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.
Not sure they are actually.
Anyway, you can make a three phase inverter for AC motors to run on DC
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.
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?
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.
Aircraft have gone through several generations of power supply,
First of all there were engines with magneto ignition and no power to
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
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
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
The 400Hz and 1600Hz instrumentation supplies continued, supplied by DC
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.
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.
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.
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
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.
On 19 Mar 2006 20:19:19 -0800, firstname.lastname@example.org 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
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".
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
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