# Quick Electric Machines

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Hi Frank, thanks for your interest!

Thermal engines tend to be less efficient than the fuel cells' 60%, but they improved quickly in the past two decades, and the difference is small now. As thermal engines are much lighter than fuel cells, the alternative must be considered, sure. 600kg less would sell 6 seats more, big difference.

Hydrogen is difficult to bring to a combustion chamber.

• Injected liquid in a piston engine prior to compression, it freezes air's water vapour and possibly the carbon dioxide, and as hydrogen vaporizes, the extra volume to be compressed spoils the engine's efficiency.
• Injected gaseous and lukewarm in a piston engine prior to compression, the extra volume to be compressed spoils the engine's efficiency. Still bad.
• Injected gaseous after compression is as bad as before compression.
• Injected liquid after compression is the least bad option at a piston engine. Hot air components won't freeze, and this squanders the least power. However, it demands a damn strong injection pump, much worse than the difficult pump of a common Diesel engine, and the whole pump and circuit must work at 20K. Design is badly difficult AND operations get complicated, as the whole circuit must be cooled before start.
• In a turbomachine, hydrogen should be injected after air compression for the same reasons, but this demands a pump much more powerful than now, and again pre-cooling before starting the engine. I put there
and especially this message for hydrogen alone
http://www.chemicalforums.com/index.php?topic=91121.msg333549#msg333549
some pumping cycles with the necessary power, and they inject the hydrogen already hot in the chamber, which helps stabilize the flame. Meant for ramjets and scramjets but fit turbomachines too. It's more rocket than aeroplane technology, and hasn't been developed for airliners up to now. Understand: long and costly. Simpler cycles are possible with turbomachines.

Hot output from fuel cells: the 40% wasted energy must go somewhere, yes. My doubt is: how heavy is a heat exchanger to make use of this? Each time I tried, heat exchangers were too heavy to outperform alternative solutions. And if you can operate the fuel cell at a pressure higher than the chamber of the thermal engine that uses the waste heat, to inject the fuel cell outlet in the chamber without a heat exchanger, then you must first pump the air and the hydrogen to that pressure, which is nearly as bad and pretty complicated too.

The rest is less of a problem, for instance hydrogen combustion is already known from the more difficult scramjets.

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Thanks!  For short hops flights, maybe consider compressed hydrogen instead, still higher volumetric energy density than Li-ion batteries and much lighter.  Then there's Skylon for LH2 powered spaceflight - Skylon (spacecraft) - Wikipedia:  https://en.wikipedia.org/wiki/Skylon_(spacecraft)  Hydrogen Turbojet engines are being developed.

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I put thoughts in the liquid hydrogen tank because compressed hydrogen needs a tank too heavy.

Also, liquid hydrogen is less dangerous than compressed one.

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Hydrogen powered, fuel cell electric aircraft company: Electric Air Taxi – HY4:  http://hy4.org/sample-page

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Here's an example of gearless ring motor for the described Dornier 328: 1400kW at 20Hz.

10mm thick Neorec53b magnets (Nd-Fe-B from Tdk) at the D=0.84m rotor move with 52.8m/s and provide 1.16T at +60°C in 2mm gap, of which prestressed graphite composite wound filaments occupy 1mm to hold the magnets.

The L=0.16m rotor and 3-phase stator have 24 pole pairs (though differing numbers would cog less). The conductors pass once per pole through an own slot, so the peak voltage is 468V. The magnets are 80% as wide as the poles, so the waveform provides a fundamental amplitude of 1.21*468Vpk. No fractional pitch here, but the slot skew is +-1/7 the pole width, multiplying by 0.967 the fundamental si H1=548Vpk=388Veff and 1400kW take 1203Aeff=1701Apk per phase.

The inverter needs inconvenient 1100VDC (if no H3). Multi-turn coils in parallel are more usual and may enable <500VDC, but then only a very symmetric machine avoids stray currents. Fractional pitch would bring known advantages and stay <3000VDC.

The bars superimpose 48 flat enamelled e=0.5mm W=6mm conductors that may be lasercut or etched from much bigger sheets for easier turns. I hope to introduce them sequentially in the slots. Estimated with optimism at 0.25m per bar, they show 1.35mohm per phase and lose 5.9kW in total.

The 480Hz azimuthal induction in the slots is 0.31Tpk near the teeth, so the eddy currents in the conductors lose 1.4kW in total. I didn't evaluate the radial induction component.

The stamped 4mil lamination are of Supermendur (Fe-Co from MKMagnetics) used at 1.87Tpk. This enables three 7mm wide slots per 55mm pole. They lose 40W/kg at 480Hz, or 2.7kW between the slots and 2.2kW outside.

All these losses, 12.2kW=0.87%*1400kW, can be conducted away by the bars within 22K if some grease makes the contact with the laminations. Outside the slots, the conductors are spread to cool by ventilation, and they rotate one position in the slot between two slots, as a simpler form or "Litz" (=braided) wire.

The stator currents superimpose 94mT at the gap, needing additional 45Vpk in quadrature per phase. The laminations don't saturate. The induction through the slots need additional 71Vpk in quadrature per phase. The machine could be shorter, thicker, lighter. Fractional pitch would ease here.

