Kitchen lighting

[MdB]

If the dimmer is using just using half of each half cycle (first and third quadrants) then the current will be lead the voltage by 30 degs or so. Not as much as 90 degs that you will get with an inductive load but there will be a PF change. Do a FT of the current waveform and you will see.

My guess is that domestic meters don't measure VArs as the units written on the front of most is kWhrs rather than kVAhrs.

 

[Flyfisher]

If the dimmer is getting very hot then perhaps it's an old-fashioned variable resistor, in which case the 'lost' light power may just be dissipated by the dimmer and therefore there would be no overall power saving (unless anyone wants to argue that the heating effect means that energy can be saved by reduced space-heating :wink: )

Modern dimmers might get slightly warm (because nothing is ever 100% efficient) but I doubt it's a significant amount of power, certainly nowhere near the (roughly) 30W saved by dimming a 60W lamp to half power.

 

[Flyfisher]

If the dimmer is using just using half of each half cycle (first and third quadrants) then the current will be lead the voltage by 30 degs or so. Not as much as 90 degs that you will get with an inductive load but there will be a PF change. Do a FT of the current waveform and you will see.

Well, I admit to being out of practice, but I don't see how a purely resistive load like a filament lamp can cause the voltage and current to be out of phase. Maybe a filament lamp is not purely resistive?

My guess is that domestic meters don't measure VArs as the units written on the front of most is kWhrs rather than kVAhrs.

I think this is correct, which is why the electricity suppliers (and various electrical standards) insist on power factor correction to a lesser or greater degree, which eliminates/reduces the reactive power (KVAr) to an insignificant level so that only the active power (KW) is significant.

Why is nothing ever simple?

 

[Lime]

My guess is that domestic meters don't measure VArs as the units written on the front of most is kWhrs rather than kVAhrs.

The standard mechanical single phase domestic tariff meter is unable to measure leading or lagging current at all.


As a matter of interest recently I purchased from Maplin a Plug-In Mains Power & Energy Monitor which measures voltage (V), amps (A), watts (W), volt-amps(VA), hertz (Hz) and power factor (PF) and a few other things.
http://tinyurl.com/w78kh
It is able to monitor electricity used over time so total power consumption can be found.

It surprised me how some appliances varied from one another.

as far as dimmers are concerned the power factor can vary depending on load and also on the reactance of the dimmer.

 

[bournonite]

I wonder if any of you guys can tell me why my meter suddenly jumped from a reading of 14136 to 74140 - seems they might want a sum of around £20,000 for a months power. Might be due to a transformer blowing in the field next door that had the power down for days - anyone heard of meters suddenly going mad??????? We hooked the generator on in the end - I dont know if that turned it backwards or something - powered the house ok...

Pete

 

[Penners]

Oh cor blimey!! I thought I was asking a simple question - so simple, in fact, that I was almost embarrassed to ask it!

The answer isn't actually that important, chaps. I was only asking because it's a question that has occurred to me, from time to time. However, thank you all for going to so much trouble with your answers.

Brad - I do apologise for diverting your thread up a highly esoteric branchline!

 

[biffvernon]

Our meter exploded when it was hit by lightning.

 

[NT]

I think most of these qs are answered by a few of the articles on the wiki page, specifically the articles:
halogen lighting
fluorescent lighting
Rewiring tips
Bad Ideas lighting

Halogens under kitchen cabinets will reflect off the worktop to produce glare. They also provide hotspots whch isnt good for food storage. They also produce a room temperature rise in already hot summers.

Fl can produce much better light if and only if decent quality tubes are used. The article explains it all.

I would basically overloook the fact that you've bought already, as if you change away from halogen the savings over product life will be over 10x the cost of another set of light fittings, so its not an unnecessary expense, more an optional high return saving.

I'd recommend a switchbank whatever you put in. If the wallswitch wiring isnt there for one and it cant practically be put in, a switchbank close to the lights is next best.

2.2m is high enough for some uplighting if its what you want.

