Why 28V DC?

[Bob Martin]

I'm working in the avionics integration test facility at Gulfstream...
one of the engineers had a question for me (being an airplane person
instead of an electrical/computer guy) that I couldn't answer... why
do airplanes use 28V DC systems (or 14V)? He says most industrial
applications use 24V DC. I tried googling on it but nowhere did it
suggest any reason why, just that it is.

[Roger Hamlett]

"Bob Martin" > wrote in message
om...
> I'm working in the avionics integration test facility at Gulfstream...
> one of the engineers had a question for me (being an airplane person
> instead of an electrical/computer guy) that I couldn't answer... why
> do airplanes use 28V DC systems (or 14V)? He says most industrial
> applications use 24V DC. I tried googling on it but nowhere did it
> suggest any reason why, just that it is.
Battery voltage.
The '12v' electrics on your motor car, actually run at typically 13.8v
(normal charging voltage of 6 lead acid cells). Hence much '12v' electronics
is 'mislabelled'. On aircraft, the naming was made to co-incide better with
the real voltage present.
In fact much industrial electronics running on '24v', is also a 28v system
(whenever the system has a lead-acid standby ability).

Best Wishes

[Ron Rosenfeld]

On 14 Jan 2004 06:19:24 -0800, (Bob Martin) wrote:

>I'm working in the avionics integration test facility at Gulfstream...
>one of the engineers had a question for me (being an airplane person
>instead of an electrical/computer guy) that I couldn't answer... why
>do airplanes use 28V DC systems (or 14V)? He says most industrial
>applications use 24V DC. I tried googling on it but nowhere did it
>suggest any reason why, just that it is.


It's only nomenclature.

In the airplanes, they are "naming" the voltage based on the charging
voltage; whereas in the other industrial applications they are "naming" the
voltage based on the *nominal* battery voltage.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

[Jim Weir]

Because the aircraft industry standardized on the nominal CHARGING voltage of 28
volts rather than the DISCHARGED voltage of 24 volts. 24-28?? Same animal with
a different nametag.

Now, why 24/28 volts? Because the aircraft needed to be lighter for military
performance reasons. Two 12 volt batteries in series comes nowhere near the
weight you can save in a fairly complex airplane (say, for example, a P-51) by
using a lighter weight copper wire for the same wattage load (double the voltage
= half the amperage for a given wattage). Remember, wire is sized by amperage,
not by voltage. INSULATION is sized by voltage.

So why was there 12 volts to begin with? Because Detroit started making cars
with a much higher compression ratio and to turn the starters over, the old 6
volt batteries weren't cutting it. Bingo. Two 6 volters in series gives 12
volts and that was close enough for Detroit gummint work.

The REAL question is who decided on 6 volts (3 each 2 volt lead-acid cells in
series) to begin with.

And the inquisitive student might ask, if 24/28 was so good, why not go 3 in
series and get 36 volt systems...or like the phone company with 4 in series for
48 volts? Because, grasshopper, the calculation WAS made to find out the most
efficient combination of voltage/current/wire size and at the time (WWII) it
came out just shy of 30 volts. Rather than dick around with special 30 volt (15
cell) batteries, the decision was made to use off-the-shelf dual 12 or single 24
volt "industrial" batteries.

Then there is the 115v 3ph 400Hz. discussion...

Jim



(Bob Martin)
shared these priceless pearls of wisdom:

->I'm working in the avionics integration test facility at Gulfstream...
->one of the engineers had a question for me (being an airplane person
->instead of an electrical/computer guy) that I couldn't answer... why
->do airplanes use 28V DC systems (or 14V)? He says most industrial
->applications use 24V DC. I tried googling on it but nowhere did it
->suggest any reason why, just that it is.

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

[Todd Pattist]

Jim Weir > wrote:

>And the inquisitive student might ask, if 24/28 was so good, why not go 3 in
>series and get 36 volt systems...or like the phone company with 4 in series for
>48 volts? Because, grasshopper, the calculation WAS made to find out the most
>efficient combination of voltage/current/wire size and at the time (WWII) it
>came out just shy of 30 volts. Rather than dick around with special 30 volt (15
>cell) batteries, the decision was made to use off-the-shelf dual 12 or single 24
>volt "industrial" batteries.

