Richard Hammond said wiring to bulbs gets hotter when they're dimmed.

[Dave W]

I've just watched "Richard Hammond's Engineering Connections" on BBC2 about a new skyscraper in Dubai. Some expert told him that capacitors and inductors were installed to smooth out the chopped up current from the lighting dimmers, to stop the wiring overheating. We were treated to a ridiculous demonstration in which hot wires ignited cotton wool soaked in inflammable liquid, producing a spectacular destruction of a wooden shed in a field.

My whole working life has been spent in electronics design, but I have never heard of such a heating effect, and I still cannot understand what it was all about. Can anybody provide a convincing technical explanation?

Dave W

[alexander.keys1]

It's called Power Factor Correction, dimmers and many other appliances, computers are bad for this too, draw a current that is out of phase with the voltage, resulting in excessive 'wattless current' flowing in the wiring, the supply mains, and the transformers, if capacitors are not fitted near the load to absorb the 'reactive power'.

[cynic]

The whole shebang was an exercise in stupidity. There really ought to be a law against lettiing the unknowing spout their drivel in the public media.

Where did they get their "expert" as if he had any sort of professional status his learned society ought to take him to task.

[Dave Plowman (News)]

The public, apparently, won't watch anything without special FX or stunts. So this sort of prog has to work one in as a 'demonstration' regardless of how tenuous the connection.

[Brian Morrison]

It also has to be narrated by someone in a breathless, excited voice by someone who the masses are familiar with.

Please can we bring back the old 1970s Horizon delivery, with equations (and people with beards!).

[Serendipity]

The whole shebang was an exercise in stupidity. There really ought to be a law against lettiing the unknowing spout their drivel in the public media.

Where did they get their "expert" as if he had any sort of professional status his learned society ought to take him to task.

"Ex" is a has-been, and "spert" is a drip under pressure!

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember cynic saying something like:

In all probability, the 'expert' did actually give some sort of feasible explanation but it went over the luvvies' heads entirely, which left the door open for a whizz-bang demo of burning stuff.

Either that, or the 'expert' was just a bloke down the pub, who one of the producer's mates spoke to one drunken night.

[Brian]

Or at least Raymond Baxter :-)

[JNugent]

Correct.

The supply boards used to charge extra for energy supplied to commercial and industrial custonmers if the supply was "seeing" a highly inductive or highly capacitive load. PFC (by placing a massively cpacative or massively inductive load across the supply) was a way of getting he power demand back more nearly in phase (and thereby lowering the PF surcharge).

[The Natural Philosopher]

Its quite correct. The model aircraft boys have found that about 80-90% throttle is the absolute worst case for controller heating.

I tried to analyse it but there were too many imponderables with an inductive load with a variable back emf. The bulb case is much simpler to analyse.

The bulb will cool, and therefore peak currents will be higher. How MUCH of that peak is NOT smoothed by the interference choke is the issue, but there's bound to be most of it.

Heating is I^2 * R *TF where R is the on resistance of the controller and TF is the on time duty cycle.

as I gets bigger with falling output a lot faster than the time interval its on gets smaller, there is definitely a peak at less than full power.

With motors, is that fact you are sending full voltage in bursts to a motor that is not generating enough back EMF cos its too slow..instead of the bulb not being hot enough, that causes the peak currents to rise.

[Brian Morrison]

That was TW though wasn't it? I don't recall him doing Horizon...

[Lordgnome]

Not so, not so. There are a majority of folk around who would much prefer sensible programmes, particularly the sort where you can actually hear the speech above background muzak.

The British Biased Company even admit to constantly getting mail asking them to desist - but of course those "clever" artistic types know what the public should get and nothing changes.

Les.

[Old Codger]

From where does the heat come if it is "wattless" current?

[Steve Walker]

Often referred to as VARs (Volt-Amps Reactive I think).

And fluorescent lighting - some of which can be dimmed of course.

Or by the use of synchronous motors, which can appear electrically to be capacitors.

SteveW

[Brian Morrison]

Losses in the conductors due to the increased peak current flowing? So it's "wattless" not wattless.

[Roy Gbiv]

"Steve Walker"

Na, volts amps resistance.

[Ian Jackson]

The 'wattless' current produces no power in the device it is feeding. However, it still produces 'I squared R' loss power loss (and 'I x R' voltage drop) in the conductors in which it flows.

[The Natural Philosopher]

although in this case it produces power in both..

[Tim Watts]

I don't think I've seen an equation on TV for >20 years.

Wow...

[Steve Walker]

No, VA would refer to a pure resistance. VARs are the reactive load of having current and voltage out of phase due to the inductive or capacitive effects of the load.

SteveW