Electric Vehicle Design

[thunderhead]

Bill541 wrote


Thunderhead, since we are getting off the thread subject a bit, perhaps we should start another thread. Fun to discuss for sure.

Indeed it is.  We're a little off-topic, but I hope the bystanders don't mind. :-)  Anybody that drives is welcome to an opinion, since a lot of this is concerned with the "man machine interface" - that is, how to produce an EV that would pass the "mother test" (one that my mother could drive without needing to know it was an EV).

One idea I would like to share would be to use the vehicles brake system in the feedback loop for regen. One thought was to use the normal brake switch and a pressure sensor in the brake lines. The switch would trigger an A-D conversion of the pressure sensor and when the circuit noticed an increase in pressure, it would apply more regen, if the pressure drops, the regen would also drop.

I've driven cars which used the brake light sensor as a trigger for "engine braking": the old Daf 33 CVT used to use the brake light switch to "change gear" and provide additional braking.  There was no analogue sensing, just on or off.  My dad used to have one: it drove OK, even without analogue sensors.  

Because the brushes will be off-centre anyway, the motors can only generate maybe 1/2 the torque in reverse that they can going forward - which is pretty small potatoes as braking forces go.  But even if the motors were completely reversible, it wouldn't be much.  Most cars can brake more than twice as hard as they can accelerate: 60-0 times are normally less than 4 seconds, but a car that can do 0-60 in less than 8 is quick, especially for an EV.  For this reason, just applying regeneration when the accelerator is released may be enough.

Another possibility is to reproduce the conventional automatic transmission, with the "P-R-N-D-L" type selection.  When 'D' is selected, only a small amount of engine braking would be applied when the accelerator was released, but when 'L' is selected, much more would be applied.  'R' is 'L' but with the motors reversed.  'N' has the motors disconnected.  The only part that cannot be reproduced electronically is the 'P', which is the 'N' but with the motor gearbox mechanically wedged in some way.

I suspect there's going to be a lot of fooling around with different algorithms for accelerator response, and so I'm planning to use a microprocessor to control motor current.  The same sort of argument applies to charging and fuel metering, which are big problems as far as EVs are concerned.

      If the batteries are already full, then the regen circuit lays dormant until needed. The vehicles brakes still operate normally but during regen, they would seem to work even better. The goal of the regen circuit would be to keep the brake line pressure constant.

Most battery technologies will not allow rapid charging when the batteries are at anything more than 80% capacity: so for more than 20% of the journey, regeneration is not going to be possible.  What is worse, if you have 75% capacity as you crest the summit, it will cease to be available halfway down the hill, which could be ... exciting.

I can't help thinking of that as "the vehicle's brakes operate normally but when the batteries are full they don't work properly", which is what it is going to seem like to my mother.  "I was driving down the hill outside the house, and when I tried to brake for the roundabout at the bottom of the hill, I couldn't stop in time.  Sorry about the car."  It doesn't pass the "mother test".  Hence the desire for a dump resistor to take the regeneration current when the batteries are full.

By the way, my work is in electronics engineering as well. Mostly sensor interfacing with microcontrollers and automotive networks. Not much in the high power realm, for the most part under 1000 Watts.

My work is mainly in telecoms and embedded control, but a project spent designing battery charging systems for mass production has taught me a lot about the chemistries and algorithms.  It is a racing certainty that someone reading this has a mobile phone whose battery management software was written by me.

Most of my electronics has been small stuff, too, which is another reason to find the idea of using a modular design attractive.  I can't buy the phat semiconductors I need to control 600A anyway, certainly not at a reasonable price, so building a +100A-30A traction control unit and then stacking four to six in parallel seems like the way to go.  How many to use would depend on the design.

I do want a charger that runs on AC or DC, though: if I ever do end up offline, converting DC to AC just to convert it back again seems silly, especially when it's several kW of power being converted.

My old electrical engineering lecturer at college used to tell me that anything less than 1kW was noise.  I think he'd approve of this project...

[jacquesm]

I have driven a honda civic hybrid for about a year, and it would have certainly passed your 'mother' test, but then again it's not an all electric vehicle. However he Toyota prius does have an all electric mode and is highly popular with the female gender, and none of them seem to have any problem whatsoever to adapt.