And since WWII the calculation has shifted, so you now find
cars being designed for 36 or 48 volts. The reason is still
cost/weight/size of wiring. When you want lots of power,
and don't want heavy/expensive/fat copper wiring you want a
higher voltage. Engines are being sold with valves that are
lifted by electromechanical actuators instead of mechanical
cams. The designers would go to even higher voltage if not
for fear that the mechanics/owners out there who're only
familiar with 12/24 volt systems in vehicles would be
electrocuted.

Todd Pattist
(Remove DONTSPAMME from address to email reply.)
___
Make a commitment to learn something from every flight.
Share what you learn.

[Roger Hamlett]

"Jim Weir" > wrote in message
...
> Because the aircraft industry standardized on the nominal CHARGING voltage
of 28
> volts rather than the DISCHARGED voltage of 24 volts. 24-28?? Same
animal with
> a different nametag.
>
> Now, why 24/28 volts? Because the aircraft needed to be lighter for
military
> performance reasons. Two 12 volt batteries in series comes nowhere near
the
> weight you can save in a fairly complex airplane (say, for example, a
P-51) by
> using a lighter weight copper wire for the same wattage load (double the
voltage
> = half the amperage for a given wattage). Remember, wire is sized by
amperage,
> not by voltage. INSULATION is sized by voltage.
>
> So why was there 12 volts to begin with? Because Detroit started making
cars
> with a much higher compression ratio and to turn the starters over, the
old 6
> volt batteries weren't cutting it. Bingo. Two 6 volters in series gives
12
> volts and that was close enough for Detroit gummint work.
>
> The REAL question is who decided on 6 volts (3 each 2 volt lead-acid cells
in
> series) to begin with.
>
> And the inquisitive student might ask, if 24/28 was so good, why not go 3
in
> series and get 36 volt systems...or like the phone company with 4 in
series for
> 48 volts? Because, grasshopper, the calculation WAS made to find out the
most
> efficient combination of voltage/current/wire size and at the time (WWII)
it
> came out just shy of 30 volts. Rather than dick around with special 30
volt (15
> cell) batteries, the decision was made to use off-the-shelf dual 12 or
single 24
> volt "industrial" batteries.
>
> Then there is the 115v 3ph 400Hz. discussion...
And the calculation here is shifting again.
With the increasing use of quite powerful electric parts in some new cars
(things like electric power-assisted steering), there is a new standard
making the rounds for systems operating just under 50v DC. However these
will use an unusual number of cells to avoid going over 50v (in some
countries the certifiation requirements increase massively at this point),
and are often based upon different battery types.

Best Wishes

[Jon Woellhaf]

Jim Weir wrote an interesting response to the subject, then added, "Then
there is the 115v 3ph 400Hz. discussion..."

Let's hear it!

Jon

[Charlie]

> Jim Weir wrote an interesting response to the subject, then added, "Then
> there is the 115v 3ph 400Hz. discussion..."
>
> Let's hear it!
>
> Jon
>

Possibly because AC is a more efficient distribution technique that DC for
long runs in large aircraft ? And 400Hz instead of 60Hz allows for smaller
transformers to step down the voltage ? Just guessing here.

[Larry Fransson]

On 2004-01-14 10:34:04 -0800, "Charlie" > said:

> > Jim Weir wrote an interesting response to the subject, then added,
"Then
> > there is the 115v 3ph 400Hz. discussion..."
> >
> > Let's hear it!
>
> Possibly because AC is a more efficient distribution technique that DC
for
> long runs in large aircraft ? And 400Hz instead of 60Hz allows for
smaller
> transformers to step down the voltage ? Just guessing here.

All of the gyros in the plane I fly (Lear 35) are powered by 400 Hz AC.
The inverters aren't more than 30 feet away. There are two transformers
(one on each bus) to provide 26 volts for the oil pressure guages, RMIs,
nav radios, and a few other things.

Back in my navy nuclear power days, I knew the reason for three phases. I
think it has something to do with power density - smaller, lighter, cheaper
is the AC mantra. Anyway.... 400 Hz provides higher power density and is
much cleaner than 60 Hz.

Am I on the right track?

