Two- or three-speed gearbox?

Do you own or use a EV. Then this is a good place to discuss things.
MalcolmB
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Postby MalcolmB » Mon May 19, 2008 11:04 am

Hi Peter,
My thinking goes like this (please let me know if my reasoning is fuddled): I'm looking for a top speed of around 70 mph and an average speed of around half that. To get the best efficiency from a DC motor I would gear the motor so that the vehicle reaches 70 just below the motor's no-load speed. This means the motor is running fairly close to peak efficiency at its average speed.

The problem with this approach is that torque tapers off to nothing as you approach the no-load speed, so acceleration is very poor in the upper speed range. The simplest way to get round this and maintain efficiency, as I see it, is to add a higher gear that would give me 70 mph at, say, two-thirds of no-load speed. Of course this means that my theoretical top speed is higher than 70 mph, but I would never exceed that normally since it would be illegal, and more importantly I would only get a few minutes' range :)

I think the tip-over point you mention depends on the size of the motor. If you use a big motor and gear it for a higher speed than you need, you can get away with a single ratio. But to get maximum efficiency and range I'm guessing a smaller motor and two-speed gearbox would win, assuming your gearbox doesn't add a massive weight penalty or sap too much power itself.

Another option would be to splash out for an AC system, which has a much flatter torque curve.

bobc
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Postby bobc » Mon May 19, 2008 11:45 am

Hyve - your power/speed maths seems a little muddled - might be just "finger trouble" -
1) drag is a force. & drag is proportional to speed squared.
2) power = force x speed so power is proportional to speed CUBED.
so in your example, neglecting friction losses, if 6kw is needed for 35mph then 6x2x2x2 = 48kW is needed for 70mph!!!

I'm hoping that much less power than this is actually needed - the 'greenpower' racers use 1/2kW at 35mph & I'm assuming 10kW will get my kitten up to near 60 (eventually - it would help if its aerodynamics weren't so poor....)

Malc - the AC systems have a flat torque characteristic up to "base speed" then an agreeably constant power region up to 3 or 4 times that, which shuold make a gearbox unnecessary... - I'll find out in about 6 months when I've done my inverter.... so my kitten should have full torque up to about 15mph (like 1st gear) & tail off above that
cheers
Bob

MalcolmB
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Postby MalcolmB » Mon May 19, 2008 1:53 pm

Bob
Are you building your own inverter? :shock: Which motor are you using?

Another idea I just found in the back of my head gathering dust is the Killacycle approach. The idea is to use two smallish motors with a series/parallel contactor.
Each motor drives a half shaft independently, so you do away with the need for a diff. Both motors are driven by the same controller. You start off with both motors connected in series, so they each see half the voltage and full current, which means plenty of torque. Once you've reached a certain speed you switch the motors in parallel, so they each see half the current and full voltage, taking you up to maximum speed. I really like the elegance of this system as it gets rid of even more greasy bits, but I'm not sure how you would arrange reliable switching. The Killacycle does it automatically using a Zilla controller, which is a bit out of my price range.

bobc
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Postby bobc » Mon May 19, 2008 6:45 pm

yeah Malcolm,
it will be a liquid cooled mosfet inverter (both firsts for me....) (I do industrial IGBT inverters for my day job).
My motor is a standard 5.5kW TEFC induction motor, I had it rewound by ASK rewinds in Altham to 70V - that cost me £250. I'm converting the motor to liquid cooling by fixing 8mm microbore tubing between the fins with PU adhesive (there's a pic on my website). I must say, it all looks the part, but the proof of the pudding will come over the next year or so when I try to get it moving under its own steam (or electrons....).
I will certainly be designing the inverter in proper 'industrial' fashion so that I will be able to make more if there's a demand. The induction motors do have plenty in their favour - & some of the 'fancy' EVs (e.g. tesla) seem to be using them.
My employers (siemens) also make copper- rotored induction motors for high efficiency applications so I'm on the lookout to get a copper rotor if it's possible ;^)
Bob

MalcolmB
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Postby MalcolmB » Mon May 19, 2008 8:35 pm

Some great stuff on your site Bob. Won't pretend I understand all the inverter tech, but I like your ideas for modular EV components and especially the daisy-chained single cell chargers. I think there would be a good market for those.

That's a hell of a lot of work you've put into rebuilding the Kitten, I look forward to hearing how it performs. Your efficiency figures for the motor look so good I wonder if you'll really need liquid cooling?

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qdos
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Postby qdos » Mon May 19, 2008 10:38 pm

bobc wrote:Hyve - your power/speed maths seems a little muddled - might be just "finger trouble" -
1) drag is a force. & drag is proportional to speed squared.
2) power = force x speed so power is proportional to speed CUBED.
so in your example, neglecting friction losses, if 6kw is needed for 35mph then 6x2x2x2 = 48kW is needed for 70mph!!!

I'm hoping that much less power than this is actually needed - the 'greenpower' racers use 1/2kW at 35mph & I'm assuming 10kW will get my kitten up to near 60 (eventually - it would help if its aerodynamics weren't so poor....)



Hi I've been away all day and not long been home. I can tell you from driving the Zest which is a 15kw petrol driven quadricycle that I have done over 120km/h in it I can not get over that but that's also down to the rev limiter on the engine which cuts in at about 3800rpm.

The Zest is anything but aerodynamically efficient as it's totally open top. However being such a low powered machine you can notice quite clearly the effect on aerodynamics but playing with the shape of the car.