In fact, the ones that I know of tell me all about how they try to maximize battery life and reduce their fuel consumption, as well as intricate tricks of how to make it through the longest stretch of city driving.

Give your mom some credit, people are surprisingly capable of adapting and as long as brakes have a mechanical override at the end of their pedal travel I don't think there should be any problem at all with having your mom drive an EV.

As long as the user interface is reasonably similar it will be allright. It can't be any worse than 'stick shifting' and millions of european (grand)mothers seem to be able to cope with it quite well (at considerably better mileage than their american grand-maternal counterparts who use automatics, and far larger cars).

No insult to your mom intended !! (but you started it

[RobD]

I like the idea of pressure sensitive breaking/charging. Dumping the load into another source would help with the fully charged battery problem but wouldn't eliminate it. To bad the new capacitor technology isn't up to 'snuff' just yet, it would be great to be able to dump it into a large bank of caps. with no current limit. How does the Prius do it?

RobD

[BrianK]

I went to the Van Wert county fair in Ohio, U.S.A. They had on display a chevy s10 pickup that had been converted to electric. Looked to me that they used the trany that was in it built an adapter plate to bolt the electric motor to. The one thing that I thought about was a solar charger they didn't do anything to the bed of the truck looks to me like a great space for a large solar array

   

[BrianK]

I forgot to mention that the truck was built by Vantage Career Center In Van Wert, Oh

[thunderhead]

My mother refuses to drive "manual" (that is, stick shift) cars, and has done for as long as I've known her.  Some ladies do fine (my wife is smarter and better qualified than I am) but others do not.

I don't think she'd have any objection to admitting it.  She wants cars simple, which is unfortunate given that my father is every bit as much a tinkerer as I am.

[thunderhead]

My understanding is that the battery in the Prius is nickel-metal-hydride, and since the battery is not charged from the mains it is rarely fully charged.  If it were lead-acid, of course, being left partially discharged 24/7 would cream it in pretty short order.

According to Toyota's website, if it were run electric-only it would have a range of less than a mile.  So the storage isn't so much - just enough to be useful for engine braking.

http://pressroom.toyota.com/photo_library/display_release.html?id=20040623

It might be worth using a Prius-style battery for the regeneration, separate to the main traction battery.  But that's something I'll leave until I have the basic model running.

[jacquesm]

From what I've seen eV's, even early day prototypes, they are easier rather than harder to drive than a gasoline car (or a hybrid for that matter). The hardest part is will be the choice between learning to operate an EV or not driving at all, until then I think everyone that wants to avoid a change into the unknown will find gas a pretty good alternative. I think our 'moms' are pretty compatible in that respect, mine drives an automatic too and refuses to even try stick shift (I offered many times). She has a hard enough time keeping up with the traffic, if the complication of shifting gets added in things could get messy.

Honda and Toyota have done an amazing job at 'packaging' the hybrid technology, if it weren't for the green 'hybrid' badge at the back of the car, a few new dials on the dash and a very weird feeling the first time your engine cuts out when you stop for a traffic light you'd never know you are in a state of the art vehicle. The civic is only available with cvt, I don't know about the prius, I haven't driven one (but I know a few people who do and they all have automatics). Somehow I doubt they have a stick version but I'm too lazy to look it up.

The transmission in the civic is on the very light side of being useable, it can propell the car, but that's it. NO towing capability at all, in fact adding a towing package immediately voids the warranty.

What's not so good about hybrid technology is the same thing about it that is good, the fuel consumption. It's good when compared to the same model car with a gasoline only engine. But compared to the Daihatsu Charade 3 cylinder 800 cc turbo diesel I had some 12 years ago it's pretty bad. That was old school technology and it did far better mpg than the civic and had lots more torque. They were pretty popular as a 'van' (different road tax) until the dutch govt put a stop to that. Another thing that's bad about hybrids is that they still have a gas engine

What with an electric car (direct drive type) not having any gearbox at all the stick gets tossed right from the start. Of course there will be differences, but they'll be for the most part in procedural things like charging and so on, not in the actual driving. Also, especially in the beginning insurance will be sky high. Whatever I saved on gas running the hybrid I more than paid for in insurance. Pretty raw deal.