--
Larry Fransson
Seattle, WA

[S Narayan]

"Larry Fransson" > wrote in message
news:2004011412081416807%lfransson@comcastnet...
> On 2004-01-14 10:34:04 -0800, "Charlie" > said:
>
> > > Jim Weir wrote an interesting response to the subject, then added,
> "Then
> > > there is the 115v 3ph 400Hz. discussion..."
> > >
> > > Let's hear it!
> >
> > Possibly because AC is a more efficient distribution technique that DC
> for
> > long runs in large aircraft ? And 400Hz instead of 60Hz allows for
> smaller
> > transformers to step down the voltage ? Just guessing here.
>
> All of the gyros in the plane I fly (Lear 35) are powered by 400 Hz AC.
> The inverters aren't more than 30 feet away. There are two transformers
> (one on each bus) to provide 26 volts for the oil pressure guages, RMIs,
> nav radios, and a few other things.
>
> Back in my navy nuclear power days, I knew the reason for three phases. I
> think it has something to do with power density - smaller, lighter,
cheaper
> is the AC mantra. Anyway.... 400 Hz provides higher power density and is
> much cleaner than 60 Hz.
>
> Am I on the right track?

If memory serves, 3-phase power is more efficiently distributed, if all 3
phases have equal loads (either as a star or delta connected network), there
is no current in the return path (ground). That is, sum of all phases is 0.
So you can save one conductor for the same power transmitted. The generation
of 3 phase power is also easy and I believe it may also be more efficient in
terms of the generator design. The 400Hz transformer, compared to a 50/60Hz
one, requires less "iron" for the same flux generation (or less turns) since
the mutual inductance is proportional to frequency. Hence they are lighter.
However, they may have more losses due to eddy currents etc.

[Ben Jackson]

In article >,
S Narayan > wrote:
>
>If memory serves, 3-phase power is more efficiently distributed

I thought the advantage of 3 phase was that the peaks of the sine waves
were evenly distributed making it more efficient for running motors.
But this was in the context of home/shop power where the single phase
in a standard home is inefficient with big motors.

--
Ben Jackson
>
http://www.ben.com/

[Kyler Laird]

(Ben Jackson) writes:
>>
>>If memory serves, 3-phase power is more efficiently distributed

>I thought the advantage of 3 phase was that the peaks of the sine waves
>were evenly distributed making it more efficient for running motors.

I feel like one of the blind men describing an elephant, but...

I thought three-phase was used to eliminate the need for the (existance
and maintenance of) starting windings, etc.

(It's been years since my Ag. Electricity course but I'm going to take
it again next semester...long story.)

--kyler

[S Narayan]

"Ben Jackson" > wrote in message
news:veiNb.66937$xy6.125476@attbi_s02...
> In article >,
> S Narayan > wrote:
> >
> >If memory serves, 3-phase power is more efficiently distributed
>
> I thought the advantage of 3 phase was that the peaks of the sine waves
> were evenly distributed making it more efficient for running motors.
> But this was in the context of home/shop power where the single phase
> in a standard home is inefficient with big motors.
>

Yes, I imagine that's another reason to use 3phase for high power motors,
where the torque is more uniformly available. I am probably getting out of
my league here but, separate windings carry different current phases which
translates to thinner copper for the same power and smoother operation, if
this makes sense. The same argument applies for a generator.

[S Narayan]

"Kyler Laird" > wrote in message
...
> (Ben Jackson) writes:
> >>
> >>If memory serves, 3-phase power is more efficiently distributed
>
> >I thought the advantage of 3 phase was that the peaks of the sine waves
> >were evenly distributed making it more efficient for running motors.
>
> I feel like one of the blind men describing an elephant, but...
>
> I thought three-phase was used to eliminate the need for the (existance
> and maintenance of) starting windings, etc.
>

Maybe a good side effect of 3 phase power, but that's not the main reason
for switching to 3 phase power.

[Gerry Caron]

"Ben Jackson" > wrote in message
news:veiNb.66937$xy6.125476@attbi_s02...
> In article >,
> S Narayan > wrote:
> >
> >If memory serves, 3-phase power is more efficiently distributed
>
> I thought the advantage of 3 phase was that the peaks of the sine waves
> were evenly distributed making it more efficient for running motors.
> But this was in the context of home/shop power where the single phase
> in a standard home is inefficient with big motors.

