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Topic Title: pf of fluroescent tubes
Topic Summary: Is it a myth that fluroescent fitments can compensate for poor inductive pf?
Created On: 16 February 2009 02:23 PM
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 16 February 2009 02:23 PM
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TCSC

Posts: 175
Joined: 25 April 2007

Somebody told me recently, that it was possible to use fluroescent fittings to compensate for a poor inductive pf caused by motors etc. that were not compensated with capacitors. This did bring up some vague memories from my college days (60's!) that this was the case.

However, looking at present day fluroescent fitments, I don't see how this could happen, particularly with there being a choke in the tube circuit. There are largish capacitors across lamps to either compensate for pf or perhaps for other purposes.

Is anybody able to throw any light on the matter? Is the use of fluroescent tubes to compensate inductive pf an urban myth?
 16 February 2009 02:59 PM
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rocknroll

Posts: 9549
Joined: 03 October 2005

"The test of a true myth is that each time you return
to it, new insights and interpretations arise."

Why dont you draw out an RLC circuit and a phasor diagram, throw in a few figures and you will often see that myths do really come true.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 16 February 2009 04:49 PM
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TCSC

Posts: 175
Joined: 25 April 2007

The only accessible example of fluroesecent fitment I have has a cap of 3.5 microF, but I don't yet know what the inductance of the choke is.

I have a cheap Maplins pf meter. This shows a pf of 0.91 with the C in place but I don't know if this is leading or lagging pf. If I remove the C, the pf falls to 0.51.

However this is just one of several types of fluroescent tube circuit. I am not fully conversant with the other types of tube starter / run circuitry.
 16 February 2009 05:54 PM
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OMS

Posts: 22426
Joined: 23 March 2004

Are you thinking of the old lead/lag type luminaires which were essentially "self compensating" for inductive power factor ?

OMS

-------------------------
Let the wind blow you, across a big floor.
 16 February 2009 07:14 PM
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normcall

Posts: 8520
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I used to use a lot of those 6ft 'twin' single fittings in shops - sold as matched pairs, Thorn PP675 if I remember correctly.

-------------------------
Norman
 16 February 2009 07:21 PM
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hertzal123

Posts: 472
Joined: 26 August 2007

I can,t see how caps in florrys will affect another circuit,since they are only rated to correct the pf of the florry.
regards,hz
 16 February 2009 08:45 PM
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TCSC

Posts: 175
Joined: 25 April 2007

Originally posted by: hertzal123

I can,t see how caps in florrys will affect another circuit,since they are only rated to correct the pf of the florry.

regards,hz


That is what I was thinking, unless the C had to be larger than required for simply compensating pf. In this case my question was "is the C only there for pf, or something else?"
 16 February 2009 09:01 PM
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TCSC

Posts: 175
Joined: 25 April 2007

Originally posted by: OMS

Are you thinking of the old lead/lag type luminaires which were essentially "self compensating" for inductive power factor ?



I have been looking at these on the web and maybe they are what I was looking for.
Cheers
 17 February 2009 09:55 AM
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OMS

Posts: 22426
Joined: 23 March 2004

I guess what you are talking about is the old series capacitor-choke ballasts - in rough numbers, the capacitor alone at 50Hz won't work but if you series connect a capacitor and a choke so the capacitive reactance is approx double the inductive reactance you can run a fluorescent tube (the were best suited to "longer" tubes such as 6' or 8')

As a single unit, this will give you a leading power factor of around 0.7 (and has the advantage that the current is pretty constant so voltage changes don't result in increased/decreased lumen output).

As Norman suggested, you could mix them up with choke ballasts (in a lead/lag fashion) to give a lighting installation around unity or use all leading fittings to compensate for motors etc within an installation - which is your original statement.

Obviously, this is now "old skool" - HF compensated ballasts are the norm today

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 20 April 2017 07:51 PM
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kellyselectric

Posts: 186
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I went I to a factory recently and noticed that a number of the twin tube fittings had only one side lit I assumed it was because of faulty lead caps I just thought it looked a bit odd but reading this could this have made the whole install have a really bad lagging PF? Also the fittings weren't that old maybe 10 years at most
 20 April 2017 09:44 PM
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mapj1

Posts: 9670
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To further the general education (!) It it perfectly possible to size a capacitive dropper in series with a tube to drop the voltage on the tube to the 90-100V or so needed when running and have no inductor. There was a brave plug in unit sold that did exactly this with a bayonet base that went into a conventional lamp holder and held up a 40 watt 4 ft tube of the old 38mm diameter, a flash starter and what must have been half a dozen microfarads or so of rolled waxed paper capacitor.
By being choke-less it was light enough to hang off the bayonet lamp fitting - well presumably the one the designers tested it on anyway. Perhaps they had brass fitting, because after a bitter experience, involving broken glass, lots of apologies and a broom later, it became clear it was not so good if suspended from ageing Bakelite.
Then after a few weeks the other design weakness became apparent. Unlike the choke version, in this one, when the lamp is first turned on, and the capacitor is flat, but we are near the top of the voltage waveform when the starter fires, there is very large inrush, which leads to a short and unhappy life for the tube filaments, and the starter too, as they see full mains voltage between them for the first part of the cycle until the capacitor fills up.

