Design limits of electric motors?

[Mjolinor]

"Mjolinor" <mjolinor@hotmail.com> wrote in message
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"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
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John G wrote:
"DaveC" <me@privacy.net> wrote in message
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On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote
(in article <10c9cn7hrsm07cb@corp.supernews.com&gt:


You could get around the magnetic problem with a pneumatic motor --

the

rim of your 2cm rotor is only traveling at 100 m/s, which is only

225

miles per hour, after all.

The thing that prompted my original question was seeing that QMII
documentary. I began wondering if air flight could ever use electric

motors

to drive turbines that would provide equivalent thrust of jet turbine

engines

(let's put aside the question of a source of electric power; for now,

let's

say it's infinite).

I realize that low-speed electric motors could drive propellers, but

is there

any hope of an electric motor being able to drive a high-speed

turbine?

The quick answer is NO
No usefull purpose can be achieved by driving a turbine.
The turbine and its heat are the source of the power.
Where does you electric motor get its electricity from?

Further most engines from old piston to modern turbines spin too fast
for propellors and have to be geared down to drive an aeroplane.

I believe that the OP wasn't going to spin the turbine to spin a prop,
he was more interested in spinning the turbine to drive the aircraft.

In theory a high-bypass fanjet motor could produce pretty much the same
thrust if you spun the fan with an electric motor as with it's built-in
turbine engine, and you'd get the same kinds of high-speed efficiency
gains that you do from using a fanjet.

The real rub would be that "infinite source of electrical power" -- so
far the only thing that really beats hydrocarbon fuels for power density
is atomics, and while the US was crazy enough to seriously investigate
atomic-powered craft in the 50's that would stay up for days they
weren't crazy enough to continue the experiment once they developed
intercontinental missiles. Even there they were going to use hot air
from the reactor to drive the turbines; the weren't going to generate
electricity then use motors.

--

I can't understand this conversation, surely an aerofoil shaped bladed
rotor
would achieve the same whether it was inside a tube or outside it. If you
dont have significantly higher pressure gas on one side of the blade then
you will reach a speed where the effect is to create vacumn on the "high
pressure side" rather than pressure increase at the back side (similar to
cavitation on a water propellor), this will still create small ammounts of
thrust I suppose but it would pretty quickly reach a maximum that you
couldn't get past.

Even if I visualise the pressures in a thing with 10 or more rotors with
different pitch (shaped) blades I can't see how it would work at all. I
end
up back at one "screw" pulling or pushing it's way through the air with
all
the limitations that standard propellors have. Enlighten me please.

To me the only difference between a jet engine and a rocket engine is that

the jet is the most complicated one way valve immaginable so that the
explosion is directed out the back but one of the two components for
combustion can still get in the front. Maybe I am looking at it wrong as I
know less about aviation than I do about electronics . So with my view in
mind how is an electric motor going to replace some/all of the jet engine or
is my understanding wrong.

 

[Clifford Heath]

Tim Auton wrote:

You might be able to compress the air and send it out the back at
supersonic velocities.

Yes.

(as the density goes up so does the speed of sound)

No. SoS is related to the average molecular mass and the square root
of the temperature in degrees Kelvin - nothing else. This is
because temperature is based on average molecular kinetic energy
of collision, which is m*v^2 - and SoS is directly related to v.

Air expanding with a pressure ratio of about 1:0.58 reaches sonic
velocity (this ratio varies slightly with the gamma of the gas,
which is related to the moment of angular inertia of the molecules).
Above this pressure ratio it's possible to use a divergent
nozzle to further accelerate the flow to supersonic.

However, a supersonic exhaust is a waste of energy unless you're
travelling supersonic - you can use the energy better by exhausting
more gas at a lower speed (higher momentum transfer, less wasted
kinetic energy). Hence airliners use high-bypass fans.

That's why the shuttle solid rocket booster burns a kind of rubber
to produce a high molecular mass, lower velocity exhaust, during the
early parts of the flight, and relies on LH2/LO2 fuel for a lower
MM, higher velocity exhaust when it's travelling faster.

Basic rocket science .