I've seen a prototype charging station for an ev that used a fixed 'parking spot' with half of a transformer embedded in the road, the other half in the car. Combine it with a parking meter and you pay for parking and gassing up in one go.

It's just a matter of getting enough volume out there, once that is established all these details will be taken care of.

Before that will happen though there will have to be a revolution in storage technology first, or the creation of a hydrogen distribution network (as well as production on scales that are not even on the drawing board right now). The speed with which you fill up your gass tank with joules is pretty impressive when compared to charging a battery.

Some people (for instance the writer of a book called 'hubert's peak' (sp?) claim that we are rapidly running out of time to make the shift. I don't know about that (he's got an awful lot of evidence to back it up though) but one thing is for sure, the way we burn fossiil fuel will end, the debate is about when, not if. Oil prices are skyrocketing and burning it in cars and powerplants is probably the worst possible use. The plastics industry needs it pretty badly and they do not have any viable alternatives (but the transportation industry does).

[thunderhead]

Most home built EVs use a manual gearbox (stick-shift transmission) and replace the ICE with an electric motor.  If motors weigh more than transmissions, that probably makes sense.  Such a vehicle wouldn't pass the "mother test" in our family!

I'm planning to use a motor for each front wheel with a fixed 6:1 reduction (probably a chain or toothed belt) and remove the transmission altogether.  But the motors I'm planning to use, two motors weigh less than one Ford gearbox, I'd wager.

[jacquesm]

pretty far out in left field, but how about adapting the 'flat' windmill rotor design to driving each wheel individually straight on the hub ? Four smaller motors are much easier than 1 huge one with a drive train. True four wheel drive too. save tons of weight. All you'd have to do is use some kind of inverter to produce 3 phase power, reverse the field direction to go backwards. traction control could be all electronic (no more spinning wheels in the winter). that would be pretty ambitious project to undertake !

[jacquesm]

There is an italian company that sells a pot box that will do regenerative breaking into the main traction battery (which is where you want your energy to go, otherwise all you can do with the 'saved' energy is run your acessories). I'll see if I can find the reference. I saw it on some guys website that did an electric conversion of an automatic american car by simply bolting an electric motor on an adapter plate bolted on to his automatic gearbox. The space saved under the hood held a bunch of batteries and so did the trunk. Emissions tests were no longer a problem

[jacquesm]

found it : here the brand is 'zapi', it may not be italian after all (but the 'zivan' charger he is using certainly is)

[RobD]

I like direct drive but the problem is torque and rpms. It would have to spin very slowly in motor terms which means the motor would have to be quite large in diameter. Gearing down has its own problems of course. My favorite system is a generator driven by a constant power (Stirling or whatever) that keeps the batteries charged or without batteries runs the motors directly like trains do.

How about a 'booster' motor that helps the wheel motors get the car moving and also acts as a battery generator on breaking? This way the direct drive wheel motors wouldn't have to develop so much torque.

[jacquesm]

maximum torque in an electric motor is at standstill ! no need for a booster motor, four 4 Kw motors should be able to get a decent size car moving. You would probably have to limit the current somehow (but if they're ac the inverter will take care of that). The idea here is that a stationary DC motor acts like a dead short, max amps no rpm, lots of torque. As the motor speeds up it draws less and less current until a balance is achieved. The very first electric cars had clutches, but it was found quickly that that is not necessary. Also adding a booster motor will need a whole extra drivetrain just for that purpose which you have to lug along and disengage when not in use (or live with the added friction). Also the 'wheel' motors would each have to develop only 1/4 the torque required to get the vehicle moving.

[laskey]

Trains, and boats like ice breakers are run by direct drive electric motors.  The have Diesel generators that provide the current for a big Wound  rotor Induction motor.  (I guess that makes them Hybrids.)  Speed control is achived via a process known as Cyclo-convertion.  Basically you control the motor's speed by changing the drive frequency.  WRIM's are AC motors so to make it's speed variable they use normal current and normal voltage, and feed it variable frequency.  This way you get an almost constant torque over the entire speed range.

