Power factor of mains diode bridge

[Fluorescence]

Hello,

I just bought an 8p Compact Fluorescent Bulb (11W) from Tesco, UK.

http://en.wikipedia.org/wiki/Compact_fluorescent_lamp

They are supposed to be excellent at being much more efficient than the usual incandescent bulb.

I sawed the base out to see what kind of ballast it was using.

-After the mains connection , there is a diode bridge followed by a 3u3, 450V electrolytic capacitor.

Then there is a switching stage which switches the DC bus using two small high voltage transistors (13003 type) and what looks like an inductor -it looks like some kind of self-oscillating high frequency driver for the bulb.

Anyway, given that the first stage is a diode bridge followed by a smoothing capacitor.....the power factor could surely be no more then 0.6 ?

-this means that in real terms this bulb is at a very maximum, only 60% efficient ?

so after all, i wonder if this bulb really is as efficient as its made out to be?....since the poor power factor will simply incur losses within the electrical supply system ?

Any thoughts most gratefully appreciated indeed

 

[electricuwe]

For equipment consuming a larger amount of power (compared to a fluorescent bulb) a PFC is mandatory because the harmonics, which would be generated otherwise by rectifiers with capacitive smooting, can cause a lot of problems in the grid and for equipment in the grid, partially due to causing localised heating / hot spots. But in a total breakdown this will not come close to the losses in all the PFCs. From energy efficiency PFCs are a step backward, but they are necessary to keep the system running.

Maybe this confusion stems from the fact that power factor correction capacitors as well as Power Electronic Frontends are both refered as PFC, but whereas the former work nearly free of loss, the others have efficiencies in the range of 80% to 98% (the smaller the power, the poorer the efficiency)

Beside this the marketing people are just doing their job. Currently you can sell anything better than improved efficiency.

 

[Fluorescence]

Hello

"Just to be clear: the "reflected" power concept is completely wrong. "

....the thing is, if you please see the following article..

http://www.scribd.com/doc/7694921/On-Switch-Mode-Power-Supply-Reference-Manual

..please see page 32, left-hand-side, 2nd paragraph down..

-It clearly states that devices with poor power factor literally reflect power back into the mains system.

 

[ScottyUK]

The author is almost certainly an excellent electronics guy but possibly hasn't studied power since university. He also have very limited space to embark on a lengthy discussion of classical power engineering theory. Either way what is written there is not accurate.

Reflections occur when a transmission line is not terminated in its characteristic impedance. In this context I'm referring to 'transmission lines' in the general sense as derived from Maxwell's Equations, not in the sense of a long series of towers strung with conductors. 'Transmission lines' in the general sense could be anything from a microwave waveguide to, well, a long series of towers strung with conductors. It is a misconception that reactive power is related to such reflections; reactive power is a means of describing the power flow in a circuit where the current and voltage are displaced in phase. Reactive power is also a means of describing power flow where voltages and currents of different harmonic orders exist within a system. It does not involve reflections.

For every reference you find in an electronics App note which mis-applies the term 'reflected' you will find hundreds of good power engineering texts which do not. Have a look at David Beach's excellent faq238-1287 and buy or loan some of the books. We can argue this all day and all night but if you don't want to read up on power engineering theory which was established long before any of us here were born and which has been proven time and again by greater minds than mine then I don't know where to go next with this discussion.


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If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

Admire your patience Scotty.

Reflected power is a simple pedagogical trick that has sometimes been used instead of telling pupils what it really is about. I wish it were never "invented". It does not lead anywhere in low frequency power applications.

I suggest we put an end to this thread. The OP does not seem to accept the facts - and we cannot force him.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

If you want a simpler definition then reactive current is current that does not do any useful work. So, it is flowing through the power system loading up valuable resources (the transmission lines and transformers) yet the utility can not bill the customer for it. The utility can only bill for the real current which is used to do work. So, the utility comes up with penalties for customers with poor power factor so they can still bill them something for having to provide the reactive current.

In your discussion of SWPS and PFC front ends do you just not see the big picture?

Adding another active stage (the PFC stage) will decrease the efficiency of the power supply.

Think of an office tower with floors of computers and fluorescent lamps, all using switching power supplies without any type of harmonic correction. Almost the whole building load will be this type of equipment and combining that much poor power factor harmonic producing load could cause power system problems. Do you think that could cause problems which could be prevented with PFC stages?

So, you have to decide. Is the clean power better than a lower efficiency power supply?