 

[Tzortzakakis Dimitrios]

It depens on number of poles.(for asynchronous motors).
Double pole->3000 rpm (at 50 Hz) ->3600 rpm (at 60 Hz) 4 pole->1500 rpm or
1800 rpm.
A DC motor, either series, shunt or compound excitation runs at about the
same speed.Or slower, of course.
--
Dimitris Tzortzakakis,Iraklion Crete,Greece
Analogue technology rules-digital sucks
http://www.patriko-kreta.com
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Warning:all offending emails will be deleted, and the offender/spammer
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After watching the PBS special on the building of the ocean liner Queen
Mary
II, I have no question of the size that electric motors that can be built.
Three (or is it four) huge motors in rotating pods push this behemoth ship
at
record speeds across the Atlantic.

But how fast can an electric motor potentially turn (though not
necessarily
the ones that drive the QMII)? Examples on-line?

Thanks,
--
DaveC
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[Tzortzakakis Dimitrios]

Are you joking?GEARED?Steam turbine?They are on a single-cast shaft.THAT
shaft is expensive, thus it connects the turbine and generator.Imagine a
gear for 2,500,000 hp (usual power of a nuclear plant generator).The
generator and turbine are designed to run at the same speed.Even train
locomotives use diesel-electric transmission, and the traction motors are
directly coupled on the wheels.So must be happening at the ships, too.

--
Dimitris Tzortzakakis,Iraklion Crete,Greece
Analogue technology rules-digital sucks
http://www.patriko-kreta.com
dimtzort AT otenet DOT gr the return adress is corrupted
Warning:all offending emails will be deleted, and the offender/spammer
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On Mon, 7 Jun 2004 08:21:34 -0700, DaveC <me@privacy.net> wrote:

After watching the PBS special on the building of the ocean liner Queen
Mary
II, I have no question of the size that electric motors that can be
built.
Three (or is it four) huge motors in rotating pods push this behemoth
ship at
record speeds across the Atlantic.

But how fast can an electric motor potentially turn (though not
necessarily
the ones that drive the QMII)? Examples on-line?

Thanks,


Ultracentrifuges and high-vacuum turbopumps use high-frequency
AC-driven induction motors, with rotational speeds measured in KHz...
million RPM territory as I recall. The limit is the destruction stress
on the rotor.

Big ships are slow... 100 RPM and thereabouts. I wonder if the QEII
motors are geared? The main reduction gear on a big steam turbine
plant costs about a million dollars.


John

 

[Tzortzakakis Dimitrios]

No.The thrust of a jet engine is not created solebly by the high rpm, but
also from the high speed of the exhaust gases, coming from the back of the
engine.The jet engine intakes air, compresses it, the fuel is ignited in the
combustion chamber, and the gases simultaneously rotate the turbine and
propel the plane.

--
Dimitris Tzortzakakis,Iraklion Crete,Greece
Analogue technology rules-digital sucks
http://www.patriko-kreta.com
dimtzort AT otenet DOT gr the return adress is corrupted
Warning:all offending emails will be deleted, and the offender/spammer
will be put on my personal "black list".
? "DaveC" <me@privacy.net> ?????? ??? ??????
news:0001HW.BCEA26AB00030638F03055B0@news.individual.net...

On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote
(in article <10c9cn7hrsm07cb@corp.supernews.com&gt:

You could get around the magnetic problem with a pneumatic motor -- the
rim of your 2cm rotor is only traveling at 100 m/s, which is only 225
miles per hour, after all.

The thing that prompted my original question was seeing that QMII
documentary. I began wondering if air flight could ever use electric
motors
to drive turbines that would provide equivalent thrust of jet turbine
engines
(let's put aside the question of a source of electric power; for now,
let's
say it's infinite).

I realize that low-speed electric motors could drive propellers, but is
there
any hope of an electric motor being able to drive a high-speed turbine?

Thanks,
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

 

[Tzortzakakis Dimitrios]

GEARED down?I have seen a WWII airplane engine, and the crankshaft is
directly coupled to the propeller.The jet engines are ~2500 hp, so it's
impossible to gear.The pilot controls only the fuel supply.

--
Dimitris Tzortzakakis,Iraklion Crete,Greece
Analogue technology rules-digital sucks
http://www.patriko-kreta.com
dimtzort AT otenet DOT gr the return adress is corrupted
Warning:all offending emails will be deleted, and the offender/spammer
will be put on my personal "black list".
Ď "John G" <Greentest@ozemail.com.au> Ýăńářĺ óôď ěŢíőěá
news:Mh7xc.5$tx5.501@nnrp1.ozemail.com.au...

