how capacitors improve power factor

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ammarawadallah

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when improving power factor using a capacitors what exactly happens or what is the direction of the capacitor current at any moment, my understanding that capacitor supplies leading current to the inductor when the inductor is oposing line current ,but my question is what is the direction of this capacitor current in the presence of incoming line current ? it seems to me as if line currents and capacitor current are opposing at the tie up point.please explain
 
If you have an ideal inductor, the current in it will lag behind the voltage by exactly 90 degrees.
If you have an ideal capacitor, the current in it will lead the voltage by exactly 90 degrees.
That puts the two currents a full 180 degrees out of phase with each other.
If you choose the capacitor value properly, the magnitudes of the two currents will be equal and *no* current will flow in the line.
This is called a "parallel resonant circuit".
If there is also a resistive component to the load, that is going to be the only thing drawing line current and you will have a power factor of 1.
In practice you do not want to cancel the inductive current completely though.

Tapatalk!
 
To add to GD's reply...

The power factor of the inductive load by itself does not change. The majority of its reactive power is stored and released locally by the capacitor, rather than being transferred through the supply line.
 
But one could equally well say that the reactive power of the capacitor is being stored locally in the inductor.
Energy is moving back and forth in a way that is symmetrical except for the fact that the inductor was there first. (Being an integral part of the motor or transformer you installed.)

Tapatalk!
 
if capacitor supplies magnetizing current only ? how load current is supplied to the inductor? does it flow along side the capacitor current thus adds up at the inductor?may be answering with drawings with arrows could help?
 
ammarawadallah said:
if capacitor supplies magnetizing current only ? how load current is supplied to the inductor? does it flow along side the capacitor current thus adds up at the inductor?may be answering with drawings with arrows could help?
Click to expand...
Let's start with simple, ideal components (word drawings :p).

Say you have an AC voltage source and an inductor in a circuit loop. Current will lag the voltage by 90 degrees.

Say you have an AC voltage source and a capacitor in a circuit loop. Current will lead the voltage by 90 degrees.

Say you have an AC voltage source, an inductor, and a capacitor all connected in parallel. Current can be anywhere between + or - 90 degrees... depends on the properties of the inductor and capacitor...

If the inductor and capacitor properties create a "resonant" circuit, i.e. perfectly tuned for the AC frequency, there will be some initial current to charge the circuit, but after that, there will be no current flowing from the voltage source to either the inductor or capacitor. Current will simply flow back and forth between the inductor and capacitor. With the three component circuit, you have two (2), three-conductor nodes. Perform a KCL analysis on either of the nodes

Couple this principle with less-than-ideal, real-world components and presto... :D

One wayto get a better understanding is, find yourself a decent circuit simulator app, setup the described circuit, and graph the currents for each "leg" of a node.
 
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The inductor and the capacitor are not perfect and will have some resistive component to them which will result in current being used from the supply. The reactive current is all that is exchanged between the inductor and capacitor. If the inductor is motor windings, or a solenoid coil, it still will draw other current (true power) from the source that is performing actual work.
 
GoldDigger said:
If you have an ideal inductor, the current in it will lag behind the voltage by exactly 90 degrees.
If you have an ideal capacitor, the current in it will lead the voltage by exactly 90 degrees.
That puts the two currents a full 180 degrees out of phase with each other.
If you choose the capacitor value properly, the magnitudes of the two currents will be equal and *no* current will flow in the line.
This is called a "parallel resonant circuit".
If there is also a resistive component to the load, that is going to be the only thing drawing line current and you will have a power factor of 1.
In practice you do not want to cancel the inductive current completely though.

Tapatalk!
Click to expand...


Ive always wondered why. Whats the reasoning?
 
One risk I have seen mentioned but never explained is that a ferro-resonant condition might cause high currents or voltages within the circuit. But I cannot understand or explain that other than to speculate that it might be related to magnetic nonlinearity (hysteresis).

Also a leading power factor is more likely to cause high current when contacts close, damaging them or even welding them.

Tapatalk!
 
Don't know the details, but regarding motors, overcorrection beyond 95% lagging may result in self-excitation.
 
Besoeker said:
We fit a PFC contactor to obviate that possible condition.
Click to expand...
Ahh...the resolution! A lot of info that tells one not to over correct fails to tell one that a PFC contactor eliminates the necessity to not over correct (within reason).

Correction to not more than 95% is just another one of those factual-based myths. ;)
 
Smart $ said:
Ahh...the resolution! A lot of info that tells one not to over correct fails to tell one that a PFC contactor eliminates the necessity to not over correct (within reason).

