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

 

[dpc]

Low power factor does not mean low efficiency. Efficiency and power factor are two different things. Low power factor does increase current draw and this will increase losses on the conductors, but this is a secondary effect.

You can draw NO conclusions regarding efficiency just from the power factor.

 

[Fluorescence]

Hi,

thankyou for returning your reply...

I can appreciate that the efficiency of the product itself will not be affected by it's power factor.....but i wish to look at the wider picture here....

i want to consider the losses incurred by the product in the entire electrical system, going back to the power station.

I can well appreciate that the domestic consumer will not have to pay any extra for the poor power factor.

However, i was always told that a power factor of 0.6 meant that only 60% of the energy arriving at the mains terminals is actually used.....the other 40% is "reflected" back into the power system where it will just be "burned up" as heat in the resistance of the wires of the electrical distribution system ?

There is , please correct me if i am wrong, no "storage" capacity in the mains electrical distribution system, so any energy reflected back due to poor power factor will just end up being wasted ?

I would be grateful for assistance concerning this "overall" efficiency consideration of this Fluorescent bulb.

 

[itsmoked]

However, i was always told that a power factor of 0.6 meant that only 60% of the energy arriving at the mains terminals is actually used.....the other 40% is "reflected" back into the power system where it will just be "burned up" as heat in the resistance of the wires of the electrical distribution system ?

You were wrongly informed. This is not what occurs.

What you have is more current moving in the 'wires' with a non-unity power factor than you would otherwise. That added current difference can incite more I²R system loses. The farther from unity the greater the loses.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

First I've ever heard of "reflected" energy.

Power factor of 0.6 just means for each VA of input "power", you get 0.6W of real power out and 0.8VAr of reactive power. The current necessary to support that 0.8VAr does cause I²R losses and therefore some loss of efficiency, but no where near 40%. Also, the two factors that go into total power, power factor and reactive factor, are in quadrature to each other and never add to the total, just like 0.6 and 0.8 don't add to 1.0 arithmetically, but they do if you take the square root of the sum of the squares.

 

[davidbeach]

Darn it, once I start typing on a response nobody else is supposed to be able to type in a response ahead of mine; I'm sure I was there first....

;-);-);-)

 

[itsmoked]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Comcokid]

You’ve found one of the dirty little secrets of the Compact Fluorescent Bulbs - poor power factor. The other, that's not discussed much? - mercury!

 

[dpc]

Just to be clear: the "reflected" power concept is completely wrong. whoever has been telling you this doesn't have a clue as to what they are talking about.

I actually have no idea as the true efficiency of a CFL, but considering it generally replaces an incandescent bulb that would have an efficiency of about 1% (assuming it isn't a HEAT lamp), almost anything is an improvement.

 

[burnt2x]

From the electrical point of view, CFL's do have poor power factor. But what matters most is the "efficiency" in the energy to light conversion of the device. I think the efficiency being considered when talking about fluorescent lamps is that CFL's do away with the usual ballast (use electronic ballasts instead).
Electronic ballasts also operate fluorescent lamps more efficiently at frequencies greater than 20,000 Hz (reason for the electronics you saw inside the base). The resulting increase in lamp efficacy, combined with reduced ballast losses boosts fluorescent system efficacy by up to 30 percent; compared to 4- 6% efficiency of filament bulbs.

 

[Skogsgurra]

Also, many flourescent lights sometimes have a PFC (Power Factor Correction) function built-in. That means that your current will be very close to a sinewave. Not at all like the typical bridge/capacitor current.

In your case, however, it seems to be the old, more primitive input stage.

FYI, there are two power factors in use. The W/VA and the displacement power factor. The W/VA is also called lambda while the displacement power factor is called cos(phi1), where 1 stands for fundamental (first harmonic). There can be a big difference between the two. Cos(phi1) is usually close to unity while lambda can be very low. A lot less than the .6 you assume.

And, yes, forget about "reflected power". This is 50 or 60 Hz - not microwaves.

Gunnar Englund

http://www.gke.org

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

 

[LionelHutz]

Maybe I could expand a little further.

power factor = W / VA
VA = sqrt(W^2 + VAr^2)
A poor power factor results in VAr in the power system

You have an 11W bulb and I'll assume 220VAC in Europe
Assume this 11W bulb operates at 0.6 power factor
That calculates to 18VA and 14VAr
So, the light bulb will cause 14VAr to flow in the power system.
This 14VAr will cause 14VAr/220V = 0.065A

Conclusion
Your lamp causes 0.065A of reactive current to flow. This reactive current does no work and only causes heating in the wiring and transformers of the distribution system.

