of my students questioned as to why you would do this instead of in series stumped me and unfortunately the book I was teaching from lacked the reasoning behind the action as well. Can any one give me reason. I understand capacitors are additive in parallel but when youre dealing with a series parallel circuit that would be formed (say a capacitor that is in parallel connected to a (resistor + inductor in series)) how would you go about explaining that? Thanks.

 

[charlie b]

Since this question was not related to the thread into which it was originally posted, I have moved it to a thread of its own.

 

[charlie b]

Without the capacitor in place, the magnetic field of the power source (i.e., generator, transformer, etc.) will be exchanging energy with the magnetic field of the inductive load. That is the ?reactive power? portion of the total energy of the system.

After you put the capacitor in place, presuming it is sized correctly, the magnetic field of the inductive load will be exchanging energy with the electric field of the capacitor. That energy exchange takes place only within the two components, and the current flow begins and ends at the points at which the two are connected. The upstream power source does not see this energy exchange, nor the current flow between the two components. That is why the overall power level, as seen by the source, goes down. That is what is gained, by using capacitors in the first place.

But for this to work, the capacitor has be in parallel with the inductive load (whether or not there is also a resistive component in series with the inductive load). The current flowing in a capacitor is proportional to the rate of change of the voltage imposed across it. That is what makes the capacitor?s current be out of phase with the inductor?s current, and that in turn is what allows the energy exchange between the two components. If the capacitor were in series with the inductor, the two could not exchange energy, and you would not get a power factor correction. Instead, the power source would see the capacitor as just another load.

 

[Besoeker]

I'm an instructor myself and we teach to correct power factor with capacitive reactance by hooking the appropriate size capacitor in parallel with the inductive load. One of my students questioned as to why you would do this instead of in series stumped me and unfortunately the book I was teaching from lacked the reasoning behind the action as well. Can any one give me reason. I understand capacitors are additive in parallel but when youre dealing with a series parallel circuit that would be formed (say a capacitor that is in parallel connected to a (resistor + inductor in series)) how would you go about explaining that? Thanks.

Maybe thinking about the purpose of PFC?
It's used to reduce the current in the supply to an inductive load but not change the voltage supplied to it which is what would happen if you put the capacitor in series with it.
And that's not usually a desirable outcome.

 

[maguire.david.c]

Thanks

Thanks

I get it now. Since with a poor power factor you'll have a high current draw. Say if I was pushing from a genset 120 with a poor power factor I would always be pushing 120 but with a high current draw from a lagging power factor. To combat that we install the capacitors in parallel to our load in order to correct the current draw. Now if only I could find a schematic to demonstrate the whole process (both before and after correction)

 

[gar]

110410-2031 EDT

Try looking at the problem in the two following ways:

Assumptions:

The same inductor is used in both circuits.
All the load power is represented by the equivalent series or parallel resistance of the inductor.
Both circuits are tuned to resonance. This is represented by a power factor of 1. Usually one does not correct power factor this completely, but it serves the illustration purpose here. Also in the parallel resonant case we will move the resistive component from being in series with the inductance to parallel for the equivalent circuit.

Normally the power factor correction capacitor is in parallel with the load inductor. At resonance the parallel capacitor and inductor have an infinite impedance. Thus, the power source only sees the resistive load, and the inductor and capacitor only see the applied source voltage.

Now change the circuit to a series resonant circuit. At resonance the series capacitor and inductor look like a short circuit. Thus, the power source only sees the series resistive load, but the capacitor and inductor may see much larger voltages than the source voltage. This will depend upon the current thru the load resistor.

The parallel arrangement is generally much better. But note in a capacitor start or run single phase motor the series method is used for other reasons, but does have the effect of improving power factor.

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