I believe it's actually the converse on an aircraft. It's the generator
that's big and heavy and by generating 3 phase, you can get more power per
pound of generator.

To make things more interesting, the industry is now going to 120V, variable
frequency AC power. There are two variants, IIRC they are 320 to 480 Hz and
280 to 620 Hz (or thereabouts.)

Why? Because to get a fixed 400Hz, you have to have a constant speed drive
off the engine to drive the generator. Those drives are expensive, heavy,
and require lots of maintenance. Getting rid of them saves money, but now
frequency varies with engine speed.

So now we get to put variable frequency power supplies in all our new
avionics.

Gerry

[Tiffani]

Jim Weir wrote:

> So why was there 12 volts to begin with? Because Detroit started making cars
> with a much higher compression ratio and to turn the starters over, the old 6
> volt batteries weren't cutting it. Bingo. Two 6 volters in series gives 12
> volts and that was close enough for Detroit gummint work.

Actually the auto industry is moving to a 42 volt system in the future to handle
increasing electrical requirements, plus more accessories (e.g. oil pump) may be
electric in the future. Where can you find more information on this? Why
http://www.42volt.org of course.

[Orval Fairbairn]

In article >, Tiffani > wrote:

> Jim Weir wrote:
>
> > So why was there 12 volts to begin with? Because Detroit started making
> > cars
> > with a much higher compression ratio and to turn the starters over, the old
> > 6
> > volt batteries weren't cutting it. Bingo. Two 6 volters in series gives
> > 12
> > volts and that was close enough for Detroit gummint work.
>
> Actually the auto industry is moving to a 42 volt system in the future to
> handle
> increasing electrical requirements, plus more accessories (e.g. oil pump) may
> be
> electric in the future. Where can you find more information on this? Why
> http://www.42volt.org of course.
>
>

Just what we need in a car -- an electric oil pump! That would be fine
for pre-oiling, but for regular operation? It degrades reliability by
making the lubrication system dependent on another system.

[S Narayan]

"Tiffani" > wrote in message ...
>
>
> Jim Weir wrote:
>
> > So why was there 12 volts to begin with? Because Detroit started making
cars
> > with a much higher compression ratio and to turn the starters over, the
old 6
> > volt batteries weren't cutting it. Bingo. Two 6 volters in series
gives 12
> > volts and that was close enough for Detroit gummint work.
>
> Actually the auto industry is moving to a 42 volt system in the future to
handle
> increasing electrical requirements, plus more accessories (e.g. oil pump)
may be
> electric in the future. Where can you find more information on this?
Why
> http://www.42volt.org of course.
>

A 42V DC? shock could be pretty bad. The new Toyota Prius uses 273V for the
traction motor, but that is very local to the hybrid system. Maybe a low
voltage higher frequency AC system might be a solution as someone suggested
earlier in the thread. Conversion efficiencies are pretty good these days
with more advanced controllers. Then individual components could step up the
voltage and convert to higher DC voltages as needed.

[mikem]

Bob Martin wrote:
> I'm working in the avionics integration test facility at Gulfstream...
> one of the engineers had a question for me (being an airplane person
> instead of an electrical/computer guy) that I couldn't answer... why
> do airplanes use 28V DC systems (or 14V)? He says most industrial
> applications use 24V DC. I tried googling on it but nowhere did it
> suggest any reason why, just that it is.

Lead-Acid chemistry batteries need 13.8 to 14.4V for "charging". The
voltage regulator on the alternator is usually set to a "compromise"
setting of 14.25V, which causes the bus voltage of the aircraft to be
near this voltage all of the time the engine is running.

All of the avionics and instrumentation is running off said 14.2V for
the duration of the flight. Double all of the above voltages if the
airframe has a 12 cell battery instead of a six cell.

What happens if the alternator fails in flight? Almost immediately, the
battery voltage sags down to under 12.6V, so the avionics has to work
over a voltage range of 11.5 to 15V...