Here are masses in radius order:

 54kg Fe-Co external
34kg Cu
68kg Fe-Co between slots
1kg Graphite
25kg Magnets
42kg Steel
------
224kg Sum

The shaft, rings, bearings, casing, fans are not included. More poles would enlighten the motor.

Marc Schaefer, aka Enthalpy

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I described here on Apr 08, 2013 an APU that tells how smaller a motor is if it rotates fast
http://www.scienceforums.net/topic/73798-quick-electric-machines/?do=findComment&comment=737931
1270kW machine too, but estimated 16kg for 813Hz instead of 224kg for 20Hz. The two-stage gear needs maintenance and weighs a bit too.

An intermediate solution, with a single-stage gear and a medium diameter motor, may be considered too.

Planetary (epicyclic) gears are compact and powerful at small electric motors. Would they be good for electric aeroplanes?
wikipedia

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1 hour ago, Enthalpy said:

Planetary (epicyclic) gears are compact and powerful at small electric motors. Would they be good for electric aeroplanes?

Since planetary gears are used in turboprop engines and/or turbofan engines, I think yes.

Also, I've noticed a sort of upper limit to existing permanent magnet motors of ~400 Hz or so, is there an induction resonance or high power electronic switching limit, or in other words, what limits the top frequency of electric motors?

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No hard limit. It's a matter of diameter, of an angular speed convenient for the other items on the shaft, and of optimization rather than clear barriers.

Magnetic materials can operate at kHz, MHz, even GHz (though less useful at many GHz). But over few kHz, the good materials use to be brittle, which I'd dislike at an aeroplane. One can misuse iron laminations at higher frequencies, but at some point the higher frequency becomes a drawback then, not an advantage.

Power electronics can still switch efficiently at 100kHz for instance, but in the MW or GW domain, lower frequencies are better. Then you want to switch must faster than the produced waveform that powers the machine, which puts a strong limit. My waveforms are a new solution to this
https://www.scienceforums.net/topic/110665-quasi-sine-generator/?do=findComment&comment=1041409

Electric machines improve with the peripheral speed, which is limited by feasibility (in a dam), centrifugal force, and possibly the speed of sound and aerodynamic losses. Presently, a sleeve of hardened steel circles the fastest rotors. I proposed to wind graphite+matrix filaments instead. 50-100m/s is common in 1GW turbogenerators, 200m/s is reasonable with permanent magnets, while variable reluctance can be much faster. A small centrifuge can rotate much faster than 400Hz, but a D=1.3m turbogenerator rotor at 100m/s only achieves 25Hz.

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A battery-powered aeroplane can tow airgliders. Towing is the major cost of a glider flight, as the aircraft used presently are too big, designed for a higher speed, and consume lots of expensive special gasoline. Batteries power the very short flight easily and use cheap electricity. The market isn't huge, but an electric tugplane is simple to design and build, its use saves much money, so it could be a first commercial success for electric planes.

The typical flight profile is to manoeuvre on the runway, wait for operations on the cable and the glider, take off, climb in <5min to ~800m, release the glider, plunge to the airfield and land. Some Li batteries are designed for quick charge and discharge; I take data from Saft's VL25PFe, safer thanks to Li-phosphate and efficient enough here: 28Ah, 0.94kg, 360kJ/kg, 1700W/kg (yes, 200s). They show the feasibility but are not an optimized choice, nor did I check the availability and price.

300kg batteries provide 108MJ and up to 510kW. Electronics, a geared motor and the propeller convert just 120kWe (163HP) to 90kW traction. Neighbours would appreciate a silent propeller.

Several features shall avoid messing with the towing cable. The bigger wing at the rear shall counter vertical forces by the glider better. An additional rudder can fit at the bottom.

The design has a fixed landing gear and no landing flaps for safety and cost. 35m/s operation and 25m/s stalling permit the 44kg/m2. The D>1.9m propeller pulls 2.6kN at 35m/s frame speed.

350kg frame, 300kg battery and 100kg pilot total 750kg tugplane flight mass. Climbing at 5m/s with a 600kg glider takes 1.9kN, moving the L/D>35 glider 0.2kN more, leaving 0.5kN to move the tugplane, for which L/D>15 suffices - the aspect suggests L/D>25.

30s equivalent full-power of ground operations and take-off, then 160s climbing, use 23MJ or 21% discharge depth at 4*C pace, sparing the batteries meant for 17*C. They should last >15 000 cycles or about 10 years; that cost remains to check. Electricity costs only 2€ per cycle. Regenerative braking by the propeller would be useful. A 100kW cable must be brought to the runway.

85MJe left after towing can still fly the tugplane for 130km and 1h at L/D=15, or rather 220km and 1.7h at L/D=25. That's important for safety, and to convey the tugplane, and for secondary uses.

