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#11
11-06-2010, 10:01 AM
 lloydn Junior Member Join Date: Dec 2009 Posts: 7
Re: Smart Fortwo EV..... High power version!!

That looks like an interesting equation there Duncan.

Just wondering where you got it from, and the units you are using, because I can't seem to get it to work with numbers I've been running, and I'd be quite interested in it.

Just to illustrate, I have an 02-05 Ford Fiesta currently. Weight is about 1045kg. Max Torque from the 1.25 is 110Nm. Wheel radius is 0.292m.
 Motor Torque (Nm) Gear Ratio Wheel Torque (Nm) Wheel Force (N) 110 15.2866 1682 5760 110 8.2411 907 3106 110 5.4656 601 2058 110 4.0565 446 1527 110 3.2452 357 1223
"Vehicle weight on the driven end x tire coefficient of friction (0.8 for "inexpensive" road tires = Max force the tires can use"

and an estimate of 60% weight on the driven wheels (627kg)

627 * 0.8 = 501.6N

Much lower than the force available in any gear. And Force divided by mass gives acceleration, so that would be 0.48 m/s^2, or a 0-62 of about a minute. Pretty sure my car isn't that slow... So I think it must be a different units thing, please let me know what you're equation is using.

What I've been using to judge the maximum usable torque is:

Torque = distance (radius of wheels) x mass (kg) x acceleration (m/s^2)

Then looking at other road cars to see what a reasonable max acceleration is. The maximum seems to be about 0.5g for FWD, and 1g for RWD.

Using that equation on my Fiesta gives about 1497Nm (about 5 times the force of the previous equation). First gear makes more sense with that result.

And for Yabert:

On the Smart, 0.287 radius wheels, 750kg, RWD, gives 2112Nm maximum. I'm not sure the smart is really set up to handle that much though, so just think of it as an absolute maximum.

In order to achieve 0-50 in 5.2 seconds (0-60 in 6.3) with direct drive, you'd need about 1200Nm of Wheel Torque, or 300Nm of Motor Torque with a 4:1 ratio. That is about 56kW of power.

If 90mph is the max speed, and 4:1 is the ratio, then 60mph is 3440rpm; so you need a motor that can develop 300Nm or more Torque from 0-3440rpm. That's 0-60 in 6.3s ish.

If you did get the torque of a warp 11 at 1000 Amps (434Nm, according to evsource), you'd get better than Tesla Roadster level acceleration.

I don't know much about motors, so hope for one of the more knowledgeable guys to come along, but this is my best guess with what you have.

The 9.6kW at 48v is 200A continuous, similar to the warp motors. The commutator section looks quite big, so should overload quite well, and will probably take the 1000Amps if they Kelly really supplies it, for a short time. I don't know how much torque per amp you will get though, if there is a torque rating on the motor, please let us know.

5000-6000rpm for that size motor would seem like a reasonable limit. If the motor has an rpm rating, it would help to know that too.

My concern would be that at 150v it might be spinning too fast, or if it is spinning at a good speed, would not be producing the torque that you need.

#12
11-06-2010, 03:44 PM
 Yabert Senior Member Join Date: Feb 2010 Location: Québec, Canada Posts: 1,484
Re: Smart Fortwo EV..... High power version!!

Quote:
 Originally Posted by MalcolmB What motor rpm would this require to get you to 90 mph?
My wheel have a diameter of 22.58" so at 90mph with 3.7:1 ratio that give 4955 motor rpm.

Thanks a lot for the link for the performance calculator.
But I see one strange thing when I played with the prameter of this calculator. Why at 125v (48s sag to 2.6v) the Warp 9 give his max torque from 0 to 3750 rpm and the Warp 11 can give his max torque only from 0 to 2750 rpm. When I played with gear ratio that give me the same 1/4 miles time. Strange?!!? Why a smaller motor will has a wider power band?

One conclution I can see from this calculator was it's a big advantage of try to go higher in voltage to have a wider power band. In my case go to 54s 7P vs 48s 8P would be a great advantage. What do you think?

Quote:
 Originally Posted by duncan No reverse just push it back
Hahahaha!! I like your idea, but I think the idea to use a old reversing forklift contactor will be the better one.

