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Transformers from 240 v to multiple 12v luminaires?

Transformers from 240 v to multiple 12v luminaires?

Transformers from 240 v to multiple 12v luminaires?

(OP)
I'm based in the UK and intending to install display lighting to a long narrow exhibition space app 80ft. x 18 ft.
To minimise cost I want to use standard Quartz Halogen 12 V spot reflector bulbs suspended from pairs of catenary stretched-wire conductors by a simple system of my own devising, using brass double-screw connectors of various sizes (cut out of the ordinary polythene connector blocks that you can buy in strips anywhere)- the first pair of these are the largest size available, and clamp L-shaped brass rods (3mm brazing rod)to the stretched conductors; the next pair connect short, flexible - but thick - copper wire links to the ends of the brass rods; and the third pair connect the copper wires to the pins of the spot-bulbs.

This way I can manually twist the spots to point in any direction I choose - and they'll stay there - without the expense of a fancy shop-bought connection system - with 4 different spotlight spread angles available this gives me a very versatile system.

The conductors will be either high strength braided brass  picture-hanging wire - the stuff they use in public art galleries to hang big pictures -or if that turns out not to be strong enough, or too stretchy if it heats up with the current, stainless steel rigging-wire - the high-conductivity stuff that they use to double for radio aerials on yachts.

There will be perhaps 100 of these spots at up to 50 Watts each,in perhaps 2 - 4 switched groups - or possibly ALL fed off one 12v supply if it's feasible, so I need a transformer from 240 v mains, capable of carrying up to perhaps 400 amps load in the 12 v secondary (ouch - sounds a lot! well- maybe 200 amps, or 100 amps for 4 groups of 25 lamps)The transformers that are specifically designed for use with 12v lighting typically can only cope with 3 or 4 lamps at most, so I need something industrial:

Questions (I'm not an electrical engineer!):
 
1) Is this a sensible proposition at all?

2) Where can I source such a transformer in the UK?

3) Am I right in thinking that, for transformers, bigger=more efficient?

4)Air-cooled? Oil-cooled??

5) Is there a welding transformer with as low as 12V output that might do the trick at lower cost?

6) Does anyone know of a source of high-conductivity + high tensile strengh catenary cable designed for this application?

7) will the catenary wire melt with these currents? I could always run solid copper bus-bars parallel with the catenaries and cross link at intervals - but now it's getting silly!!

- Those are my practical questions - now for some theoretical ones that have occurred to me as a result of puzzling over this:

8)With alternating current, the voltage, hence current, drops to zero every half-cycle(?) 100times? 50times? per second; so the temperature/luminosity of the filament of an incandescent bulb is constantly going up and down - the bulb is 'out' for part of each cycle. (Hence 50 Hz stroboscopic effects).
Would, therefore, direct current at the same 240 voltage produce more perceived light? Would the filaments, spared the presumed stress of constantly heating-up and cooling down, last longer?
(None of my electrical engineer acquaintances has been able to answer this one - but they're all high-voltage distribution-grid specialists.)

9) Supposing direct current WOULD result in greater efficiency in converting electrical energy into light, and/or a longer life for the bulb filaments, would the losses involved in having a large power rectifier in the circuit negate any advantages? (not to mention the capital cost of the rectifier!)?

9) If NOT, and there WOULD be significant advantages to DC, where can I get such a rectifier, to match the transformer?

Any advice and comments gratefully received - any rebukes for naive stupidity humbly borne.

Dick Q

RE: Transformers from 240 v to multiple 12v luminaires?

Hi Dick

I also want to connect up a large number of 12V halogens without the expense of hundreds of expensive transformers - Please see my post about series wiring 12V lamps, it may give you something to think about as an alternative solution... but be aware I'm not yet sure it will work :) and you couldn't use bare wires.

I don't think any thin catenary can carry 100A plus!! (without voltage drop, and the wire perhaps getting hot enough to give out more light than the fittings - I think you want white light, not bright orange!!

Well, not unless it's the kind of steel rigging wire you'd find on the QEII <G> - above 10-16 square millimetres cross section.

I doubt such a big 240:12V transformer exists, and if it did, it would be extremely expensive, heavy and probably noisy.