There is a small wind deflector / windscreen which if you remove it will give you almost 10km/h on top of your max speed also if you drive with the toneau on you get a better top speed and I also dropped the driver's seat too which lowers me about 4" out of the air flow over the top of the car. I would also like to enclose the rear wheels at some time too and see what effect that has. It's all part of the fun of driving a low powered car. I've owned Lotus's BMW's and other sports/race cars but I've never had as much fun with aerodynamics as I've had with the Zest.

hmmm that's a rather long winded way of saying 15kw should get you to 70mph really isn't it LOL

MalcolmB
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Postby MalcolmB » Tue May 20, 2008 7:33 am

Your 15 kW fits in well with what I've found for my bike Kelvin. That uses about 5 kW at a steady 50 mph. To get to 70 mph (which would mean changing the gear ratio) I would need roughly 5 x 1.4 cubed kW, which comes to 14 kW.
I know you've messed about with plenty of kit cars in the past. What would you recommend as the best candidate for an electric conversion? I'm looking for a second car that I can use as a daily short-distance runabout, with a roof and doors, at least two seats and some stowage space for shopping. It obviously needs to be as light as possible, and preferably based on a donor that has readily available parts. I was thinking of something along the lines of the Midas Bronze. (This will obviously tip the weight well over my 400 kg target).

hyve
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Postby hyve » Tue May 20, 2008 7:45 am

Bobc,
Speaking of fuddled maths: 6 cubed is 6x6x6 when I went to school; this makes 216kw by your reasoning - clearly not correct.
The source I quoted for the formula I mentioned also listed a number of real cars for which these sums had shown agreement. I am no mathematician and simply use stuff which has been shown to be reliable, such as Pythagoras theorem and so on.
Peter Ph

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qdos
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Postby qdos » Tue May 20, 2008 8:35 am

MalcolmB wrote:Your 15 kW fits in well with what I've found for my bike Kelvin. That uses about 5 kW at a steady 50 mph. To get to 70 mph (which would mean changing the gear ratio) I would need roughly 5 x 1.4 cubed kW, which comes to 14 kW.
I know you've messed about with plenty of kit cars in the past. What would you recommend as the best candidate for an electric conversion? I'm looking for a second car that I can use as a daily short-distance runabout, with a roof and doors, at least two seats and some stowage space for shopping. It obviously needs to be as light as possible, and preferably based on a donor that has readily available parts. I was thinking of something along the lines of the Midas Bronze. (This will obviously tip the weight well over my 400 kg target).


I'd love to say a car we're working on but it's a bit early for that! Teh Midas is still about and funnily enough I did speak to them at the show :wink:

Currently there's just the R2 and the Jimini available and on the road but there will be more. Most kits thugh are sports car orientated. The Quantum is a lovely practical Fiesta based car but it is a tad heavy Chris B converted a Fiesta which Alan and Honar Ward now own. There are also Quadricycles available which are ideal too though lots feel they are too small for them to feel safe.

At the end of the day it depends on what you are looking for out of the car Kit cars are superb they are extremely green as you are recycling otherwise cars which would be scrapped and I can tell you I drove my first for over 12 years every day and only got rid of it because I was bored drivng the same car all the time!

hyve
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Postby hyve » Tue May 20, 2008 8:48 am

Hi Malcolm,
Going back to your response to my offering: not only does torque fall off to nothing at the no-load speed, horsepower does too, being a product of torque and revs. Since at top speed you need a lot of power, the gearing, or electricity utilisation at this point will need to be rather different.

These questions get to the heart of the whole problem of trying to use electric motors for vehicle propulsion: their characteristics are so different to IC engines, which increase power along with revs to a much more marked extent. This rather suits vehicle propulsion at the higher end, but not starting from rest.
But there are so many variations on electric motor configuration that a lot more is possible and getting to grips with all this is taking me some time. Your idea of switching power for 2 motors is a fine example, though as you say it sounds like a big bunch of contactors will be needed.

Returning to the physics, you really need power curve graphs for the different motors you're looking at. The only way to gear the vehicle is to get maximum output power/kw at max. road speed, if you're trying to keep the motor size, battery pack and finished weight/cost within reason.
What actual power you'll need at 70mph is the hard question ! Examples of similar machines is about your best hope for arriving at this figure, but frontal area has a lot to do with it.

Having worked this out you will then have a gear ratio to multiply the torque figure by. It's important to remember that torque versus weight is what governs acceleration, not horsepower. There is a lot of misinformation in the popular motoring jargon, talking about horsepower per ton. This ratio is fairly irrelevant. Torque is what accelerates the vehicle; bhp comes at just one point on the rev band and affects top speed mainly, unless you have an infinitely variable transmission and can keep the motor at that speed constantly.

By comparing the result of your sums with some figures for ordinary IC cars with known gear ratios, torque figures and allup weight, you should be able to predict whether your planned vehicle will have enough torque to cope without a gearbox.
There is a complication in this approach, of course: a lot depends on what type of motor you use. This is where my query about tipping point comes in. So far my feeling is that we need a series wound DC motor to give that huge start-off torque, then change it's wiring to an AC motor for high speed ! Such a motor would compare with the IC version so very differently that making this comparison will not really help.
Whether anything like this is possible I leave to you electric boffins, because I just don't yet understand or know enough about what can be done. I'm just drawing conclusions from the requirements vs. the characteristics. Someone else has put it well in saying that electric cars use clever electronics instead of old fashioned mechanical gearboxes to create the same result.

Good luck with it anyway.
Peter Ph


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