Quote:
 Originally Posted by dimitri I would recommend to design your battery pack such that you can add more cells in parallel to existing 48 groups later on, if/when you realize you need more capacity to get your range, so you can get from 48S8P to say 10P or 12P without having to redo a lot of work.
I don't have the space and the money to put more cells in paralel. If My calcul is right I will need around 80A to drive at 50mph and that give me a range of 40 miles at 80% DOD. So if my range will be less than 40 miles I can't do anything and I will do with. Less range maybe, but it's less money for an electric car and that is the most important thing.

Well, in conlusion, because the Crown motor is around 80% of the lengh of a Wrap 11 or similar motor I think I need a bigger motor to perform a realy fast 0-60 mph. The torque capacity of this short Crown motor is probably more comparable to a Warp 9 and it isn't what I need.
#13
11-06-2010, 04:20 PM
 Duncan Administrator Join Date: Dec 2008 Location: Southland New Zealand Posts: 4,485
Re: Smart Fortwo EV..... High power version!!

Hi Lloydn

and an estimate of 60% weight on the driven wheels (627kg)

627 * 0.8 = 501.6N

You changed units from Kg to Newtons - 1 kg exerts 9.81 Newtons downwards due to gravity (I normally use 10 as a conversion)

This becomes

and an estimate of 60% weight on the driven wheels (627kg)

627 * 0.8 = 5016 N

Which means that you should be able to spin the tires in first gear but probably not in second!

My "car" (partly built)

Vehicle weight - 650Kg / 50/50 weight distribution 325 Kg on back wheels
325 x 10 x 0.8 = 2600N

or 325 x 2.2 x 0.8 = 572 lbs force

Tires are 1 ft in radius therefore 572 ftlbs torque

Final drive 4.1 : 1

prop-shaft torque required is 572 / 4.1 = 140 ft lbs

I have a 48volt 10Kw Hitachi forklift motor - 11inches diameter 14 inches long weighing 102 Kg

I think (guess) that it will be comparable to a Warp11
The Warp11 graph goes to 140 ftlbs at about 450 amps

So I decided not to go for a gearbox!

I have ordered an OpenRevolt controller kit - 500 amps

I don't know what max revs - 5000rpm is about 80 mph - should be good enough - if I blow it up I will have to find another one!!

#14
11-07-2010, 04:41 PM
 Yabert Senior Member Join Date: Feb 2010 Location: Québec, Canada Posts: 1,484
Re: Smart Fortwo EV..... High power version!!

Hi all

What do you think about dual motor. Something like one 8" motor coupled to each rear wheel by 4:1 ratio.
I have 350\$ budget for a helical LSD, but with two motor, I don't need LSD. I only need to coupled each motor in parallel (500A, 154v each) and they give me similar torque to each tire. This set-up will probably weight a bit less than big 11" motor+ helical LSD, but I'm not sure about power capacity.

Comment?
#15
11-07-2010, 05:08 PM
 lloydn Junior Member Join Date: Dec 2009 Posts: 7
Re: Smart Fortwo EV..... High power version!!

Thanks Duncan, I see I mixed up weight and mass. That gives a more reasonable number.

It's surprisingly hard to find good numbers for static coefficient of friction on the internet. I've seen anything from 0.7 top 1.0 for normal cars on dry tarmac, and 0.3 to 0.8 for wet.

I think it's quite important to include weight transfer if you are using the equation you are, as most of the recorded acceleration times, especially of RWD cars, are not possible with the equation as it stands. For Yabert's Smart, it would mean 0-60 in less than 7s is impossible.

Regarding battery setup, I just had a look at the Kelly website, and the max operating voltage is 180v. A 48s pack is the maximum you can really run for that, charging each cell to 3.75v. If you put any more in series, then the Kelly would not work with the fully charged voltage. 48s8p is a good set up though.

If your motor is 80% of the size of a warp 11, and if you get 80% of the torque, then you should get better than your required acceleration (less that 6s 0-60 if you're lucky). The big two ifs are if the motor is like a warp11 with 80% torque, and can the smart put the torque down? Only way to find out is to test it really.

Range wise, I think you'll make your 40 miles quite happily at 50mph, though I don't have a calulator worked out for a real driving cycle. That's just calulcated from rolling and air resistance, and 80% of the charge of the battery.

You might want to consider a seperate fan for cooling the motor, for low speeds, and it can't hurt when driving it harder too.