Cya,

Chris

[gibsonfvse]

Studies on this have been done already:

"The car is propelled by electric motors at each of the wheels, eliminating the need for heavy components of a conventional car such as driveshaft, differential and transaxle. This shrinks the weight further, making it possible to supply that electricity from a relatively small bank of fuel cells -- which also helps keep costs down."

However, I'm still wondering when (if?) such a design will finally come to market.

[jacquesm]

nice article, thank you ! It's over a year and a half old, I wonder what they are up to now (the article states that they want to have a roadworthy prototype by the end of 2004)

The idea of driving at the hubs is probably pretty old (hardly an original thought in my head anyway , what I would like to see is a working machine. Wonder if there is one out there somewhere and how it performs. It seems like the most natural solution to the drivetrain problem, the differential is just a kludge because it's impractical to have two ice's driving one wheel each in a conventional setup. (let alone four in a fourwheel drive system). But if we can have brakediscs on all four wheels then I really don't see why we can't have another rotor on the same shaft to drive it (and use it for regenerative braking at the same time)

[bill541]

This is a simplistic diagram of a possible EV motor controller with regenerative braking. Regeneration could be fully automatic based on battery voltage and braking conditions. It could also be enabled by simply using a switch similar to the ones used on diesel trucks with compression brakes, in which case regen would occur any time the throttle was lifted. Current monitoring during drive and regen would be required to prevent overloading the motor and/or the controller.

During forward drive, the REGEN PWM  is off and FWD PWM signal turns on Q1 causing current to flow from the battery to the motor. The PWM (Pulse Width Modulation) duty cycle will determine the speed and torque at which the motor will turn.

During Regen, the FWD PWM is off and REGEN PWM turns on Q2. This shorts out the motor causing current to rise in the motor windings (because the motor and vehicle is still moving). When Q2 is turned off, the motor field collapses and current flows from the motor through D1 back to the battery and/or capacitor bank. The REGEN PWM duty cycle controls the amount of regenerative braking applied. The frequency of REGEN PWM should be chosen based on the motors inductance for optimal performance. D1 would need to be of adequate size and heat-sinked well as it is used for the catch (freewheeling) diode during FWD drive as well as for the regenerative braking current path.

If you were real tricky during regen, you could even turn on Q1 when Q2 was in its off cycle to provide a lower resistance path to the battery as the motor field collapses. The charge pump would need to respond quickly so that the gate voltage on Q1 was always above the source voltage. A forward biased N-channel MOSFET has a much lower voltage drop than the diode and should produce lower heating (I2R) losses. One benefit of a MOSFET is that current can flow source to drain or drain to source as long as the gate is forward biased.

If a series wound DC motor is used, some means of providing field excitation current may be required. This may be accomplished by a few pulses of the FWD PWM just prior to applying the REGEN PWM signal. Series inductance could be added between the controller and motor + input and a small value of capacitance added to form a pi filter.  This would decrease the EMI and add a bit of inductance to the motor during regen.

By using MOSFETs, you can run at higher switching speeds than you can with IGBTs, making it a bit easier to filter EMI. Of coarse, a full evaluation of the circuit would be required to determine weather using MOSFETs or IGBTs would be the most efficient overall.

-Bill-

[bill541]

Jacque,

I think this is a modification Jerry did to add regenerative braking to his Zappi controlled motor. This is similar to what Otmar (maker of the Zilla motor controllers) did on an earlier EV with an older Curtis controller.

http://cafeelectric.com

http://cafeelectric.com/curtis/regen/index.html

Adding regen to a low side switching controller can get a bit messy when done with external contactors. But it can be done!

-Bill-

[jacquesm]

It certainly looks similar. Is there any reason why EV's use DC motors ? PWM control of AC 3 phase motors (using H bridges) should give you pretty good efficiency and does away with the field and the brushes, or am I missing something obvious ? Civic and Prius both use DC brushless motors. this article states the main reason is the simplicity of the controller as the limiting factor (cost). In the long run though it seems the money is on AC systems.

[bill541]

As you mentioned, I think developing a 12KW plus size inverter to run the AC motors compared to a simple PWM DC controller is one major reason. The other may be that regeneration is more difficult to do with an AC induction motor. A PM 3-phase motor may not be as bad for regen though.

