Design limits of electric motors?

[DaveC]

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.

 

[Mjolinor]

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]

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.
--
DaveC
[email protected]
This is an invalid return address
Please reply in the news group

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]

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]

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,

 

[Roger Hamlett]

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

 

[John Larkin]

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.

I was referring to a steamship. Prop RPMs are in the 100 range, the
small high-pressure turbine spins maybe 12 grand - it makes 80% of the
power - and the huge LP turbine runs roughly three or four. The main
bull gear is typically about 30 feet in diameter or so... I saw one
being ground at DeLaval, and I designed a number of steamship throttle
control systems. The LASH ships I worked on made 32,000 shaft
horsepower at 120 RPM. If the prop falls off, there's a good chance
the turbine will disintegrate.

Direct-coupled reversing diesels are popular in ships nowadays because
they are simpler and more compact than a high-efficiency steam plant.
I think the steam plants are still more efficient, and the stuff they
burn - essentially asphalt - is nasty and dirt cheap.

John

 

[John Larkin]

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.

My Dremel tool has a series-wound brush motor, and runs at 30,000 or
so.

John

 

[Tim Wescott]

--snip--


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.

A turbofan engine (which is used by most airliners these days) gets most
(about 80% IIRC) of it's thrust from the fan and about 20% from the
turbine engine buried inside of it.

Ducted fans are used because a the tips of a large, fast propeller
exceed the speed of sound and ruin the propeller efficiency. A
relatively small-diameter prop with lots of blades ends up throwing a
lot of air radially off its travel path. The answer is to put a
multi-bladed propeller into a tube, and turn it with a turbine engine -
that's a turbo fan.

So if you take that turbofan engine and replace it's turbine section
with a big-ass electric motor you'll get almost as much thrust as
before, but on electric power instead of kerosene.

Note that none of this applies to low-bypass engines, like the ones used
in older jet fighters and the concord: Those engines get _all_ of their
thrust directly from the hot, fast exhaust. It's great for supersonic
flight because the exhaust is going so very fast, but for slower travel
it's not good for fuel efficiency because a lot of air moving slowly
produces more thrust than a little bit of air moving fast.

 

[Tim Wescott]

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.

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!

 

[John Larkin]

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.

The single-engine VTOL Joint Strike Fighter runs a shaft fore-aft,
from the engine to the front lift fan, which blows down. There is
right-angle gearing at the fan casing. They shoot 32,000 horsepower
down this shaft; there's a clutch somewhere, too. They're using my VME
arbitrary waveform generators to simulate all the sensor inputs (shaft
speed, torque, displacements) into the control computers now being
designed.

Most jet engines have internal gearing. Jet helicopters obviously have
gears.

John

 

[Winfield Hill]

John Larkin wrote...

The single-engine VTOL Joint Strike Fighter runs a shaft fore-aft,
from the engine to the front lift fan, which blows down. There is
right-angle gearing at the fan casing. They shoot 32,000 horsepower
down this shaft; there's a clutch somewhere, too. They're using my
VME arbitrary waveform generators to simulate all the sensor inputs
(shaft speed, torque, displacements) into the control computers now
being designed.

Most jet engines have internal gearing. Jet helicopters obviously
have gears.

What happens when a tooth breaks? I suppose there's a mandatory
replacement schedule. What's a typical mandated gear life?

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

 

[daestrom]

But what about conventional (fuel) turbine engines. Surely they turn in the
100,000 rpm range, and use ball bearings.

Actually, I think you'll find the high speed shafts are in journal bearings.
And they have a continuous supply of oil, pumped by an oil pump driven from
an auxilary shaft. The aux shaft is at right-angle to the main shaft and
driven by bevel gearing at a slower speed.

daestrom

 

[daestrom]

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.

Hate to burst your bubble, but they *do* make gearing for this kind of
power. Typical steamships use reduction gears between the IP/LP turbines
(in thousands of RPM) and the main shaft (hundreds of RPM). And smaller
gearing between the HP and IP turbines. Bull-gears, the final output gear
connected to the propeller shaft are large with double helix cut. Often use
double-reduction with 'quill' shafts between successive gear stages.