MikeM

[Peter Gottlieb]

"S Narayan" > wrote in message
...
>
> A 42V DC? shock could be pretty bad. The new Toyota Prius uses 273V for
the
> traction motor, but that is very local to the hybrid system. Maybe a low
> voltage higher frequency AC system might be a solution as someone
suggested
> earlier in the thread. Conversion efficiencies are pretty good these days
> with more advanced controllers. Then individual components could step up
the
> voltage and convert to higher DC voltages as needed.
>

I've worked on a lot of 48 Volt systems and it is more like an unpleasant
tingle than a bad shock.

Higher voltage means thinner wiring, cutting weight.

[Ron Natalie]

"Peter Gottlieb" > wrote in message . net...
> I've worked on a lot of 48 Volt systems and it is more like an unpleasant
> tingle than a bad shock.
>
It is until ringing voltage is applied :-)

[Lisa Hughes]

S Narayan wrote:

> "Tiffani" > wrote in message ...
> >
> >
> > Jim Weir wrote:
> >
> > > So why was there 12 volts to begin with? Because Detroit started making
> cars
> > > with a much higher compression ratio and to turn the starters over, the
> old 6
> > > volt batteries weren't cutting it. Bingo. Two 6 volters in series
> gives 12
> > > volts and that was close enough for Detroit gummint work.
> >
> > Actually the auto industry is moving to a 42 volt system in the future to
> handle
> > increasing electrical requirements, plus more accessories (e.g. oil pump)
> may be
> > electric in the future. Where can you find more information on this?
> Why
> > http://www.42volt.org of course.
> >
>
> A 42V DC? shock could be pretty bad.

Not really. Touching a spark plug could feel pretty bad. Touching 42 volts
might give you a bzzz! However you wouldn't normally become part of the
circuit, anymore than you would with your domestic 120 volts or your telephone
(about 53 volts on hook, maybe double while ringing). Remember even if you did
get a shock, it's DC not AC. AC is worse for shocks because it can cause the
muscles to hold on to the conductor. DC tends to do the opposite. And it is
the amps that are most damaging to humans, not the volts.

[Peter Gottlieb]

"Ron Natalie" > wrote in message
...
>
> "Peter Gottlieb" > wrote in message
. net...
> > I've worked on a lot of 48 Volt systems and it is more like an
unpleasant
> > tingle than a bad shock.
> >
> It is until ringing voltage is applied :-)
>

Ah well the systems I worked on had rather substantial batteries and the
real hazard was shorting something out rather than touching it. Some people
hated the feeling but unless I was sweaty I would just touch things to see
if they were live.

The 20 Hz ring was interesting because you can quite distinctly feel each
half cycle go by. The original systems I messed with used motor-generators
to make the 90 Volt 20 Hz and every so often when I am near an old elevator
the smell reminds me of those old dynamotors.

[G.R. Patterson III]

Lisa Hughes wrote:
>
> Not really. Touching a spark plug could feel pretty bad.

Spark plugs run on extremely high voltage. The 12v battery power is fed through
a step-up tranformer (known as a "coil" on most systems) to increase the voltage.
Voltage at the plug will be anywhere from 40,000 to 100,000 volts.

http://www.howstuffworks.com/ignition-system2.htm

George Patterson
Great discoveries are not announced with "Eureka!". What's usually said is
"Hummmmm... That's interesting...."

[Dan Thomas]

"S Narayan" > wrote in message >...
The generation
> of 3 phase power is also easy and I believe it may also be more efficient in
> terms of the generator design.

That's probably why the alternators in your car and airplane both
are three-phase. A 60-amp alternator weighs less than an old 25-amp
generator.

Dan

[Jim Weir]

Jeez...that's something I've never considered. You are RIGHT, internal to that
alternator there IS 3-phase AC that is regulated and rectified to produce the
DC...

And now with three small transformers and a little surgery on the alternator, we
could have 3-phase 115VAC to run those military gyros...I wonder...yeah, I know
the frequency is a function of engine RPM...but still...

