Design limits of electric motors?

Tim Wescott wrote:


Don't go sit next to the engines in such a plane. Unless it is made of quite thick lead. Hmmm...
Thomas
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Tim Wescott wrote:

Project "Pluto" in the late '50s through '64. The engine (a ramjet, rather than turbo-jet) was intended to power ICBMs and was tested (statically) for the duration needed (7 minutes, IIRC). ISTR that it was also carried aboard a test B36, but un-powered, and perhaps even un-fueled.
http://www.nv.doe.gov/news&pubs/publications/historyreports/news&views/pluto.htm
--
Keith

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wrote:

Interesting. Sort of a turbine assisted ramjet.
- YD.
--
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On Mon, 7 Jun 2004 16:57:28 -0700, John G wrote

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.

Again, we're not talking about propellers, but turbines.
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DaveC
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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.
d Pearce Consulting http://www.pearce.uk.com
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On Mon, 7 Jun 2004 22:50:51 -0700, Don Pearce wrote

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.
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On Tue, 8 Jun 2004 06:40:33 -0700, DaveC wrote

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.
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without
do.
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.
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DaveC wrote:

Your electric motor will probably be most efficient at speeds higher than want to drive your prop -- so you'll still want to gear the motor down to the prop.
And I disagree about the turbofan assertion -- assuming that you've got the motor to do it, if you want to fly at jetliner speeds a propeller is going to be horribly inefficient, which is why jetliners use turbofans and not turboprops. Since only 20% or so of the thrust of a turbofan is from the turbine I think you _could_ use a motor, keeping in mind that it's going to be a _long_ time before this is a better solution than just burning jet fuel in a turbine!
--

Tim Wescott
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without
do.
isn't
requirement.
wouldnt additional pole-pairs be a more efficient way of reducing motor top speed?

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Terry Given wrote:

-- snip --

Not really -- motor torque as a function of power input has a lot to do with the magnetic materials you use and how much of them you use. You _may_ gain something by increasing the motor diameter and shortening it, but I'm not sure. The motor torque doesn't have much to do with the number of poles. DC Motor torque is generally limited by the amount of current that you can push through it without warming it up too much or demagnetizing it, so motor power is generally dependent on how fast you can make the thing spin.
--

Tim Wescott
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top
It seems almost intuitive that if a 4-pole gets n rpm, then a 24-pole would get n/6; is that incorrect? And I'd think the torque would change proportionally, i.e. 6x as well, all with losses taken into account, of course. (so not necessarily exactly 6, ...)
And wouldn't including superconductor magnets be cool? They do give the most magnetism per pound that can be had, don't they?
Thanks, Rich
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Rich Grise wrote:

For a given drive frequency that's true, and it will be more difficult to drive a multi-pole motor fast. As far as the torque goes your slots get smaller when you have more poles, so you can't stuff as much copper in there, so your torque per pole goes down as fast as the number of poles goes up.
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Well, yes - but is it proportional? If so, then the torque would be constant, within a reasonable experimental error. ;-) Or so it looks to me.
Thanks, Rich
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Rich Grise wrote:

It depends on what you want to remain constant. If you are keeping the horsepower constant, then reducing the rpm by half will cause the torque to be twice as much. power = angular velocity x torque x (some correction factor to make all the units work together) using horsepower, foot pounds, and rpm HP=(rpm)(torque)/5260.
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without
do.
Look up "Muffin Fans." Imagine scaling one up big enough to fly an airplane.
Or imagine one right out of the catalog flying a small model plane. I think it's pretty feasible, after all, given the dilithium-antimatter battery pack.
:-) Rich
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Rich Grise wrote:

Muffin fans stink for that -- they're designed for reliability and quietness. Check out http://www.hobby-lobby.com/ for electric ducted fans that really work (except that they tend to be big, low velocity things).
--

Tim Wescott
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You would indeed do far better with just the propeller.
d Pearce Consulting http://www.pearce.uk.com
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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
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On Tue, 8 Jun 2004 07:19:34 -0700, Tam/WB2TT wrote

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
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