If you can fit the two motors and gearing, that might be a viable option too. It might be hard to get two reasonably well matched motors.
#16
11-07-2010, 08:55 PM
 Duncan Administrator Join Date: Dec 2008 Location: Southland New Zealand Posts: 4,485
Re: Smart Fortwo EV..... High power version!!

Hi Yabert

Which diff are you looking at - I just scored a complete front and rear setup including an LSD for a Subaru for \$200

Subaru LSD's seem very common here - they are made by Fijutsi Heavy industry and I think are used in Nissan's as well

If you get a cheap LSD - a single motor would be easiest - (my motor cost \$100 - did I say I am a Scotsman?)

Hi Lloyd
Weight transfer (fore-aft)
Force at tire contact x height of center of mass = wheelbase x wt transfer

So a dragster can have a high C of G,
I like cornering so I am going for a low C of G

For my car which is quite long with a very low C of G it gave very little weight transfer - I think it was 10%

It's surprisingly hard to find good numbers for static coefficient of friction on the internet. I've seen anything from 0.7 top 1.0 for normal cars on dry tarmac, and 0.3 to 0.8 for wet.

Thats what I found which is why I went for 0.8 - for cheap tires (did I say I am a Scotsman?)

I don't think less than 7 seconds to 60mph and cheap tires go together!!
#17
11-08-2010, 07:29 AM
 MalcolmB Senior Member Join Date: Jun 2008 Location: Newcastle upon Tyne, UK Posts: 561
Re: Smart Fortwo EV..... High power version!!

Quote:
 Originally Posted by Yabert What do you think about dual motor /snip/ I only need to coupled each motor in parallel (500A, 154v each) and they give me similar torque to each tire.
Dual motors sound good. I've also been considering running two motors in parallel but recently saw this comment from CroDriver that left me a little uncertain: (http://www.diyelectriccar.com/forums...l#post208510): "the problem is that there is no way to make both motors perfectly same, the wires are not the same and the connections are not the same. There will always be a different resistance from the controller to the motor. That will kill one motor if you're close to the limit of both motors. That happened to me. I fried one motor while the other one was like new."

I guess the only way to find out is to try for myself, but I will definitely be making cables exactly the same length and installing a pair of ammeters on the dash to monitor both motors carefully.
#18
11-08-2010, 08:22 AM
 rwaudio Senior Member Join Date: May 2008 Location: Calgary Canada Posts: 1,285
Re: Smart Fortwo EV..... High power version!!

Quote:
 Originally Posted by MalcolmB Dual motors sound good. I've also been considering running two motors in parallel but recently saw this comment from CroDriver that left me a little uncertain: (http://www.diyelectriccar.com/forums...l#post208510): "the problem is that there is no way to make both motors perfectly same, the wires are not the same and the connections are not the same. There will always be a different resistance from the controller to the motor. That will kill one motor if you're close to the limit of both motors. That happened to me. I fried one motor while the other one was like new." I guess the only way to find out is to try for myself, but I will definitely be making cables exactly the same length and installing a pair of ammeters on the dash to monitor both motors carefully.
If you don't already have the controller you could do what Crodriver recommends and put the motors in series, and use a higher voltage lower current controller. That way both motors get the same current, regardless of load/rpm/traction/motor differences etc. Total power can remain the same, and for reliability it seems like a good choice.
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#19
11-08-2010, 09:24 AM
 Yabert Senior Member Join Date: Feb 2010 Location: Québec, Canada Posts: 1,484
Re: Smart Fortwo EV..... High power version!!

I have the controller right now! 180v max. I will charge each cells at 3.65v, but it drop to around 3.34v after I stop charger.

At 1000A connected the motor in series this give around 65 volts to each motor. It's probably not enought to sustain the max torque at least to 60 mph.
#20
11-08-2010, 09:32 AM
 rwaudio Senior Member Join Date: May 2008 Location: Calgary Canada Posts: 1,285
Re: Smart Fortwo EV..... High power version!!

Quote:
 Originally Posted by Yabert I have the controller right now! 180v max. I will charge each cells at 3.65v, but it drop to around 3.34v after I stop charger. At 1000A connected the motor in series this give around 65 volts to each motor. It's probably not enought to sustain the max torque at least to 60 mph.
That's too bad, you are correct the voltage wouldn't be near enough in series. If you did have the voltage though, 1000A through both motors would give you lots of power in a little car. (if you could keep the tires from spinning uncontrollably that is)
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