The brush type DC motors are getting a lot of mileage out of the brushes these days, I think they change the brushes about the same time it is due for a bearing change. I guess this would depend on how hard you were hitting the motor.

I think where the AC motors really shine is when the battery bank voltages really get up there around the 1000V mark. Of coarse, the DC brushless motors can get up there too.

Basically I think it is the economics involved. Is it cheaper to do high power commutation with electronics or mechanical brushes? One advantage I can see with electronic commutation is that you can set the advance dynamically all via electronics.

-Bill-

[bill541]

Jacque,

You got me thinking about the AC drive requirements and how to do regen. After a bit of research, I think the simple explanation is that the controller drives a frequency lower than the motor is turning. The part I'm not so sure about is how the current is forced back to the batteries without damage to the inverter. I'll post back after some more digging.

The AC drive system that Siemens makes for EVs certainly looks like a very nice combination. Some information is available at http://www.metricmind.com/index1.htm

I'm not sure I agree with all of his arguments, but there are some good points.

-Bill-

[jacquesm]

I think that to do it properly you'd need some kind of encoder on each shaft, fairly low res, enough to be able to make a servo loop that is phase locked with the actual situation on the hub, that way you could always 'lead' the rotor by just so much to produce optimum torque.

Possibly a hall effect or gmre sensor right next to the magnets on the rotor
would do. (cheap and reliable, no need for a spinning glass disc or something
like that)

The regen may not be that hard in fact, as long as you make sure you keep your H bridge protected with diodes in series with the coils and fet outputs they are to all intents and purposes disconnected as soon as you stop driving them. Then you could open up the output lead from a full wave bridge that is permanently connected to the three phases of the motor and start drawing current to the battery. From what I get all it would take would be a fairly small inductor in series with every coil before the rectifier to go recover the energy.

No relays, just a chunk of high current devices (or a good sized bank of smaller ones).

The easiest would be to give each motor it's own inverter. More reliable as well (if you fry one it could be taken offline, the car would still drive and at the next opportunity you can switch it with a good unit).

For a pickup the rear wheel drive ones could be a little bigger, for a regular
car they could all be equal.

[thunderhead]

My approximate calculations are based on needing 20kW to do 75mph, and the vehicle weighing something like a ton.  I have a front-wheel-drive car, and I'm not going to fool around with driving the rear wheels at this time - perhaps for a later project.

So that makes each wheel motor a minimum of 10kW.

The problem is acceleration.  To get a 1-ton car from 0 to 60 in 15 seconds requires that you push it with a force of 1800N - about 400lbf - neglecting air resistance, which reaches about 300N at 60mph. If you've got 13 inch wheels you need a torque of about 300Nm.  To get a top speed of 75mph you need to get the wheels to 1000rpm.

The motors I'm looking at deliver 56Nm at 400A, and are rated to a maximum 6500rpm and 35kW each.  They're AVT modifications of the Lynch LMC-200 (data http://www.lemcoltd.com/performance.htm, but scale up for 120v).  To get 75mph the maximum reduction is about 6.5:1, and to get 300Nm the minimum reduction is 5.35:1.  I prefer acceleration to speed, since it's more useful in real driving and you don't lose your license for excessive acceleration, so I'll aim for 6.5:1.

Eliminating the reduction gear and driving the wheels directly has three problems I can see.  The first is that there is a direct relation between torque and armature diameter, and in a practical design the force between rubber and road will be more than the force on the armature.  So to build a motor that can drive the wheel hard enough to spin it at the lights probably means a motor that is bigger than the wheel.

Another problem is that fitting the motor to the hub adds it to the unsprung weight, which becomes a problem when trying to maintain grip on bumpy roads.

Lastly, here in sunny England we've been experiencing an August in which I've had to drive through water over the hubcaps several times on my journey home.

For these reasons, my plan was to put a reduction unit on the other end of the half-shafts, the other side of two universal joints and so away from the unsprung weight, and use the shape of the reduction unit to mount the motors up high, away from floodwater.

[thunderhead]

...especially in the beginning insurance will be sky high. Whatever I saved on gas running the hybrid I more than paid for in insurance. Pretty raw deal.