The large cat 2 fluorescent downlighters would suffer some of the same problems as any other downlighter, but certainly arent as bad as halogen in some respects. I can see those looking right in a minimalist futuristic building, but a cottage??

Theres lots more in those articles, I think they pretty much cover it all.


NT

 

[Brad]

No problem Penners. I got lost long ago........!!!!

 

[Brad]

Got lost. Now I'm found! Thanks NT - will look at the articles.

 

[Flyfisher]

Oh cor blimey!! I thought I was asking a simple question - so simple, in fact, that I was almost embarrassed to ask it!

That's the thing about simple, or apparently simple, questions - never be afraid to ask. I guess most simple questions can be answered as yes or no . . . . but - and it's the 'but' that we seem to be having some fun with (if you'll pardon the expression :shock: ).

I've revised a bit more forgotten theory and think I the full answer lies in harmonic distortion of the mains sine wave. I believe I was right about an incandescent lamp being purely resistive, so current and voltage remains in phase and there is no power factor involved (or, more correctly, the power factor is 1).

But I missed the point that this is only true for a pure sine wave . . . which (I think) is the point that MdB was getting at regarding the effect of the dimmer switching circuit, which effectively distorts the normally pure sine wave, making the current and voltage out of phase, which is the definition of power factor in the first place.

It is possible to implement power factor correction circuits, though, and I know that recent EU legislation introduced a Power Correction Directive a few years ago because it affected the choice of switched-mode power supplies in some projects I was working on at the time. Anyway, I checked the EN61000-3 standards and it seems that domestic light dimmers are exempt, presumably because of their low power, so I assume that no power factor correction is actually included in light dimmers.

So where are we with this simplest of questions?

1. Lamp dimmers that chop the mains AC waveform (other types are available but let's not go there!) introduce harmonic distortion to the waveform, giving rise to active power (KW) and reactive power (KVAr) - even though the load (the lamp) is purely resistive.

2. The power company has to supply both the active and reactive power but, if Lime is correct, the domestic meter can only measure the active (KW) power.

3. Industrial installations are required to implement power factor correction in order to reduce the amount of reactive power required and recent EU legislation has required the increasing number of reactive domestic applicances (mainly those powered by switched-mode power supplies, e.g. PCs, TVs etc) to implement power factor correction in order to reduce the reactive power load on the grid - which can be a big problem for the power companies.

So (I think!), if you use a typical modern lamp dimmer to halve the power to a filament lamp, it may actually draw more than half its rated power because of the power factor effect, but you'll only have to pay for half its rated power because the reactive power component can't be measured by the meter.

So I'm sticking to my original answer of "Yes . . . . probably (!)" :wink:

Phew! Anyone care to pick the bones out of all that?


OK Penners . . . let's have another embarrassingly simple question :lol:

 

[Nemesis]

Penrose - don't you dare!

 

[Brad]

Halogens under kitchen cabinets will reflect off the worktop to produce glare. They also provide hotspots whch isnt good for food storage. They also produce a room temperature rise in already hot summers.

Fl can produce much better light if and only if decent quality tubes are used. The article explains it all.

I would basically overloook the fact that you've bought already, as if you change away from halogen the savings over product life will be over 10x the cost of another set of light fittings, so its not an unnecessary expense, more an optional high return saving.

I hope I've done this quato properly. I'm a new kid on the block.

I'm probably being a bit thick here but what you are saying is that good quality tubes will last longer - yes?

So can you recommend decent quality tubes?

 

[Brad]

Well I obviously didn't do the 'quato' properly!!! But I think you can get the drift..........

 

[MdB]

... long post just above ...

Yep, agree with all of that.

... perhaps it's an old-fashioned variable resistor, in which case the 'lost' light power may just be dissipated by the dimmer and therefore there would be no overall power saving ...

Not quite correct, the overall power does go down a bit as the pot is turned if you do the sums correctly. It is very inefficient though and will get hot.

I wonder if any of you guys can tell me why my meter suddenly jumped from a reading of 14136 to 74140 - seems they might want a sum of around £20,000 for a months power.

