Which Residential Voltage & Frequency Arrangement Is Best?

[Don Kelly]

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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?

[Don Kelly]

Aluminum cable, steel reinforced, replaced copper in the 1940's, and copper, with or without a steel core, simply doesn't compete in terms of weight and cost for a given current capacity. It's choice, around the world was not a mistake, but a deliberate decision based on economics and performance.

Aluminum creep has been a problem with domestic wiring but not with transmission system wires where proper attention (and expense) is paid to splices and connectors.

[Owain]

Yeah, but us Brits like a nice cuppa tea afterwards

Owain

[Owain]

There is; I don't know whether in the actual NEC but there certainly are in many local regulations.

In the UK 20 or 30 double 13A sockets per 32A ring circuit would not be unusual or excessive.

Lake County, Painesville, Ohio - requirements additional to National Electric Code, 2005: General receptacle outlets combined with lighting outlets shall have a (maximum) total of ten (10) outlets per circuit. (Amends Section 210.23) Two or more small appliance branch circuits shall be provided for all receptacles in kitchen, pantry, breakfast, and dining rooms. The number of receptacle outlets on each small appliance branch circuit shall be limited to four (4) outlets. Receptacle outlets installed in the kitchen shall be supplied by not less than two small appliance branch circuits. Such circuits shall have no other outlets. Counter top receptacle outlets installed in the kitchen shall be supplied by not less than two small appliance branch circuits. Sections 220.18, 210.52(B) (C), & 220.52

City Limits of Belleville, Illinois: Not more than four (4) outlets per circuit permitted in kitchen and dining area. All others, up to eight (8) outlets per circuit

New Mexico Electrical Code 14 NMAC 10.4: 10.2.2 210-11 (a). Number of Branch Circuits. Add, In dwelling occupancies, circuits for general purpose receptacles shall be limited to a maximum of eight current consuming outlets. Single and duplex receptacle outlets are considered to be one current consuming outlet. 10.2.3 210-11 (c) (1). Small Appliance Branch Circuits Dwelling Unit. Add, Not more than four (4) current consuming outlets shall be connected to these circuits. Single and duplex receptacle outlets are considered to be one current consuming outlet. [7-1-99]

San Marcos, Texas: (C) No more than 12 outlets will be installed per circuit serving general lighting circuits. No more than six outlets will be installed on each GFCI circuit. Outlets serving the small appliance branch circuit for counter tops are limited to three outlets per circuit

A maximum of 8 duplex receptacles on a normal wiring circuit; a better recommendation is a maximum of 4. Refer to the code for specific formulas.

Owain

[dennis]

After. You need a good cuppa after the first three or four hours to get your strength back for the rest of the night.

[Bud--]

The NEC has no limit on the number of outlets per circuit for residential. It specifies the number of circuits and the distance between the outlets and a competent installer will distribute the outlets between circuits. Local regulations can modify any part of the code in any way making discussions a bit difficult.

For non-dwelling, in general, an outlet (single or dupex) is figured to be 180VA, or 10 max on a 15A circuit.

bud--

[Andy Dingley]

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.

[David Hansen]

On Tue, 21 Mar 2006 23:41:57 +0000 someone who may be Andy Dingley wrote this:-

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".

[The Natural Philosopher]

Its real problem. Its why the commutators and brushes of DC motors last about 30 seconds and no DC motors are made that run over 5A or 50 RPM ;-)

Its that switching of high current DC that does it.

My car starter motor solenoid can barely manage 5 starts at 400A 12v DC before I need a new one.

That's why all modern cars run on 400V 10kHz AC.

[The Natural Philosopher]

Is there something in the water?

Sure, 400V DC will sustain an arc better than 400V AC, but its HARDLY a huge problem to design a switch that will handle 40V DC at 75A?

[meow2222]

I gotta agree with you on that one. A dc switch requires faster opening, larger contgact separation and bulkier contacts. But none of these are in any way difficult to do!

Ac switches are made dc-inadequate because slow opening performs better on ac, and smaller contacts are 0.1p cheaper.

NT

[Andy Dingley]

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.

[Don Kelly]

---------------------- I assume that what you are saying is tongue in cheek.

As for the switching of high current DC in a DC motor, please note that the whole rotor winding is not switched so the voltage across the coil being switched is small(ideally 0) and the brushes short this coil at the time, making a controlled reversal of current in this single coil. Sometimes things aren't perfect and at heavy currents rather nasty arcing can occur. There is also a reason why brushes are made of carbon. DC motors are fine but are also high maintenance devices compared to AC machines.

However one can get switches for 48VDC, 25A or 100A. Not a problem but expensive compared to AC switches. For household use- there is no point in going to DC. --

Don Kelly @shawcross.ca remove the X to answer

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[Don Kelly]

Remember the old switches which could handle DC?. A 15A switch was somewhat bigger than the present AC switch but had a bloody good spring which was essential in quickly opening up a gap sufficient to extiguish the arc. This was noisy, subjest to failure and comparatively hard to operate. AC switches depend on the arc becoming unstable at the current zero and don't need this cumbersome mechanism. It's not that slower opening performs better on AC but it can be tolerated as the switch doesn't have to force a current zero and the arc is likely extinguished just as quickly as with the old switches.

[Member, Takoma Park Volunteer]

David Hansen wrote:

I had a hard time convincing one foreman that we could not use AC only snap switches on the emergency lighting circuits in an older building we were remodeling. As soon as I saw that the emergency lighting panels were single phase 208/120 supplied by locking AC coil contacters I looked around for the battery room. There in all their ancient glory were 120 two volt wet cells wired in series that the contacters dropped out to when the AC power fails. Those batteries had the emergency lighting Grounded Current Carrying Conductors tapped of between batteries 60 and 61 which was also the ground reference for the DC supply. It was a true Edison circuit. All of the emergency lights were incandescent. All of the existing switches had a large T stamped into the strap. The T indicates that the switch can safely close on the inrush current of it's rated load in tungsten filaments. That mark is only found on specification grade DC switches. Later in that same project some brilliant soul decided to use up some left over unit battery pack exit lights as replacements for the old two bulb incandescent fixtures. The first time the power failed the power supplies for the unit battery chargers all burned up when the 120 volt DC hit the transformers. The contractor then sold the customer on the necessity of upgrading to inverter supplied AC for the emergency lighting circuits. The batteries were regrouped into three 48 volt banks to supply three inverters that were interconnected so as to supply

208/120 volt AC.

-- Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison

[Andrew Gabriel]

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.

[Jim Michaels]

But the winding is optional and therefore can still lead to a cord dangling over the edge. I assume British children are not able to withstand a 3kw deep fat fryer full of hot oil coming down on them without harm. Remove SPAMX from email address

[tony sayer]

Thanks for that, very interesting,

I wonder what the electricity bill for a Jumbo comes out at;-))

[Tony Williams]

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.

[David Hansen]

On 21 Mar 2006 17:19:21 -0800 someone who may be [e-mail] wrote this:-

Precisely.

Nowhere did I claim that it is difficult to produce DC switchgear. However, it is bulky compared to an AC only version. Certainly not worthwhile installing in residential properties.