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BCEA26AB00030638F03055B0@news.individual.net...
On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote
(in article <10c9cn7hrsm07cb@corp.supernews.com&gt:

You could get around the magnetic problem with a pneumatic motor --
the
rim of your 2cm rotor is only traveling at 100 m/s, which is only
225
miles per hour, after all.

The thing that prompted my original question was seeing that QMII
documentary. I began wondering if air flight could ever use electric
motors
to drive turbines that would provide equivalent thrust of jet turbine
engines
(let's put aside the question of a source of electric power; for now,
let's
say it's infinite).

I realize that low-speed electric motors could drive propellers, but
is there
any hope of an electric motor being able to drive a high-speed
turbine?

The quick answer is NO
No usefull purpose can be achieved by driving a turbine.
The turbine and its heat are the source of the power.
Where does you electric motor get its electricity from?

Further most engines from old piston to modern turbines spin too fast
for propellors and have to be geared down to drive an aeroplane.
--
John G

Wot's Your Real Problem?

 

[krw]

In article <ka-dnT6UTba1flndRVn-jw@comcast.com>, t-tammaru@c0mca$t.net
says...

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BCEA26AB00030638F03055B0@news.individual.net...
On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote
(in article <10c9cn7hrsm07cb@corp.supernews.com&gt:

You could get around the magnetic problem with a pneumatic motor -- the
rim of your 2cm rotor is only traveling at 100 m/s, which is only 225
miles per hour, after all.

The thing that prompted my original question was seeing that QMII
documentary. I began wondering if air flight could ever use electric
motors
to drive turbines that would provide equivalent thrust of jet turbine
engines
(let's put aside the question of a source of electric power; for now,
let's
say it's infinite).

I realize that low-speed electric motors could drive propellers, but is
there
any hope of an electric motor being able to drive a high-speed turbine?

Thanks,
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

Dave,
There are pilotless drone aircraft that use electric motors, though they
drive low speed propellers. The cheapest high speed electric motor I can
think of is a vacuum cleaner motor, about 10000 rpm. Gear it up?

25000RPM isn't unusual for an unloaded router. ...some a little
higher.

--
Keith

 

[Tim Auton]

Clifford Heath <no@spam.please.net> wrote:

Tim Auton wrote:
You might be able to compress the air and send it out the back at
supersonic velocities.

Yes.

(as the density goes up so does the speed of sound)

No. SoS is related to the average molecular mass and the square root
of the temperature in degrees Kelvin - nothing else. This is
because temperature is based on average molecular kinetic energy
of collision, which is m*v^2 - and SoS is directly related to v.

You're quite right of course. I shouldn't do my inventing while tired
and drunk


Tim
--
Love is a travelator.

 

[krw]

In article <ca47dg$cr2$1@usenet.otenet.gr>,
dimtzortihatespam@nospamotenet.gr says...

It depens on number of poles.(for asynchronous motors).
Double pole->3000 rpm (at 50 Hz) ->3600 rpm (at 60 Hz) 4 pole->1500 rpm or
1800 rpm.
A DC motor, either series, shunt or compound excitation runs at about the
same speed.Or slower, of course.

DC, or universal, motors can run much faster (add a zero). As I posted
earlier, it's not uncommon for a router (woodworking tool) to have a
no-load speed of 25,000RPM or more.

example: http://www.portercable.com/index.asp?e=547&p=2810

 

[Tim Shoppa]

"John G" <Greentest@ozemail.com.au> wrote in message news:<Mh7xc.5$tx5.501@nnrp1.ozemail.com.au>...

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BCEA26AB00030638F03055B0@news.individual.net...
On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote
(in article <10c9cn7hrsm07cb@corp.supernews.com&gt:

You could get around the magnetic problem with a pneumatic motor --
the
rim of your 2cm rotor is only traveling at 100 m/s, which is only
225
miles per hour, after all.

The thing that prompted my original question was seeing that QMII
documentary. I began wondering if air flight could ever use electric
motors
to drive turbines that would provide equivalent thrust of jet turbine
engines
(let's put aside the question of a source of electric power; for now,
let's
say it's infinite).