We managed to get our one at work to stop reading anything for well over 6 months and it could have been 9. It was spotted after 3 months (two identical meter readings) and then took SWEB ages to get it replaced. Never charged us anything for our usage during that time. No idea what caused it to stop though.

I also had a mate who told the story of his parents house having all the local street lights connected to their domestic meter. Nobody believed this was possible until they demonstrated it by switching off all the street lights using their master switch. Apparently they had some interesting meter readings.

 

[biffvernon]

Nobody believed this was possible until they demonstrated it by switching off all the street lights using their master switch.

Gosh, the power. I'd love to be able to switch off the streetlights. It'd be worth paying the bill.

 

[NT]

I hope I've done this quato properly. I'm a new kid on the block.

You did, but I reckon you had the 'disable bbcode' box ticked.

I'm probably being a bit thick here but what you are saying is that good quality tubes will last longer - yes?

No, the difference is about light quality, not tube life. Some fl tubes are first class, and some are vomit inducing, some are so-so.

So can you recommend decent quality tubes?

Yes. For a more useful answer see the linked articles.


NT

 

[NT]

> I think most of these qs are answered by a few of the articles on the wiki page

I posted that without checking to see there had been more written. There seems to be a good deal of confusion in this thread so far.


An incandescent lamp at half brightness is consuming most of full power, not half. Lamp... no I've not got it on this computer but IIRC Light_Output is proportional to Power_consumed to the power of about 13. IOW nothing remotely like a linear relationship. Dimming incandescents is the worst way to reduce brightness.

Domestic meters read watts not VA.

The colour temperature of an incandescent bulb shifts a lot with dimming. To see the difference, put 220v and 240v bulbs side by side on 240v. Or a 40w bulb and a dimmed 150w set to same light output.

Normal dimmers dont produce reactive loads, v and i always stay in phase. It is of course possible to use reactive elements (capacitors or inductors) to dim lighting, but this is not done in domestic dimmers.

Filament lamps are resistive devices with a tiny amount of inductance. But this is not enough to have any visible effect on power factor at chopped mains frequencies. One would have to go high into the rf band to observe such an effect.

Plug in testers are not always accurate. Some are in fact very inaccurate with some loads.

If a meter jumps by 6000 units, either it was misread or is faulty. Or perhaps nothing but cinders remains of the house.


NT

 

[Flyfisher]

There seems to be a good deal of confusion in this thread so far.

I thought we'd finally cleared it all up . . .

An incandescent lamp at half brightness is consuming most of full power, not half. Lamp... no I've not got it on this computer but IIRC Light_Output is proportional to Power_consumed to the power of about 13. IOW nothing remotely like a linear relationship. Dimming incandescents is the worst way to reduce brightness.

Dimming a lamp is, by definition, the best way to reduce brightness - I guess you mean it's not necessarily the most power-efficient method (pedantic, I know).

Domestic meters read watts not VA.

We'd agreed that.

The colour temperature of an incandescent bulb shifts a lot with dimming. To see the difference, put 220v and 240v bulbs side by side on 240v. Or a 40w bulb and a dimmed 150w set to same light output.

MdB had already made that point quite clearly.

Normal dimmers dont produce reactive loads, v and i always stay in phase.

We went through that discussion and concluded it's not the load (lamp) that's reactive and causing the power factor issue, it's the harmonic distortion of the mains sine-wave caused by the dimming circuit 'cutting out' portions of the sine-wave to reduce the power delivered to the (purely resistive) load. V and I do go out of phase as a result of this harmonic distortion.

Filament lamps are resistive devices with a tiny amount of inductance. But this is not enough to have any visible effect on power factor at chopped mains frequencies. One would have to go high into the rf band to observe such an effect.

We already agreed that as well (OK, we didn't stray into RF territory - thank goodness!)

So I'm not sure where the outstanding confusion lies.

 

[Lime]

Nobody believed this was possible until they demonstrated it by switching off all the street lights using their master switch.

Gosh, the power. I'd love to be able to switch off the streetlights. It'd be worth paying the bill.

:lol: :lol: :lol: :lol: :lol:

Very good!