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PFC capacitors.

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alphacat

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In order to correct the power factor of the inductive load, the PFC capacitor should consume the same amount of reactive power that the inductive load consumes.
I wonder, how do these PFC devices correct power factors of different loads that consume different reactive power from the power line?
Thanks.
 

Chippie

Member
Where I work( a term I use loosely) we have an automatic power factor correction unit consisting of a number of capacitors that are switched by means of contactors that are controlled by a regulator that measures power factor, it does this by measuring the current in one of the phases...
Does this help you?
 
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alphacat

New Member
Thank you very much :)
I just read about contractors in wiki, and it seems to operate just like a relay, but there's some kind of difference between the two.

How do they differ from each other?

Moreover, dont you need many contractors for this?
 
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Chippie

Member
Thank you very much :)
I just read about contractors in wiki, and it seems to operate just like a relay, but there's some kind of difference between the two.

How do they differ from each other?

Moreover, dont you need many contractors for this?
I think you mean contactor...which is just a big relay with heavy duty contacts...

There isnt really a great difference in general...Both are electromagentically operated by a coil, both have a set (or more than one set) of contacts....

Although there are Vacuum contactors where the contacts are contained in a vacuum..often called vacuum breakers( we have them too for switching 6.6kv loads)
 

tcmtech

Banned
Most Helpful Member
A contactor is just a big relay. They are typically heavier built and designed for rougher service life. I usually consider anything small DC powered and enclosed a relay. Large AC powered and open is a contactor.

The contacts on a heavy duty 15 amp cont actor can be 10 times larger than those on a 15 amp relay even at the same rated operating voltage.

-------------------------------------------------------------
Contactor is not in the electro tech spell check dictionary. Thats one more common word found. :mad:
 

Chippie

Member
The contacts on a heavy duty 15 amp cont actor can be 10 times larger than those on a 15 amp relay even at the same rated operating voltage.
QUOTE]

Moreso if its switching DC......
 

alphacat

New Member
Thanks guys.

About this PFC device you mentioned, Chippie,
Doesnt it take many contactors to select and deselect capacitors?
 

Chippie

Member
Depends on how much PFC is needed....Got a lot of inductive loads switched on at once? then you may need a lot of C....

Where I work we have 4* 300KW compressors, 3*2.1MW compressors, all running off a 6.6KV supply...imagine trying to manage that lot...along with 3* 3.5MW gas driven generators
 

alphacat

New Member
Hey,
Thanks!

If you have for example a variety of inductive loads, from 100VAR to 10kVAR, wouldnt your device need a lot of contactors in order to have a range of 100VAR capacitor up to 10KVAR capacitor?
 

Chippie

Member
You would need whatever pfc capacitors are required to enable the PFC to be as near to unity( Not sure what we have...)Although in practice it turns out that 0.85-0.9 is about the best we achieve from memory....Its not something I look at on a daily basis
 

alphacat

New Member
Thank you mate.

Out of curiosity, what made your company to use this automatic PFC unit?
It doesnt save up money to your company, right?
I mean, all it does is to spare the electricity & power company from supplying reactive power to the inductive load, which you dont pay for anyway.
 

Hero999

Banned
Some power companies fine businesses if their power factor is below a certain level.
 

alphacat

New Member
Thanks to all of you for your help.

I had an experiment today.
I connected a plasma TV to a device that measures active power and power factor, amongst other paramters.
The devices measured 140W active power consumption and 0.725 power factor.
Therefore, I connected in parallel to the device 4 2uF capacitors, which were connected in parallel to each other.
Strangely, the device showed 0.62 power factor and 137W active power consumption.

I should add that I also connected only the 8uF capacitor (consists of four 2uF capacitors in parallel) to the device, and the device showed power factor of 0.01 and active power of 1.4W.
All I know on these capacitor that they fit to 275Vac and 50Hz and 60Hz.

Could you help please figure out why the PFC capacitors didnt correct the TV's power factor? (but only made it worst).
 

tcmtech

Banned
Most Helpful Member
Over correction likely. Too much capacitance.
Try it with only a single 2 uf capacitor and move up from there.
 

alphacat

New Member
I wonder what did i do wrong.
I calculated the Q consumed by the inductor:
Q = √[ S² - P² ] = √[ (140W / 0.725)² - (140W)² ] = 133VAR
Then I calculated the required PFC capacitor:
C = Q / (V² * 2Π * f) = 133VAR / ((233V)² * 2Π * 50Hz) = 7.8uF.

Does it look fine?
 
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tcmtech

Banned
Most Helpful Member
Theoretical Vs real life. When theoretical doesn't do it, try the real life version. 2uf take reading, Add 2uf more take reading. find happy medium.
A modern TV has a switching power supply so you cant actually consider it a reactive load like a true transformer. Its more complex than that and can often create false readings on digital power factor meters.
The old school analog units will show the real PF while the digital will show what some programmer thought would be a good number based on a theoretical formula calculation referenced to the volts and amps readings derived from a complex time referencing system and what not.

I have seen digital PF meters get their high priced butts kicked by 50 year old analog units! I dont trust the digital units after seeing that first hand one time.

Just something to consider!
 

Hero999

Banned
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alphacat

New Member
Yes, I do remember the harmonic distortion issue that we talked about.
If that is indeed the case, I assume that connecting a capacitor in parallel would only increase the bulk capacitor and wouldnt correct the power factor?
 

Hero999

Banned
That's right a capacitor probably won't help.

A harmonic filter is the answer to non-linear loads.
 

alphacat

New Member
Thank you very much.
Could you please direct me to some schematic/design of such harmonic filter, so i could understand how it works and how it connects to the load.
Somehow I havent found any on web.
 
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