Even in the more common series LC variant, the tubes are likely to blacken at the ends for the same reason, rather faster than tubes from the same batch fitted at the same time to fittings with a traditional 'proper' L-match, with the inductive dropper in series losing all the volts for you, and then the C just straight across the mains to pull the PF back to being reasonable.
So the answer to the OP is 'maybe' but only if the light fittings were specially set up with this in mind (which you don't normally do).
In such a case proper capacitors near the inductive loads may be a better bet, as there is a lot of risk associated with large numbers of out of phase circulating amps looping from power circuit back to the board via various light switches to lights that may be switched while the load is present.

In effect with the PFC you are creating a parallel L-C tuned circuit, in a bid to get maximum impedance at 50Hz (which occurs when the imaginary part is cancelled out as far as possible), with half of it , the L in the motors and the other half, the C, in the light fittings - co-location, or at least the same final circuit, is far more normal.

-------------------------
regards Mike
 20 April 2017 10:35 PM
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broadgage

Posts: 2448
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A standard fluorescent lamp circuit with a series choke has an unavoidably lagging power factor.

A less common circuit with a series capacitor had a significantly leading power factor.
As already posted, it was common practice to install equal numbers of leading and lagging fittings so as to produce a near unity overall power factor.
Alternatively and fairly unusually an installation might be specified with all leading power factor fittings so as to compensate for lagging power factor elsewhere in say a factory.

The leading power factor fittings never much favour because they were slightly more expensive, shortened the life of the lamps, and seemed to be less reliable.

BTW, I think the recollection regarding the fluorescent fittings that plugged into a BC lamp holder may be mistaken.
I never saw one with series capacitor. All the examples that I have ever seen used a series resistance, a length of insulated resistance wire along the length of the fitting.
This was not very efficient, the commonly used four foot, 40 watt lamp wasted as much as 60 watts in the resistance for a total consumption of about 100 watts. Still gave a lot more light than a 150 watt lamp.
And they worked on DC mains !

Similar fittings are still made today but with electronic ballasts.
 20 April 2017 11:03 PM
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kellyselectric

Posts: 186
Joined: 22 July 2016

I did some experiments with 2 and4 foot fluorescent lamps and a circline lamp using a bulb as ballast instead of a choke and found that starting was a little slower and the tube wasn't as bright although when I changed the 100 watt bulb to a 150 watt one it did improve things. Interestingly according to my plug in meter the PF was still laying a bit even though there was no inductive ballast
 20 April 2017 11:26 PM
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mapj1

Posts: 9670
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Well there was at least one with only a capacitor and no choke or resistor - I had the misfortune to fit the thing, then unfit it, and replace the broken BC fitting and revert to a normal lamp. So cross with it I took it to bits.
Pifco I think. Horrible idea.

I have also seen a design with a resistor dropper, in the form of a filament lamp - and I have been known to patch in a BC lamp holder to replace a blown choke with a bulb in extremis once for a few days until the spare part came..

-------------------------
regards Mike
 21 April 2017 10:11 AM
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John Peckham

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I had to carry out some practical assessments for Level 2 installation students a couple of years ago and the college test rig I was given to use was at best "cream crackered". The rig was a fluorescent fitting that the students had to measure current and voltage in different parts of the circuit. I made up a nice new rig in my home workshop inserting sockets for the voltage measurement and sheathed insulated loops for the current measurement so to minimize risks for the budding electricians. There was also a switch to switch in and out the PF correction capacitor to take measurements with it in and out of circuit. It is still in use screwed to the wall in the assessment room.

I now take the lids off LED fittings to take loop impedance readings only to find an empty box with no gubbins inside other that a connector block! I wonder what this will do to the PF when the number of fittings increases to more that the inductive loads? Will the PF correction units on big supplies get confused?

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 22 April 2017 09:10 AM
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Jaymack

Posts: 5377
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Originally posted by: TCSC
Somebody told me recently, that it was possible to use fluroescent fittings to compensate for a poor inductive pf caused by motors etc. that were not compensated with capacitors.

I seem to remember that PF capacitors only compensated for downstream equipment.

Regards
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