One lamp by itself is very insignificant. However, add enough of them and you eventually cause a significant amount of losses. Eventually, the utility will place power factor capacitors into the system near this low power factor to correct it. Either that or they would force you to correct it. This is why much of the office equipment in Europe is regulated to have power factor correcting supplies - an office building full of these lights could create a significant enough problem.

Stating that this 0.065A causes 4.4W (40% of 11W) of power to be lost in distribution system is completely wrong. You really can not predict this number unless you have detailed knowledge of the utility system. This argument is often used to "prove" CFL's use more power than what the manufacturer states.

 

[electricuwe]

The additional loss caused in the system due to low power factor of the lamp is very low.

Assuming that the loss in transmission is for sure lower than 5% in europe, the additional loss may be calculated as

18 VA * 0.05 - 11 W * 0.05 = 0.35 W

Contrary to LionelHutz's statement a power factro correction capacitor will not be very effective in curing this, since the rectifier mainly creates distortion and not displacement.

For large lighting installations it is worth while to care about power factor and distortion, but for a single fluorescent bulb or a circuit of a few, a PFC circuit would be wasted money.

 

[LionelHutz]

I don't understand that loss calculation?

I was trying to keep away from discussing the harmonics but that is very true. The rectifier will create harmonics which aren't easily corrected with just a capacitor. But as a general statement, when you have a location (say a building or a plant) with a low power factor and a high enough VAr, the utility will generally install or force you to install capacitors. Same applies if you create high enough levels of distortion. If you correct it near your location then you're not causing the additional VAr load to be seen all the way back to the utility generation.

This is why you see these lights or larger ballasts or computer power supplies, for examples, with power factor correcting built in. This eliminates the harmonics and the poor power factor presenting close to a resistive load on the power system.

 

[Fluorescence]

I'm sure anybody like me reading this would agree that these posts are the best source on PFC that i've ever read....stars all round i think.

anyway...i did stray into microwave using the term "reflected"....

..though i am not sure of which term i should have used?....my university lecturer from 7 years ago used the term "shuffling" in these exerpts from his notes on PFC and the electricity supply system.....

--------------------------------
"Electricity supply companies dislike customers who place large reactive loads on the power distribution system because this results in the current carrying capacity of the network being used up in transferring power backwards and forwards to reactive components and therefore reduces the capacity of the network to deliver active power.”


"……Furthermore the sinusoidal current drawn by each load must be in phase with the supply voltage. If this is not the case the load is drawing reactive power; i.e. power that is drawn from the supply during one part of the mains cycle and then returned back to the supply during the remainder of the cycle. This “shuffling” of unused power back and forth between the supply and the load, results in unnecessary voltage drops across the supply impedance and power losses in the supply resistance R. Moreover the power rating of the power distribution system has to be increased above the real power levels consumed by the loads.”
--------------------------

I too am working through the calculations......i am shocked at your findings that energy saved through power factor correction is not as great as i thought.

i wonder why PFC is now mandatory for 25W lighting and 50W smps?....since the PFC stage is going to be max 80% efficient and its going to mean no energy is saved overall?...though as you point out i suppose the distortion is reduced.

also, with PFC being law for 25W lighting....i suspect consumers will just go for two cheaper non-PFC 12W lights instead.

 

[ScottyUK]

Fluorescence,

One reason utilities don't like low power factors is that the equipment has to be sized to carry the load current whether it is reactive or active power. A low power factor load requires that the equipment - transformers, generators, lines, cables, switchgear - is sized to carry the current even though most of the current isn't doing any useful work. The equipment could be either reduced in size and cost if the power factor were improved, or the alternate way to look at it is that the same equipment can supply a lot more active load for the same capital outlay if the power factor is good.


----------------------------------

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

 

[Fluorescence]

thankyou,

though regarding overall efficiency....something appears to be amiss......at university we were always implied upon that Active Power Factor correction went towards energy saving.....due to the ridding of supply system losses caused by reactive currents and harmonic currents..

...but from these posts.....the contrary seems the case......since in above posts it has been calculated that the supply system losses are not that great from reactive and harmonic curretns.