BTW, yours is a fascinating question about the effects of AC or DC on lamp filaments.  I hope someone answers that!

Best regards and good luck,  Mark




RE: Transformers from 240 v to multiple 12v luminaires?

(OP)
Mark - thanks for your interest (none from anyone else, it seems!!) I understand your comments about the currents involved (it had sort of dawned on me as I formulated the question that the single transformer idea was a non-starter,anyway!)

I'd still like to get some informed opinions about the AC/DC effects on perceived light intensity and filament life - it's one of those things that nobody ever queries.

Dick Q

RE: Transformers from 240 v to multiple 12v luminaires?

I can help some with one of your questions about transformers. If you are stepping down from 240 to 12 volts then the transformer ratio 240/12 =20. If you need 400 amps out of the secondary then this means you need a 240 volt supply capable of supplying (400/20)=20 amps (with no cushion built in). The transformer VA rating needs to be 12*400=4800 VA or 4.8KVA. This is not anyhting out of the ordinary except for the secondary voltage. Hope this helps some.

RE: Transformers from 240 v to multiple 12v luminaires?

I don't believe that DC would give more light than AC.  AC voltages are usually given in RMS values, which means that they deliver the same power as a DC system of the same rating, and that the peak voltage is higher (typically by .707).  The elements in the bulb do indeed see zeros in voltage, but the thermal characteristics do not respond as fast as 50HZ, so the bulb effectively stays hot and produces light even at the zeros.

RE: Transformers from 240 v to multiple 12v luminaires?

Current new electrical regulations in South Africa restrict the ratings of transformers for 12 V lighting systems to 200 VA. This is for fire and safety reasons. There is also an overload trip set for 1.6 times of this rating and an over temperature trip for 80 degrees Centigrade. I should imagine that the UK regulations are simlar or stricter.

RE: Transformers from 240 v to multiple 12v luminaires?

Lookslike you need to engage service of a good lighting designer (not archietct alone). By your admission you are not an electrical engineer, then hire one!  How would you feel about an electrical engineer doing the job of your profession?

A few phone calls to local lighting reps and referrring to lighitng catalogs will provide you plenty of options.

All 12 lamp and luminair manufacturers offers  a series of light fixtures with 12V tranformers that are either built in each head or one common xfmr for a group of light(remote but mounted close to the ligting track or group of fixtrues).

This are standard accesories offered for low voltage lighting, do not reinvent the wheel.

Even by any engineering principles, you want to bring 240V service as close to the load (lamps) as possible before transforming to 12V.  Your desing sounds like a clap-trap with all due respect. I am sure you are good at what you have expertize in.

Trying to light  a 'exhibition' space and then skimping on lighting trasnformer cost  or Do-it-yourself concept sounds oxymoroic.

RE: Transformers from 240 v to multiple 12v luminaires?

This thing you are proposing may electrically work but is is safe? I don't think so. Try using the expensive accepted proven products or you may cause a lot of damage or a fire. Quartz Halogen lamps are hot and can cause fires.   

RE: Transformers from 240 v to multiple 12v luminaires?

I'd strongly recommend using the typical small 12v transformers for several reasons, including voltage drop, safety, and reduced wiring expense.

RE: Transformers from 240 v to multiple 12v luminaires?

in reply to q8 (i think), i'm not sure about halagen bulbs but tungsten filiment bulbs do have a shorter life due to an AC voltage, this is because the tungsten evaporates under extreem heat, this evaporation isn't even, therfore causing parts of the filiment to become thinner than others, making parts of the filiment to heat up more because it has a higher resistance (current traveling through thinner sections has a higher resistance). when bulbs are turned on it causes a power surge, this is because it needs more energy to light it up than it does to keep it alight, this power surge causes the tungsten to evaporate quicker on the thinner parts of the filiment and therfore break in a shorter time. a bulb run on DC for longer.

RE: Transformers from 240 v to multiple 12v luminaires?

As I understood things with lamps running on DC - certainly higher voltage lamps - there is a general migration of metal ions under the influence of the electric field between the two electrodes, and the metal is gradually removed atom by atom from one end of the filament, which ultimately breaks. Have I been misled by the physicists?

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