Jim


(Dan Thomas)
shared these priceless pearls of wisdom:

-> That's probably why the alternators in your car and airplane both
->are three-phase. A 60-amp alternator weighs less than an old 25-amp
->generator.
->
-> Dan

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

[Ron Natalie]

"Jim Weir" > wrote in message ...
> Jeez...that's something I've never considered. You are RIGHT, internal to that
> alternator there IS 3-phase AC that is regulated and rectified to produce the
> DC...
>
> And now with three small transformers and a little surgery on the alternator, we
> could have 3-phase 115VAC to run those military gyros...I wonder...yeah, I know
> the frequency is a function of engine RPM...but still...
>\
You think that AC coming out of those inverters is all that stable? It tends to wander
with the input voltage. Of course the real problem is that if the alternator crumps, the
battery will not put out three phase.

[Peter Gottlieb]

"Ron Natalie" > wrote in message
. ..
>
> "Jim Weir" > wrote in message
...
> > Jeez...that's something I've never considered. You are RIGHT, internal
to that
> > alternator there IS 3-phase AC that is regulated and rectified to
produce the
> > DC...
> >
> > And now with three small transformers and a little surgery on the
alternator, we
> > could have 3-phase 115VAC to run those military gyros...I wonder...yeah,
I know
> > the frequency is a function of engine RPM...but still...
> >\
> You think that AC coming out of those inverters is all that stable? It
tends to wander
> with the input voltage. Of course the real problem is that if the
alternator crumps, the
> battery will not put out three phase.
>

What?!? Your plane doesn't have 3-phase batteries? Call CBS!

[GeorgeB]

On Sun, 18 Jan 2004 19:20:48 -0500, "Ron Natalie" >
wrote:

>"Jim Weir" > wrote in message ...
>> Jeez...that's something I've never considered. You are RIGHT, internal to that
>> alternator there IS 3-phase AC that is regulated and rectified to produce the
>> DC...
>>
>> And now with three small transformers and a little surgery on the alternator, we
>> could have 3-phase 115VAC to run those military gyros...I wonder...yeah, I know
>> the frequency is a function of engine RPM...but still...
>>\
>You think that AC coming out of those inverters is all that stable? It tends to wander
>with the input voltage. Of course the real problem is that if the alternator crumps, the
>battery will not put out three phase.

The voltage is pretty stable, and is a square wave clipped by the
battery voltage ... at least while charging. I think the regulator
shuts it off when not, but am not sure. Years ago, when construction
of auto alternators was less optimized, I helped a freind rig his full
kW ham rig ... we took the alternator output into a 3 phase
transformer to get plate voltage. MUCH better than the vibrators of
the day. And we needed very little filtration with the 3 rectified
phase.

FYI, one, and perhaps the main, advantage of 3 phase rotating
machinery is that in theory, the developed torque of a motor, and load
torque of a generator is constant. Only manufacturing tolerances and
real conductors and iron give ripple.

[Lisa Hughes]

x-no-archive: yes

"G.R. Patterson III" wrote:

> Lisa Hughes wrote:
> >
> > Not really. Touching a spark plug could feel pretty bad.
>
> Spark plugs run on extremely high voltage. The 12v battery power is fed through
> a step-up tranformer (known as a "coil" on most systems) to increase the voltage.
> Voltage at the plug will be anywhere from 40,000 to 100,000 volts.

Uh, yes. That's why I said it could feel pretty bad and juxtaposed it with the
relatively harmless 42 volts. BTW: that spark plug can knock you into last week, but
it has very few amperage and will not likely cause you any long term harm (unless you
bang your head as you back away fast :)

[Ron Natalie]

"GeorgeB" > wrote in message ...
> On Sun, 18 Jan 2004 19:20:48 -0500, "Ron Natalie" >
> wrote:
>
> >"Jim Weir" > wrote in message ...
> >> Jeez...that's something I've never considered. You are RIGHT, internal to that
> >> alternator there IS 3-phase AC that is regulated and rectified to produce the
> >> DC...
> >>
> >> And now with three small transformers and a little surgery on the alternator, we
> >> could have 3-phase 115VAC to run those military gyros...I wonder...yeah, I know
> >> the frequency is a function of engine RPM...but still...
> >>\
> >You think that AC coming out of those inverters is all that stable? It tends to wander
> >with the input voltage. Of course the real problem is that if the alternator crumps, the
> >battery will not put out three phase.
>
> The voltage is pretty stable

I was referring to the freqeuncy (in response to Jim's comment about the engine RPM's).
You can actually here the whine of the inverter change pitch with the change of input voltage.

