Last nights DIY SOS

[stef]

afternoon chaps,

has anyone seen last night's DIY SOS, the episode with the eco house? They used a mains step down unit
to lower the power consumption (by lowering the supply voltage) to 220 V. This unit seemed to me very light
weight so it can't be a step down transformer so I assume it must be a MOS FET stack which is triggered
by a sine signal. A switch mode PSU would be inappropriate for a mains supply.
Has anyone any experience with such a unit? (efficiency, price, noise, heat)

TA

 

[Adam W]

What can anyone tell me about "inductive voltages"? I remember something about Halogen spots pulling more current than standard lamps of the same power output from college. Also the same for transformers. Anyone offer any information on this?

Do you mean inductive loads?
If you pass a current though a coil of wire it produces magnetism. If that's an ac current the magnetic field will move around, which is how a generator works. In this case in effect it works against the power being fed in (back emf, or 'Lenz's law').

 

[Dunc]

Those v phase things only have small befits when used with resistive loads
For that benefit to be of any use the distribution board would have to have the circuits segregated to supply only the resistive loads
At 300 or so a shot,it would take an eternity to recoup the outlay.but sales and marketing can do wonderful things for such drivel

If you have a look at the wiring diagram form a V Phase unit you'll see that you've really got to play about with the consumer unit to get it to work. You have to feed the V Phase box from a 50A MCB then send a 10mm T+E back in to the consumer unit to feed all your socket and lighting circuits. Apparently it can't handle sustained high loads and if it gets warm it will shut itself off and bypass all this wonderful technology. The one place I can see a use for it is in the countryside where I've seen some high and fluctuating voltages which a lot of equipment doesn't like.

 

[Adam W]

so in that case appliances wont work at their rated power output? there is a fair difference there with only a 25v change

That example I gave is a bit over-simplistic and in fact only actually refers to a dc circuit.
As Des says for an ac circuit you'd also need to consider Z to get a more realistic picture; nearly all appliances have inductive loads which makes the idea seem less attractive.

 

[telectrix]

so, basically it's another sinclair C5.

 

[ukpablo]

i fitted one of these the other day! seemed pretty straight forward????!!!!
all powered up ok, however im waiting for the phonecall at some point saying that board is tripping, ccts being lost etc! i would have to say that it did not fill me with confidence....................all seems too easy IMO? Have told customer to keep link cables for CU handy so it can be put bak to normal if required. Also it makes u think on how u have to split the load in the CU!

 

[Engineer54]

These Vphase units are a waste of time and money, you would be better off buying LED lamps for every light fitting in the house. You would be saving more money on them, than with one of these units, and getting a return/pay-back a dammed sight quicker too.

The high consumption loads you want to be reduced, and would make sense reducing, the Vphase unit can't handle and goes into by-pass. With resistive loads (eg, ...kettle) it will do nothing for, just takes more time to heat up, so your saving nothing, just be waiting longer to have a cuppa!! Don't give them a second thought, is my advice...lol!!!

 

[Guest123]

These Vphase units are a waste of time and money, you would be better off buying LED lamps for every light fitting in the house. You would be saving more money on them, than with one of these units, and getting a return/pay-back a dammed sight quicker too.

The high consumption loads you want to be reduced, and would make sense reducing, the Vphase unit can't handle and goes into by-pass. With resistive loads (eg, ...kettle) it will do nothing for, just takes more time to heat up, so your saving nothing, just be waiting longer to have a cuppa!! Don't give them a second thought, is my advice...lol!!!

It seems we can agree on something!

 

[Engineer54]

Haha.... We may not agree on TT systems and relying on RCDs, ...but i'm sure there are many other things in our industry we Can agree on!! lol!!

 

[ukpablo]

i agree, would not reccomend, its what the customer wanted and were supplying, so i just did as was asked. Plus takes up more wall room too!

 

[mike2790]

even slight changes in voltage affects cable calcs and cable sizes, this is why electricity is distributed at a high voltage to reduce current and therefore reduce cable sizes. reduce the voltage and current goes up, thats just the way it is, so am i right in thinking that you couldnt just chuck one of these units in on any installation as some cable csa's may not be suitable.

 

[Des 56]

If you think of a shower running on 240 volt with a particular rating and a set resistive load, it draws a set current
That same shower on 230 volt with the same resistive load will now draw less current

V=IxR voltage drops so resistance or current has to change
Resistance is set so current must drop to balance that formulae

 

[Adam W]

reduce the voltage and current goes up, thats just the way it is,

If you connect two 240v light bulbs in series with a 240v supply they will be working on 120v. They won't start using more current to compensate for the reduced voltage, they'll just be dimmer.

 

[mike2790]

as you can see from the calcs below if you reduce the voltage, the amperage goes up, meaning potentially cables with a larger CSA being needed.

7500w shower
230v supply
I=P/V

7500/230 = 32.06A

7500w shower
220v supply

7500/220 = 37.5A

all i was getting at with reducing the supply voltage, is that in certain cases, original cabling which was designed for 230v would change the charactaristics of the circuit design in respect that cable CSA's may no longer be large enough? or am i just getting the total wrong idea? i'm not familiar with these reduced voltage units as you may have guessed im just trying to gain a bit of info just incase some eco maniac wants me to install one of these in the near future.

for anybody who has experience installing these, is there anything out of the ordinary you may have to take into consideration when installing one?

cheers.

 

[Des 56]

Mike you are assuming that the KW rating remains constant,that is not so
The reduced current times the voltage now present will give a different KW rating
W=VxI

 

[telectrix]

agree with des on this one. reducing the voltage reduces the power ( kW ) for the same current, so IMO a complete waste of money that would be better spent on switching things off and going down the pub.