I realize that low-speed electric motors could drive propellers, but
is there
any hope of an electric motor being able to drive a high-speed
turbine?

The quick answer is NO
No usefull purpose can be achieved by driving a turbine.
The turbine and its heat are the source of the power.
Where does you electric motor get its electricity from?

If you read _Surely You're Joking Mr Feynman_, you will find
most of a chapter devoted to how he sold his patent for a nuclear-
powered airplane for the sum of $1.

Tim.

 

[John Fields]

On Tue, 8 Jun 2004 14:31:09 +0300, "Tzortzakakis Dimitrios"
<dimtzortihatespam@nospamotenet.gr> wrote:

GEARED down?I have seen a WWII airplane engine, and the crankshaft is
directly coupled to the propeller.The jet engines are ~2500 hp, so it's
impossible to gear.The pilot controls only the fuel supply.

---
Suggest you look at propjet engines. Check out

http://www.garrettaviation.com/

BTW, FYI there is no such thing as analog; everything is grits.

And, mostly, idiots top post.

--
John Fields

 

[DaveC]

On Tue, 8 Jun 2004 02:01:50 -0700, Rene Tschaggelar wrote
(in article <40c580c4$0$21341$5402220f@news.sunrise.ch&gt:

There are ultra high vaccuum turbines doing 100k revs or so.
Search for "turbo molecular pump"

Thanks, I'll look at that.
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

 

[Andrew Gabriel]

In article <MPG.1b2f7b6332df44db98971e@news3.prserv.net>,
krw <krw@att.biz> writes:

DC, or universal, motors can run much faster (add a zero). As I posted
earlier, it's not uncommon for a router (woodworking tool) to have a
no-load speed of 25,000RPM or more.

Universal motors from washing machines (that's european style washing
machines) initially look like a good bet if you're after a mains
motor. However, with no load and without their servo control, they can
get to speeds where they fly to pieces.

--
Andrew Gabriel

 

[DaveC]

On Mon, 7 Jun 2004 22:50:51 -0700, Don Pearce wrote
(in article <40c5525e.18783953@news.plus.net&gt:

But the turbine is the bit that gets spun by the hot gases. That in
turn drives a compressor to provide the air for the combustion
chamber. In jet engines that is it - the hot exhaust gas drives the
plane. In a turbo fan a geared down output from the compressor shaft
drives the big fan (effectively a propeller) that you see in the front
of a modern jet engine.

Which bit, out of that lot, do you propose to replace with an electric
motor? The whole thing is a bit circular (apart from the fan) and it
is hard to see how you could break the loop to put your motor in.

It looks a little as if you think that it is the rotating turbine that
provides the thrust that drives the plane. It isn't - quite the
opposite, in fact.

Your post answers several of the questions -- many, unexpressed -- that I've
been after.

So, basically, turning a fan in a tube (spinning a turbojet engine without
fuel) doesn't gain you much efficiency. If electrics are to power an
aircraft, it seems that an efficient propeller is the best that you can do.
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

 

[DaveC]

On Tue, 8 Jun 2004 06:40:33 -0700, DaveC wrote
(in article <0001HW.BCEB0FE10039AEFDF03055B0@news.individual.net&gt:

So, basically, turning a fan in a tube (spinning a turbojet engine without
fuel) doesn't gain you much efficiency. If electrics are to power an
aircraft, it seems that an efficient propeller is the best that you can do.

And then hi rpms isn't important any more. Indeed, since torque doesn't
increase with speed (I *do* have that fact right, don't I?), gearing isn't
necessary and propellers have a relatively low maximum speed requirement.
--
DaveC
me@privacy.net
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[Mjolinor]

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BCEB11070039F3F1F03055B0@news.individual.net...

On Tue, 8 Jun 2004 06:40:33 -0700, DaveC wrote
(in article <0001HW.BCEB0FE10039AEFDF03055B0@news.individual.net&gt:

So, basically, turning a fan in a tube (spinning a turbojet engine
without
fuel) doesn't gain you much efficiency. If electrics are to power an
aircraft, it seems that an efficient propeller is the best that you can
do.

And then hi rpms isn't important any more. Indeed, since torque doesn't
increase with speed (I *do* have that fact right, don't I?), gearing isn't
necessary and propellers have a relatively low maximum speed requirement.