Marc Schaefer, aka Enthalpy

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2 hours ago, Enthalpy said:

A battery-powered aeroplane can tow airgliders. Towing is the major cost of a glider flight, as the aircraft used presently are too big, designed for a higher speed, and consume lots of expensive special gasoline. Batteries power the very short flight easily and use cheap electricity. The market isn't huge, but an electric tugplane is simple to design and build, its use saves much money, so it could be a first commercial success for electric planes.

The typical flight profile is to manoeuvre on the runway, wait for operations on the cable and the glider, take off, climb in <5min to ~800m, release the glider, plunge to the airfield and land. Some Li batteries are designed for quick charge and discharge; I take data from Saft's VL25PFe, safer thanks to Li-phosphate and efficient enough here: 28Ah, 0.94kg, 360kJ/kg, 1700W/kg (yes, 200s). They show the feasibility but are not an optimized choice, nor did I check the availability and price.

Aviation fuel has an energy density > 40 MJ/kg, and the bonus of not having to fill the tank up all the way (i.e. the mass is variable) so you don't have to provide additional lift for a short flight — you just load less fuel.  Does the efficiency of a battery-powered engine make up for this?

Jet-A doesn't look to be that much more expensive than gasoline

6.75 lbs/gal, so that's a little over 3 kg, or about 120-130 MJ. For electricity, it's around $0.12 per kWh, which is 3.6 MJ. So about$3 - \$4 for 120 MJ. Perhaps half the cost, but not accounting for the extra weight you have to lug into the air because you've got all those batteries, which means you need additional fuel. (plus the amortized cost of the batteries over their useful life)

2 hours ago, Enthalpy said:

Neighbours would appreciate a silent propeller.

I'm sure they would , but what connection is there to this? I can appreciate that the electric engine would be silent, but the propeller?

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There are a few biasses in my comparison. Tugplanes used presently were designed >50 years ago while I compare with a more up-to-date design and modern materials. They are too big: most accommodate 4 people while only the pilot is needed there to tow a glider. They are too fast: meant for >200km/h or more while towing is done at 110km/h. So my comparison is unfair in that the reference is unfit for the job. Or said differently, I compare with the existing stuff, not with what a new, specialized combustion engine tugplane could achieve.

Other comparison elements are more fair. Conversion from electricity to shaft power is >95% efficient, but from fuel it's rather 40% (or less with the old engines), giving an other 2.5* advantage to batteries, over the 2* you mentioned. Then, piston engines that power the existing tugplanes use aviation gasoline, not jet fuel. With high octane rating, low water, formulated with lead for old engines, Avgas 100LL costs typically 2.5€/L here in ol' Europe, ouch.

The tugplane's descent contains almost 1/2 the energy put in the ascent, so maybe 1/3 or 1/4 can be gotten back to the battery.

A turbine sized for small aircraft would use cheap jet fuel, but presently small turbines are rare and demand a special license. A Diesel engine could burn jet fuel and be less inefficient than the present Volkswagen and Lycoming that are 0.5 century old designs - several aviation Diesel were under development 25 years ago.

Maintenance is costly with a piston engine and nearly unnecessary for an electric motor. The battery must be kept under surveillance, but electronics does it for free.

Fill less the tank: in my estimate, I wanted a battery 5* bigger than the minimum, because the pilot wants the capability to wait in the air or join an other airfield if something goes wrong. That would be the same with a fuel.

So while I didn't check how good a new design with a combustion engine would be, the usual comparison is that electricity (at the present taxes...) is easily 3* cheaper than gasoline for the same flight and design epoch.

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Battery cost: I had feared a replacement every season, due to the very frequent charges and discharges. The estimated 10 years lifespan that result from shallow discharge are a relief to me.

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Silent propeller: this is a recommendation, not a consequence from the electric motor. I should have made it clearer. Once the motor is silent, most noise comes from the propeller, which shall bear the emphasis.

The first standard means for that is a more solid propeller, with more blades, wider. This goes in the good direction for regenerative braking. The other means is to bend the blade tips backwards.

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So you're comparing a new design with an electric motor vs an old design with combustion, instead of comparing electric vs combustion in a new design. That kinda skews the analysis, don't you think?

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Yes.

On the other hand, I just compare what can be done with the existing stuff. Tugplanes presently in activity were designed 50 years ago and built 40 years ago. No newer design with a combustion engine replaced them, despite fuel to operate these antiques costs a lot.

So to imagine if an electric tugplane has a chance on the market, I compare it with with the present fleet, rather with an alternative inexistent option.

Pessimism would let say that if newer thermal designs didn't get through, the electric one won't neither. But an electric tugplane has some advantages.

If willing to compare a new electric design with a new combustion engine design, we might forecast that power is 3* cheaper with electricity, maintenance is much faster, and construction looks cheaper (far from obvious, because combustion engines are often modified car engines). But this is only extrapolation and gut feeling, as neither one nor the other exists.

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More and more aeroplanes go electric, as the Beeb reports from Le Bourget
bbc.com
some claiming far better performance from battery-powered craft than my estimates here despite their wing isn't as wide.

Hydrogen is missing in the report. Because of that, "electric" aeroplanes are said not to fly far, but the limit results from batteries. Hydrogen tanks and fuel cells give aeroplanes much more range than kerosene does.