Which Residential Voltage & Frequency Arrangement Is Best?

Andy Dingley wrote:


Sometimes I forget that normal people have no need for the types of motors I'm used to. Since I'm mainly concentrating on industrial construction right now, my mind kind of kept going with everything I'm familiar with.
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sanjian wrote:

Not sure they are actually.
Anyway, you can make a three phase inverter for AC motors to run on DC fairly cheaply.
My real point was that with the advances in semiconductors, making AC from DC and voltage transformation is not really a huge issue as it was in the 20's when all the grid went in.
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On Mon, 20 Mar 2006 10:41:10 +0000 someone who may be The Natural

They are standard equipment.
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----------------------------

-----------------
Actually it is quite an added expense for most situations. A converter station is far more expensive than a transformer. Note also that practical DC circuit breakers for higher voltages are not available so the switching flexibility of the "grid" isn't there for DC. Note that HVDC systems are used to connect AC systems where a) end to end where distances are long enough to warrant the increased station costs vs the reduced line costs. b)Ties between areas where stability problems may occur. c)ties between regions at different frequencies. Note that in all cases the links are between AC grids. What makes the AC grid possible is the ease of changing voltage levels with simple transformers and of switching. DC is a bugger to switch. Now as for household use, 48V is low for many applications. A 1200 watt toaster would require 25A DC (roughly 70% larger area wires =$). The problem of breakers is much more difficult for DC (i.e. $$$). Take a look at a simple toggle switch as used for many applications. Compare the AC and DC specifications -provided that the switch can even handle DC- most household switches intended for 15A AC would simply not handle even 2A at 48V and simply arc -not desirable unless you are into whole house heating . Frankly, 120V 60Hz or 240V 50Hz, is nicer to handle and safer in many ways than 48V DC at the same current.
Example: at 120V AC, opening a 5A current with a knife switch causes a small spark. At 120V, 5A DC, the same switch can be opened 1/2 inch, causing a hot sustained arc (possibly 3/4 inch to 1 inch long), sufficient to light a cigarette or anything inflammable nearby). 48V is a bit better but at higher currents - the arc will be heavier.
You suggest inverters. Fine, but your inverter can cause problems with your neighbor's inverter and what you suggest is an added lossy component which has a poor cost/benefits ratio for most household uses. Why put in equipment that provides no gain in any sense?
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Don Kelly @shawcross.ca
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I think the 'mains' in some modern helicopters is about a 270Vdc bus, generated by one central 3-phase rectifier on the 200V L-L AC. Saves a sigificant amount of weight in both the generation and consumers. I have no idea how they switch 270Vdc though.
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Tony Williams wrote:

MOSFETS are able to go that high..450V MOSFETS are made in vast quantities for mains voltage SMPS.
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Wonder why such a high voltage, its not that transmission losses will be all that great ?....
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wrote:

Keeps the current low, which simplifies simple DC switchgear.
If you really want to run kW domestic appliances from 48V DC, you'd be looking at solid state switches instead of cheap mechanical ones.
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So they have 270 odd volt lamps then?.
And the Avionics and instrumentation all 270 or is there a lot of step down switch mode?...

I don't think anyone would.....
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wrote:

Aircraft have gone through several generations of power supply,
First of all there were engines with magneto ignition and no power to the instruments.
By WW2 a 12V DC battery system was common, with an engine powered generator. Towards the end of WW2 this was generally upgraded to a 24V battery and 28V DC busbar system fed by variable speed generators and regulators. These ranged from about 0.5kW to 12kW power.
AC began as low-power 400Hz 3 phase or 1600Hz single phase systems to drive instrumentation and position-sensing synchros. This was "derived" power, supplied by DC-powered rotary inverters with inbuilt speed control.
Larger multi-engine aircraft demanded more power for radar, windscreen and crew suit heating, bomb releases, etc. and so the voltage increased to keep the current and the wiring manageable. These were 112V DC busbar systems, driven by a number of DC generators, one per engine. These were variable speed, voltage regulated and might offer about 20kW of power.
A 28V DC system was maintained to power the standard instrumentation developed in earlier low-voltage systems. This was fed by a rotary inverter.
High electric power for de-icing heating, gun turret traverse or the beginnings of high power systems like landing gear lowering or even flight controls gave a demand for even more power. This needed a more efficient, simpler and lighter generator and so there was a switch to AC generation. These were simple variable speed (and so variable frequency) 208V AC devices for de-icing (by far the biggest electrical load), or 104V outputs that were then rectified and fed to the 112V DC busbar system.
The 400Hz and 1600Hz instrumentation supplies continued, supplied by DC rotary inverters.
With the development of constant speed drives to the generators, it was practical to maintain a constant frequency, and to synchronise generators between engines. This was the beginning of 400Hz 3 phase as the standard high power supply in aircraft, at 200V and up to around 20kW. The 112V DC systems disappeared, but the 28V DC busbar was retained for compatibility, powering the low-power equipment. This was driven by its own constant-speed DC generator and was also battery backed up, maintaining radios and flight instruments in emergency.
All-AC aircraft appeared when specialist DC equipment no longer needed a common busbar but could provide its own DC "in cabinet". A 28V DC busbar was retained, but just to provide emergency battery backup.
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Very intesting, thanks for that.
The first computer company I worked for manufacturered a lot of equipment for air force and navy. We had whopping great 400Hz generators outdoors to power the military stuff, but I don't now recall the voltage it supplied. (Not sure why we used generators rather than rotary converts -- might have been cheaper to buy?) We also had a very hefty 52VDC supply for equipment which went into telephone exchanges.
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Andrew Gabriel

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Thanks for that, very interesting,
I wonder what the electricity bill for a Jumbo comes out at;-))
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An alternator assembly bolted to each engine, and one on the auxiliary power unit (APU) in the tail. Each alternator will probably be capable of over 100KVA. The alternators will be 2-pole, spinning at 24000rpm. A 100KVA 400Hz alternator is not that large, something like a very large starter motor. Driving the alternator is the hydraulic constant speed gearbox, with splined input shaft and bell housing for attachment to the auxiliary take-off point on the side of the engine.
That hydraulic constant speed gearbox has a large power loss and a huge weight penalty for an aircraft, which is the motive for the moves to variable frequency aircraft mains, or to that 270Vdc bus in recent helicopters.
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Tony Williams.

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Avionics is very safety conscious, so it tends to evolve rather than innovate. 115V 3-phase alternators are already in service, so the only significant mod is just a rectifier stack.
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The Natural Philosopher wrote:

Thats why I said 100V at 400Hz earlier. You can make good SMPS with ferrite cores easily.
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On Sun, 19 Mar 2006 14:49:13 UTC, "dennis@home"

OK, apart from the transmission problem.
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dennis@home wrote:

Yeah but they switch at a few Khz anyway, so what you feed them from is irrelevant.
They all transform to DC at some point first...
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The Natural Philosopher wrote:

Truth is that lots of kit still uses transformers.
Also I thought the question was what would one do starting around year 1900, and 48vdc would have been a difficult voltage for much of the century.
Andy Dingley:

easy.
Last time I switched >100A dc I used 2 copper washers. The extra complications over ac are not major, as long as its low v.
NT
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On 19 Mar 2006 20:19:19 -0800, snipped-for-privacy@care2.com wrote:

Now do it repeatedly. DC is hard to switch because the arc isn't self-quenching (when the AC drops to zero). For DC you have to have a mechanical separation that's far and fast enough to extinguihs the arc, and contacts resistant enough to survive the wear of this intervening period.
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On Tue, 21 Mar 2006 23:41:57 +0000 someone who may be Andy Dingley

Indeed. DC switchgear isn't bulky because the manufacturers are especially ripping buyers off, but because DC is difficult to switch off. If it was otherwise then the switches one sees in buildings wouldn't be marked "AC only".
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David Hansen, Edinburgh
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