Saw more than one bull gear get some broken teeth ground out. Didn't
replace the teeth, just ground down the sharp edges so they wouldn't wear
into the low-speed pinions (some sailors didn't believe the rules about
FOD). Some marine applications include clutches that can carry over 35000
hp. These ain't your standard automobile clutch, they have dozens of
friction plates and positive, splined-sleeve engagement.

Large stationary power plants have the HP and LP turbines co-linear with the
generator, that is true. But the 'shaft' is made up of several pieces, one
for each turbine section and another for the generator. Each section is
bolted to the next with flat-faced, bolted couplings. One plant (I think in
Korea) a year or so back had a failure where a fire in one bearing support
caused it to sieze. The shaft twisted right apart and in the process threw
pieces/parts all around the turbine building. The pictures were *very*
impressive.

Get a couple of mechanical engineers together in a room and they can come up
with things almost as outlandish and exotic as any EE's

daestrom

 

[Mjolinor]

Get a couple of mechanical engineers together in a room and they can come
up

with things almost as outlandish and exotic as any EE's

And from the EE side we have the Rabbit phone
For the mechanics we have the Edsel.

And as proof that the engineers can get it right but still not succeed there
is of course Betamax.

 

[daestrom]

My Dremel tool has a series-wound brush motor, and runs at 30,000 or
so.

One of the drawbacks/precautions about series-wound DC motors is that if
they are unloaded, the only thing limiting their speed is the windage and
friction losses. Some can literally tear themselves apart if run unloaded.
Of course, your Dremel is designed *not* to do that. Some older automobile
starters have been destroyed by running them on the bench to the point where
the copper bars come out of the rotor slots.

daestrom

 

[John Larkin]

John Larkin wrote...

What happens when a tooth breaks? I suppose there's a mandatory
replacement schedule. What's a typical mandated gear life?

Gosh, I don't know. I'd imagine the stress levels are pretty high.
Helicopters in particular are suicide machines. I've seen some of the
big fanjets disassembled, and they have a 4-foot wide, several inch
thick wrapping of epoxy-kevlar around the main (12 foot diameter) fan
blades to catch them if the rotor disintegrates. They actually test
this, and I'd love to see one of those tests.

This aerospace stuff looks like fun, and it is if you get to see it
but don't have to actually do it. My son-in-law works for Sandia, and
does some explosives stuff. I commented that it must be fun, and he
said, no, after all the management and paperwork and safety measures
and planning and stuff, it's not fun any more.

John

 

[John Larkin]

But what about conventional (fuel) turbine engines. Surely they turn in the
100,000 rpm range, and use ball bearings.

Big commercial jets are in the 12K RPM range, and I think some of the
military engine parts spin up to maybe 18K. There are some tiny jet
engines (coke-can size, or smaller) that run around 100K or more.
Research microturbines are pushing something like 500K.

Some steam turbines run in the teens, and they use plain
pressurized-oil bearings, not ball bearings.

John

 

[John Larkin]

Hate to burst your bubble, but they *do* make gearing for this kind of
power. Typical steamships use reduction gears between the IP/LP turbines
(in thousands of RPM) and the main shaft (hundreds of RPM). And smaller
gearing between the HP and IP turbines. Bull-gears, the final output gear
connected to the propeller shaft are large with double helix cut. Often use
double-reduction with 'quill' shafts between successive gear stages.

Saw more than one bull gear get some broken teeth ground out. Didn't
replace the teeth, just ground down the sharp edges so they wouldn't wear
into the low-speed pinions (some sailors didn't believe the rules about
FOD).

There was a gear being ground at DeLaval for a LASH ship, and the
grinder operator guy set the final grind pass to 10 mils instead of 1
mil. So this 32 foot diameter double-helix million-buck gear came out
with square edges on all the teeth. They called the shipyard
(Avondale), told them the gear would be a few weeks late, and gave the
guy a file. True story.

John

 

[Tim Auton]

And from the EE side we have the Rabbit phone

Nothing wrong with the electronics in a Rabbit phone was there?
Perhaps there was, but that's not why it failed. It was just a crap
idea. So now they're doing it again with Wi-Fi hotspots.


Tim