There are several insurers over here that specialise in kit-car insurance, which is the main reason why I chose a Quantum over the standard Fiesta upon which it is based.  Yes, it's GRP bodyshell, which makes the car 20% bigger and 6% lighter, but the main advantage is that kit-car insurers are tolerant of wierd things under the bonnet.  Since my 70kW of motors produce about the same power as the petrol XR2 engine that is in there now, their only loading will be for the cost of the batteries.  But the batteries cost me less than £250, so that will be fine.

If I were to have a rush of capitalism to the head, and start making them, I'd be up against the 500-unit limit for SVA, which would curtail production a little.  But then I don't have the infrastructure to make or sell 1,000,000 cars. I suspect I could make and sell a couple of hundred a year here in Europe, and having them designated "kit cars" would help insurance a lot.  The only difference would be that I'd use all new parts and so get a new car registration mark.  My guess is that they'd retail for about £16,000 each with the lithiums in, which is more than the cheapest cars but much less than the more expensive ones.

More likely, if I had such a rush of capitalism to the head, I'd lie down until it went away.  But if I can get to these sorts of prices, handbuilding four cars a week in a workshop somewhere, so can Ford.

Before that will happen though there will have to be a revolution in storage technology first, or the creation of a hydrogen distribution network (as well as production on scales that are not even on the drawing board right now). The speed with which you fill up your gass tank with joules is pretty impressive when compared to charging a battery.

That it is.  I have never seen a practical battery technology that can refuel at faster than the 1C rate or for more than 80% of capacity.  So an EV needs to carry a day's fuel, or a backup generator for those few occasions we drive further.  But an alternative would be charging points in the car-parks of motorway service stations and roadside cafés, so you do 3 hours driving, then plug the car in while you have something to eat.  The infrastructure to refuel an EV is much simpler than that required to refuel an ICE, which makes it cheaper and more cost-effective to offer refuelling points.  I can buy outdoor mains plugs from B&Q for less than £5 each - you won't get a petrol pump for anything less than 1000 times that cost.  To cost the same for the fuelling equipment, your petrol pump needs to refuel the car to 80% in less than 3 seconds, which is more like F1 pit-stop speeds.

I still don't believe in the "hydrogen economy" for cars, because their refuelling infrastructure is even more expensive, but even they will need motors to turn the electrical energy from the fuel cells into mechanical energy to push the car down the road.

The revolution in storage technology is called "lithium": as well as the Thunder Sky units I keep mentioning, the last three months have seen announcements from several other battery manufacturers: Valence Technology, for example; or a French company called BatScap.  I'm expecting that by the time my lead-acid AGMs wear out, that I will be able to buy lithium batteries from lots of places.

Some people (for instance the writer of a book called 'hubert's peak' (sp?) claim that we are rapidly running out of time to make the shift. I don't know about that (he's got an awful lot of evidence to back it up though) but one thing is for sure, the way we burn fossiil fuel will end, the debate is about when, not if. Oil prices are skyrocketing and burning it in cars and powerplants is probably the worst possible use. The plastics industry needs it pretty badly and they do not have any viable alternatives (but the transportation industry does).

My oil-prospector brother also believes that time is running out, and we are already past the peak.  However, be of good cheer: plastics can be made from coal (and so can oil), and it seems certain they can be made from organic oils, too.  The price of plastics may go up, but they won't disappear.  More expensive plastics will encourage people to value plastic items, rather than throwing them away, as shown by the Irish "bag tax".

It is energy that will be hit hard by the end of petrochemicals, and many countries (including my native Britain) have alternatives waiting in the wings.  I don't believe motoring will disappear, nor yet do I want it to disappear: I'm looking for a way to continue my motoring without relying on petrochemicals.

[thunderhead]

It looked overly complicated to me, with all those high current relays, and I got wondering what would happen in the "make before break" scenario.

Probably something like this:-

http://www.jerryrig.com/convert/step38.html

Circuit design should be modular, failsafe, and above all elegant.  Perhaps I'm asking too much... :-(

[jacquesm]

how about battery swap stations ?