Correction to not more than 95% is just another one of those factual-based myths. ;)
Click to expand...
Slightly off topic....
A very, very long time ago before the Dead Sea reported sick, I did some tests on self-excitation of induction motors. They were submersibles with borehole* pumps and the idea was to run the pumps as turbines to produce a cheap hydro generator. The tests were on 30kW units and worked pretty well. I can't remember what capacitors we used but, at the time, we were making force-commutated inverters so we had plenty lying about experiment with.

The idea was never pursued - we got busy with other business so it was let slide.

*And completely off topic......
I was preparing a bid for a system that included a couple of borehole pumpsets. It was in the early days of using a word processor application and it objected to borehole pumpsets. It decided that "brother dumpsites" was the way to go. I was half tempted to leave the correction in...............
 
GoldDigger said:
If you have an ideal inductor, the current in it will lag behind the voltage by exactly 90 degrees.
If you have an ideal capacitor, the current in it will lead the voltage by exactly 90 degrees.
That puts the two currents a full 180 degrees out of phase with each other.
If you choose the capacitor value properly, the magnitudes of the two currents will be equal and *no* current will flow in the line.
This is called a "parallel resonant circuit".
If there is also a resistive component to the load, that is going to be the only thing drawing line current and you will have a power factor of 1.
In practice you do not want to cancel the inductive current completely though.

Tapatalk!
Click to expand...

if capacitor is sized such as it is equall and opposite to inductor current and indeed cancels each other at every second how can we still have the magnitizing current necessary to drive the load??
 
if capacitor is sized such as it is equall and opposite to inductor current and indeed cancels each other at every second how can we still have the magnitizing current necessary to drive the load??:weeping:
 
ammarawadallah said:
if capacitor is sized such as it is equall and opposite to inductor current and indeed cancels each other at every second how can we still have the magnitizing current necessary to drive the load??:weeping:
Click to expand...
The amount of current flowing through the inductor does not change. The difference is the current flowing in the line conductor.. and that difference is going back and forth to and from the capacitor.
 
Besoeker said:
Slightly off topic....
A very, very long time ago before the Dead Sea reported sick, I did some tests on self-excitation of induction motors. They were submersibles with borehole* pumps and the idea was to run the pumps as turbines to produce a cheap hydro generator. The tests were on 30kW units and worked pretty well. I can't remember what capacitors we used but, at the time, we were making force-commutated inverters so we had plenty lying about experiment with.

The idea was never pursued - we got busy with other business so it was let slide.

*And completely off topic......
I was preparing a bid for a system that included a couple of borehole pumpsets. It was in the early days of using a word processor application and it objected to borehole pumpsets. It decided that "brother dumpsites" was the way to go. I was half tempted to leave the correction in...............
Click to expand...
You should see what happened to me when I tried to send an email on my company PC to someone and refer to the wire connection box on a motor using a very common vernacular term over here; "Peckerhead". I got a call from our HR department and was told to not use that term because it was considered "vulgar" and "sexually charged" language. They suggested that I re-take my "Employee anti-harassment" on-line webinar course again. I tried to explain to them that this was not a sexually charged term, but I could not for the life of me find any definitive reference as to where it stems from, which leads me to fear that it may indeed be what it conjures up in the mind. Since I could not find that definition, it was easier to just run the webinar course again. I always refer to it as the "motor wiring enclosure" now... I'm leery of even using the word "box", because I already have this on my "permanent record" with HR.
 
Jraef said:
You should see what happened to me when I tried to send an email on my company PC to someone and refer to the wire connection box on a motor using a very common vernacular term over here; "Peckerhead". I got a call from our HR department and was told to not use that term because it was considered "vulgar" and "sexually charged" language. They suggested that I re-take my "Employee anti-harassment" on-line webinar course again. I tried to explain to them that this was not a sexually charged term, but I could not for the life of me find any definitive reference as to where it stems from, which leads me to fear that it may indeed be what it conjures up in the mind. Since I could not find that definition, it was easier to just run the webinar course again. I always refer to it as the "motor wiring enclosure" now... I'm leery of even using the word "box", because I already have this on my "permanent record" with HR.
Click to expand...
In a related story...

I was having trouble with my computer at work, and I had an IT guy in my cubicle helping me solve it. The problem was especially persistent and I was very frustrated, and I was using some, um, uncomplimentary language. Unbeknownst to me, someone walking by heard me and reported me to HR. When the HR rep called me a few days later, they said that they had gotten a report that I was verbally abusing a coworker. I was puzzled; I don't do that. When they told me when the report came in, I remembered what the situation was. I told them that Carl had been in my cubicle, but the language I was using was directed at my computer. "Oh," she said, "we all do that. Never mind."
 
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