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Old 15th August 2010, 10:37 AM   #11
AndrewT is offline AndrewT  Scotland
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What little I think I know about power factor is to do with the phase difference between current and voltage. Industrial loads tend to be inductive and as you have said this uses current and develops current losses that are not directly related to the power consumed.
I gather that capacitors can be used to bring the phase of the current closer to that of the voltage and thereby reduce the wasted I^2R current losses that are not power.

But what is schem referring to?
That statement seems to be something quite different, and Abrax seems to be going down the same route, by saying that capacitors make the problem worse.
Explain what you are referring to.
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Last edited by AndrewT; 15th August 2010 at 10:39 AM.
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Old 15th August 2010, 10:53 AM   #12
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Quote:
Originally Posted by AndrewT View Post
What little I think I know about power factor is to do with the phase difference between current and voltage.
Yes, this is correct - the supply companies strongly prefer consumers to connect appliances with near to unity power factor - that means resistive loads are best.

Quote:
Industrial loads tend to be inductive and as you have said this uses current and develops current losses that are not directly related to the power consumed.
Inductors draw current but very little power is consumed in the load. Suppliers suffer losses due to heating relative to the square of the current drawn, not relative to the power taken. They charge domestic consumers according to power, not current.

Quote:
I gather that capacitors can be used to bring the phase of the current closer to that of the voltage and thereby reduce the wasted I^2R current losses that are not power.
If a company has loads which are mainly inductive, putting capacitors across the mains can improve the power factor. Inductors have lagging current, capacitors leading current. With the right value added cap, the leading can cancel the lagging so the result just looks near enough a resistor.

I am not sure about this, but I believe that industrial companies pay their electric bills according to VA, not Watts. Does anyone know to confirm this? If its true, then the company will pay lower bills by correcting their power factor.

Quote:
But what is schem referring to?
He's saying a big cap used on the secondary of a transformer makes for a poor power factor - large current flow, larger stray fields and more heating in the transformer. So he's discouraging specifying too big caps on the secondary.

Any clearer now?
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Old 15th August 2010, 11:21 AM   #13
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by abraxalito View Post
..........He's saying a big cap used on the secondary of a transformer makes for a poor power factor - large current flow, larger stray fields and more heating in the transformer. So he's discouraging specifying too big caps on the secondary.

Any clearer now?
no.
Quote:
.Big capacitors worsen power factor considerably
I just cannot see with my understanding where that statement comes from.
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Old 15th August 2010, 12:54 PM   #14
sreten is online now sreten  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
no.

I just cannot see with my understanding where that statement comes from.

Hi,

They are simply saying the short large peak rectified currents for an oversized
capacitor will cause more resistive losses than a "correctly" sized capacitor.

In a pre-amp / CD player this hardly matters, and CRC filtering can be used.
Small then Big in this case is poor due to the supply wiring AC ripple currents.

It does matter in high power applications, it not true that you "cannot
make the filtering capacitors too big", you can, causing excessive losses.

I agree with you though, power factor is not "conversion efficiency",
though this what is often meant when the term "power factor" is used.

Lots of wasteful resistive losses will actually improve "power factor",
but not "conversion efficiency" or "maximum supply power throughput".

rgds, sreten.

Last edited by sreten; 15th August 2010 at 01:02 PM.
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Old 15th August 2010, 02:05 PM   #15
AndrewT is offline AndrewT  Scotland
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Sreten,
are you saying that the use of the phrase "Power Factor" has nothing to do with what Schem is referring to?

BTW,
I started using psud many years ago. It clearly shows the short term current peak when voltage after the rectifier is above the voltage stored in the smoothing capacitor. That never confused me.
I also know that most transformer manufacturers recommend a transformer de-rating factor when the transformer feeds a capacitor input filter. I have seen DF between ~65% and ~75% and that is what I use to determine the maximum continuous DC current that can be drawn from the smoothing capacitors. This usually comes out at ~half the maximum AC current.
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Last edited by AndrewT; 15th August 2010 at 02:10 PM.
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Old 15th August 2010, 02:38 PM   #16
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Somehow they seem to write about the DC power supply then about the AC power supply. It's hard to keep track.
I thought that the original question was about main reserve supply capacitors and stage decoupling capacitors. But now I'm not sure.
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Old 15th August 2010, 02:46 PM   #17
AndrewT is offline AndrewT  Scotland
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Originally Posted by Speedskater View Post
Somehow they seem to write about the DC power supply then about the AC power supply. It's hard to keep track.
somehow, I am coming to the conclusion that they cannot keep track either.
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Old 15th August 2010, 02:52 PM   #18
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Quote:
Originally Posted by AndrewT View Post
I also know that most transformer manufacturers recommend a transformer de-rating factor when the transformer feeds a capacitor input filter.
I'd be surprised if it wasn't all trafo manufacturers. A capacitive load's power factor after rectification to some degree depends on the ripple voltage - higher ripple means better (higher) PF.
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Old 15th August 2010, 02:54 PM   #19
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somehow, I am coming to the conclusion that they cannot keep track either.
If by any faint chance 'they' includes me, feel free to highlight the bits in my writing where you're getting confused. I realize my writing skills leave room for improvement in clarity
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Old 15th August 2010, 03:04 PM   #20
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by abraxalito View Post
A capacitive load's power factor after rectification ........................ higher ripple means better (higher) PF.
Quote:
Originally Posted by abraxalito View Post
........feel free to highlight the bits in my writing where you're getting confused.
you're last post has two quotes that confuse me.
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