Take an active power factor corrected SMPS....
the PFC and PWM stage will have about 80% efficiency....so that means just 64% efficiency overall....thats terrible...getting like linear regulator.

if we cut out that active power factor stage we end up overall making the whole system more efficient...and thats not what we were told at university.

something seems wrong.......at university we were told (as in my above quote in 29 oct post) that power "shuffles" back and forward due to poor power factor loads....in other words...current shuffles back and forth...meaning that the harmonic and out of phase currents shuffle back and forth in the supply system until they are "used up" by their I^2R losses in the supply system resistance.

I appreciate PFC allows less distortion and smaller supply system components......but am i wrong since i thought the main drive of power factor correction was to save energy and be overall more efficient ?

 

[dpc]

The main driver for power factor correction is to free up system resources for delivery of real power instead of reactive power. The reduction of losses is quite secondary.

Maybe you were a little sleepy that day in class.

At radio frequencies, mismatched transmission line terminations create reflected voltage waves that due result in lost power. But this has nothing to do with power factor.

 

[Fluorescence]

Hello,

The above posts do indicate that putting an Active Power Factor Corrector at the front end of a power supply will actually decrease the overall efficiency. (since as we know the APFC stage is ~80% efficient and so dissipates 20% of the energy passing into it.)

However, the following articles (pse see below) all point out that Active Power Factor Correction actually increases efficiency.

Also, the last article listed here, uses the term "reflection" to describe electrical effects in the 50Hz Power system.

So i guess this theory of harmonic currents and out-of-phase currents "shuffling" back and forward in the power system creating huge losses really is duff ?

List of articles......

"--------------------------------------------

http://www.edn.com/contents/images/46463.pdf

...2nd page (page 32) , top right of page "At a Glance" text box, -see the 2nd point............

"Active power factor correction (PFC)
quashes ac line harmonic emissions and
improves efficiency."
-----------------------------------

http://www.antec.com/pdf/flyers/PFC_energy_efficiency.pdf

...see 5th line down............

..."Active PFC is a method
used in power supplies that cuts electrical waste in the power grid
and reduces overall negative impact on the environment"
--------------------------------

http://www.actapress.com/PaperInfo.aspx?PaperID=33063&reason=500

.....see following....

"Million of computers in the world operate around the hour which makes their efficiency a very critical issue. A trial of improving the efficiency by using power electronics circuits have been simulated and tested. In this case an active Power Factor Corrector (active PFC) has been used."
-------------------------

http://ww1.microchip.com/downloads/en/AppNotes/01106A.pdf

...see 3rd paragraph down , first page.....

"When this type of current is drawn from the
mains supply, the resulting network losses, the total
harmonic content, and the radiated emissions become
significantly higher"
-------------------------------------

http://www.fairchildsemi.com/an/AN/AN-42047.pdf

....page 3, bottom sentence, left hand side of page.....

"Problems are caused by the harmonics
creating additional losses and dielectric stresses in
capacitors and cables...
------------------------------------

http://66.102.9.104/search?q=cache:LDhRk1cxjv8J:

http://www.zvei.org/index.php?id=48...orrection+efficiency&hl=en&ct=clnk&cd=6&gl=uk

...first page....

"Calculations show that in 2007 the power factor correction
systems then installed in Germany reduced
network losses by about 5.5 billion kilowatt-hours."

--------------------------------------

http://www.silverstonetek.com/tech/wh_pfc.php?area=

....last sentence on page.......

"Active PFC power supply can achieve 99 percent power factor; therefore, it can save more energy"
----------------------------------

http://www.silentpcreview.com/article28-page5.html

...see under "Power Factor Correction" heading...
"....reduction of the harmonic content....allows the power distribution system to operate at maximum efficiency, which reduces energy consumption."

---------------------------------------

http://www.edn.com/contents/images/46463.pdf

....With regard to "reflections" in the Electrical Distribution Sytem....

...see page 36, 17th line down, left hand side of page....

"You may also need another choke to help
smooth the dc output rail to further limit
reflections into the ac line."
----------------------------------------------

 

[DHambley]

These numbers being discussed, 64% efficiency for PFC+PWM are just not on par with modern switching supplies. A good PFC should give you over 94% efficiency. A good switcher to step up to high voltage should have better than 85%. Total efficency should be better than 80% overall.

DH