Design limits of electric motors?

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

John

--snip--

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.

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!

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

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)

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

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

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.

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

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

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

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

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

They take sonic signatures with built-in transducers of the new installations. They then listen periodically to see what has changed. An FFT will reveal where in the gear chain the wear is happening, and with some experience on the part of the operator, how bad it is. The parts are changed on the basis of the results.

For carbon fibre parts the situation is different. The parts are stressed severely to a degree that uses about one third of their useful life. The remaining two thirds are then very accurately predictable, and bits don't get changed until they really need to.

d Pearce Consulting

I believe it. When you screw up something *that* big and expensive, they find a way to make it work anyway ;-)

daestrom

Yeah, you're right. A fan has inherent limitations; a jet adds energy to the fluid stream - so, since we've got an infinite supply of energy, drive the compressor part of a jet motor with the Electric Motor and replace the combustion chamber with a refractory heater that's arbitrarily hot, to expand the gas. Then lose the turbine, since we've got the infinite battery.

Cheers! Rich

crankshaft

supply.

horsepower

inputs

I watched a programme on the development of the JSF. The comment made was that the front lift fan is only used during vertical landing, and if anything broke you were going to have "a _really_ bad day".

Regards Ian

This isnt too hard to work round though - but of course that does add complication. Reduced voltage, dummy loading, monitoring the tacho output, and designing to avoid no-loads can all work.

Regards, NT

On the ships I saw, the access ports to the main gear were sealed with huge padlocks, and only the Chief had the keys. The gears are just too tempting a tagret for sabatoge.

John