AFAIK the required speed of the propellor can be put anywhere controlled by
the pitch and shape of the blade. It gets more difficult when the blade tips
start going faster than sound and these make a distinctive sound like the
hughes 500 helicopter, the noise it makes is different to most helicopters
because the blade tips go supersonic. Thinking about it I reckon that
propellor blade design is like antenna design, there is a lot of "magic" in
it.

 

[Tam/WB2TT]

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BCEA99CD001E0246F03055B0@news.individual.net...

On Mon, 7 Jun 2004 16:57:28 -0700, John G wrote
(in article <Mh7xc.5$tx5.501@nnrp1.ozemail.com.au&gt:

The quick answer is NO
No usefull purpose can be achieved by driving a turbine.
The turbine and its heat are the source of the power.
Where does you electric motor get its electricity from?

In my original post, forget electric power source. I'm interested *only*
in
the possibility of the motor to turn fast enough to spin a turbine to
drive
an aircraft.

Further most engines from old piston to modern turbines spin too fast
for propellors and have to be geared down to drive an aeroplane.

Again, we're not talking about propellers, but turbines.
--
DaveC
me@privacy.net
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Dave,

I think you might want to look at the ducted fan engine. Also, there were
test made where one of the jet engines on a DC9 was replaced by a turbine
engine driving an open pusher propeller with what looked like maybe 10 - 16
blades. BTW, I am 99.99% sure that the propeller on turboprop engines is
geared down. You don't want the speed at the tip of the propeller to exceed
the sped of sound. When automobile V8 engines have been installed in light
aircraft, they have been geared down to allow an engine speed of ~4500 rpm.

Tam

 

[Don Pearce]

On Tue, 8 Jun 2004 06:40:33 -0700, DaveC <me@privacy.net> wrote:

On Mon, 7 Jun 2004 22:50:51 -0700, Don Pearce wrote
(in article <40c5525e.18783953@news.plus.net&gt:

But the turbine is the bit that gets spun by the hot gases. That in
turn drives a compressor to provide the air for the combustion
chamber. In jet engines that is it - the hot exhaust gas drives the
plane. In a turbo fan a geared down output from the compressor shaft
drives the big fan (effectively a propeller) that you see in the front
of a modern jet engine.

Which bit, out of that lot, do you propose to replace with an electric
motor? The whole thing is a bit circular (apart from the fan) and it
is hard to see how you could break the loop to put your motor in.

It looks a little as if you think that it is the rotating turbine that
provides the thrust that drives the plane. It isn't - quite the
opposite, in fact.

Your post answers several of the questions -- many, unexpressed -- that I've
been after.

So, basically, turning a fan in a tube (spinning a turbojet engine without
fuel) doesn't gain you much efficiency. If electrics are to power an
aircraft, it seems that an efficient propeller is the best that you can do.

You would indeed do far better with just the propeller.

d
Pearce Consulting
http://www.pearce.uk.com

 

[DaveC]

On Tue, 8 Jun 2004 07:19:34 -0700, Tam/WB2TT wrote
(in article <9Pudnb1d4813V1jdRVn-uw@comcast.com&gt:

I think you might want to look at the ducted fan engine.

I'll move the discussion of fans & propellers, etc. to a more appropriate
group.

Any more comments on the use of high-speed electric motors, I'd be grateful
to hear.

Thanks,
--
DaveC
me@privacy.net
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[Roger Hamlett]

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BCEB2060003D8CD8F03055B0@news.individual.net...

On Tue, 8 Jun 2004 07:19:34 -0700, Tam/WB2TT wrote
(in article <9Pudnb1d4813V1jdRVn-uw@comcast.com&gt:

I think you might want to look at the ducted fan engine.

I'll move the discussion of fans & propellers, etc. to a more appropriate
group.

Any more comments on the use of high-speed electric motors, I'd be
grateful
to hear.
Have a look at the spindle motors on the lpkf PCB prototyping machines. The

fastest of these go to 100,000RPM, and are speed controlled brushless DC
motors. Brush designs, are generally rare beyond perhaps 25,000RPM, but
brushless designs are remarkably common at these speeds. Also look at:
http://www.coercive.com/dcmotor.htm

Best Wishes