[S Narayan]

"Peter Gottlieb" > wrote in message
. net...
>
> "S Narayan" > wrote in message
> ...
> >
> > A 42V DC? shock could be pretty bad. The new Toyota Prius uses 273V for
> the
> > traction motor, but that is very local to the hybrid system. Maybe a low
> > voltage higher frequency AC system might be a solution as someone
> suggested
> > earlier in the thread. Conversion efficiencies are pretty good these
days
> > with more advanced controllers. Then individual components could step up
> the
> > voltage and convert to higher DC voltages as needed.
> >
>
> I've worked on a lot of 48 Volt systems and it is more like an unpleasant
> tingle than a bad shock.
>
> Higher voltage means thinner wiring, cutting weight.

I meant if things got wet, it could be quite a tingle. Considering the
highly litigous public in this country, I can see the lawsuits if poeple got
even slightly "hurt". Agree about the copper savings. I grew up with 240VAC
power and that is something you don't fool around with. I have also had
shocks from the flyback transformer in TVs and ignition systems in cars, but
those are likely to get you physically hurt (uncontrolled muscle reaction)
than the shock itself.

[Gene Seibel]

And more easily filtered when rectified for DC.
--
Gene Seibel
Hangar 131 - http://pad39a.com/gene/plane.html
Because I fly, I envy no one.





"S Narayan" > wrote in message >...

> If memory serves, 3-phase power is more efficiently distributed, if all 3
> phases have equal loads (either as a star or delta connected network), there
> is no current in the return path (ground). That is, sum of all phases is 0.
> So you can save one conductor for the same power transmitted. The generation
> of 3 phase power is also easy and I believe it may also be more efficient in
> terms of the generator design. The 400Hz transformer, compared to a 50/60Hz
> one, requires less "iron" for the same flux generation (or less turns) since
> the mutual inductance is proportional to frequency. Hence they are lighter.
> However, they may have more losses due to eddy currents etc.

[Dan Thomas]

You used to be able to buy a "converter" for your alternator-equipped
car to turn it into a 110-volt DC supply. Good for running lights,
heaters, and series-wound motors such as those found in electric
drills, skilsaws and the like. No good for induction motors or
anything with a transformer (electronic stuff). I boought one of these
years ago, paid a handsome price for this "advanced" technology, then
took it apart to see what was in the box before I installed it.
It was the box, a household 110V duplex receptacle, a DPDT toggle
switch, an NE-2H bulb and 22K (might have been 27K) resistor. Total
value not more than eight or ten bucks, today's prices. The box would
be the most expensive item.
One pole of the switch switched the alternator field from the
regulator directly to the battery, and the other switched the
alternator output from the battery to the receptacle. A manual
throttle cable (not included) was used to set engine RPM to a value
that would fire the NE-2H, being about 110V with that resistor, and
you went to work. The battery would drain slowly as the field was
consuming about 5 or 6 amps, and you'd have to flip the switch once in
a while to recharge it. It didn't hurt the alternator, as it takes
amps, not volts, to burn it out. The diodes would suffer first, from
the voltage, I think. You'd have a hard time finding a tool that used
60 amps.
It was a handy doodad for anyone without power at the field. I
don't know if they're still made. It wouldn't work with today's
alternators with their built-in regulators.

Dan


Jim Weir > wrote in message >...
> Jeez...that's something I've never considered. You are RIGHT, internal to that
> alternator there IS 3-phase AC that is regulated and rectified to produce the
> DC...
>
> And now with three small transformers and a little surgery on the alternator, we
> could have 3-phase 115VAC to run those military gyros...I wonder...yeah, I know
> the frequency is a function of engine RPM...but still...
>
> Jim
>
>
> (Dan Thomas)
> shared these priceless pearls of wisdom:
>
> -> That's probably why the alternators in your car and airplane both
> ->are three-phase. A 60-amp alternator weighs less than an old 25-amp
> ->generator.
> ->
> -> Dan
>
> Jim Weir (A&P/IA, CFI, & other good alphabet soup)
> VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
> http://www.rst-engr.com