Keeping a large (as much as the number of cars that pass a station per day + some margin) amount of batteries charged in parallel at a gas station, an automated swap out procedure (drive over griddle in the road, griddle opens, your battery pops out and disappears underground, new battery pops out of the ground, pay depending on the difference in charge between the pack you dropped off and the one you're taking with you)

All vehicles would have one or more such bays depending on vehicle size and desired range.

[jacquesm]

yes, you make perfect sense. I never did any calculations on ev's, but the relationship between armature size and torque is pretty logical (a similar relationship exists between armature LENGTH and torque, possibly you could series two smaller motors on the same shaft ?).

The spot where the engine used to be at the other sides of the half shafts is the place to go for instead of on the hubs (my bad). In most cars there used to be an ICE there, so you should have all the room you want !

I wonder how much margin there is in those motors, and how much it would take to put a thermal feedback in there that allows you to run the motor at its rated winding temperature instead of at it's rated current.

Sort of a controlled overload situation during accelleration. That might gain you some torque for free.

How about snow mobile like belt drives ? Or maybe planetary gears or inline reducers (like the kind they use in the process industry, only one wear part and no gears). There are tooth belts with that kind of torque (kevlar inlay, hardly any stretch, very little wear, extremely strong, up to 10's of HP are readily available).

A car would probably need to be seriously overdesigned wrt starting torque, imagine starting from a standstill going uphill with four people and some luggage in the car ! (think skiiing holiday or so)

fwiw, I spent some time in Poland before the wall fell, and lots of people there drove wartburgs, serenas and trabants. These are all very old 2 stroke eastblock cars, the engines in these things were unbelievably small, about 25 hp. Yet they did surprisingly well in traffic, no problem at all. The trabant was especially popular in east germany where it was produced. It has a plastic body (with a resin so flammable that you'd wonder if this was east germany's solution to the overpopulation problem) and weighed about 630 Kg.

Judging by the fact that that was a car that was probably living at the low end of 'possible' I'd say for practical use you may need to up your torque requirements a bit.

The easiest conversion then would be a lightweight four wheel drive car. That would give you four spots to tie motors to instead of two ?

Suzuki Sidekick or so ? Subaru (justy ? are those 4WD too) makes some pretty small cars with 4WD.

[bill541]

One other problem you may run into with using two brush type DC motors, one for each wheel is getting them synchronized. With one motor running backwards, its brush advance would need to be adjusted the other way from zero. Even with this done, the motors can still lead or lag each other making it harder to stay on the road.

In robotics when they use two motors that are not tied together, they often run a counter shaft so that both motors spin the same direction.

Using AC motors which would be inherently synchronous may be a solution, but even this presents its own problem. This is the problem of cornering, you no longer have a differential to allow the wheels to spin at different rates.

In my mind a single motor driving a differential is a good thing. The differential also gives you a gear reduction for increased torque. I would think a small vehicle with an aluminum transaxle solves a bunch of problems, not to mention having quite a gear selection to choose from.

My personal selection for an EV would be a small pick-up truck, rear wheel drive with the batteries under the bed. These little trucks already have the needed suspension components for the increased weight and they are relatively light weight.

-Bill-

[bill541]

I think the load on the AC motor will keep it lagging the controller under drive conditions. When the motor is up to speed, it would draw less current but still be lagging the controller waveform.

During deceleration is when the motor can lead the controller. The controller would have two options. (1) coast and allow the motor to run at what ever rate it wanted to.(2) The controller could keep applying the same AC frequency and slow the motor all while recapturing the current generated by the motor which is trying to rotate faster.

-Bill-

[jacquesm]

hehe, and here I was thinking that getting rid of the diff was an advantage. You're right about the cornering, I didn't think of that at all. Otoh, driving the wheels with independent inverters would give you all kinds of possibillities that could take the place of a diff. A little inertial sensor or something hooked up to the steering colum could determine the current wheel angle and adjust the drive frequency of the wheels to match. That still leaves the gearing problem though.

 

[jacquesm]

I don't understand the 2nd option, doesn't that drive the motor again until the point where it will trail the field ?

[BrianK]

The electric wheel designe is allready in use if you can check out one of those locomotives that run the tracks they are really a big gennerator with electric driven wheels But then also ask how much fuel it takes or even holds I have been told about 3000 gallons.