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  #1  
Old 11-01-2010, 06:45 PM
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Yabert Yabert is offline
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Default Smart Fortwo EV..... High power version!!

Hi all

Many of you have view my motorcycle project http://www.diyelectriccar.com/forums...ric-48239.html

But it was only a test for introduce the mecanical guy in mine to electric power...
So now it's time to the real project: An electric car fun to drive!

My first idea, last year, was to convert my old mercedes 190 for have a high power RWD fun to drive car (just a step under the Cro Driver BMW...). But I performed some calcul and the cost have no sense for the performance I need.
So because my budget is a bit limited I decide to reduce all the need in part like less powerfull controller and motor, less quantity of battery and that give:

-Donor car: BUY! Smart fortwo 2005 (real 730 Kg Go-Kart) It have a lot of room under the floor!
-Battery: Headway 10Ah, 48S-8P for 154V and 1000A at 12.5C
-BMS: Mini BMS centralized (3x 16S)
-Controller: BUY! Kelly KDHD 156V 1000A
-Motor: 11" modified forklift motor for good torque
-Gear box: Direct drive with 3.5:1 to 4:1 ratio



And now I have few question for you....

-I find a crown 11" forklift motor (W11AB02). I think it is a bit small! The frame is only 7-7/8" long and it's rate 9.6 Kw at 48v. The battery are rate for 115 Kw, the controller is rate for 180 Kw, so do you think this motor will be capable enough Kw to match with battery and controller? Or I need something much close to a Warp 11?

-I think don't use the original transmission because it's probably not enought strong to pass all the torque and I need a LSD differencial. I'm probably capable to build a fix ratio transfer case with integrated limited slip but I will need 4 kilovac contactor to go in reverse. In the other way I'm probaly capable to integrated some FWD LSD transmission (Honda, Volks, or....) without clutch and set to a fix ratio to have a mecanical reverse. What will be the better choise?

-What kind of charger I can match with mini-bms on 120v outlet (1800w) can give the shorter time of charge (1800w x 0.9 effiency = 1620w / 173v (48x3.6v) = 9A...... 64Ah / 9A = 7.1h of charge...... oh! fucking long)

-What do you think about the project? Good voltage? to low? Other BMS suggestion? Motor? Other concern?


Thanks

Final product: http://www.evalbum.com/4155
December 2015. Many pictures are not shown anymore, so I've create an album here: https://imageshack.com/a/uyz4/1
























Last edited by Yabert; 12-06-2015 at 06:52 PM.
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  #2  
Old 11-02-2010, 08:34 AM
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Default Re: Smart Fortwo EV..... High power version!!

Looks like a fun project.
The motor is similar in size to my 11" for my trike but mine is 80v 12.6kW.

When I bought a mig welder from a closing down factory unit recently, the seller was a disabled trike builder. He showed me a differential unit he says is use in rally cars. LSD, quick change ratios and a reversing lever so that the diffs can be used in either rotation.

Now, annoyingly, I can't remember the manufacturer details but a quick search on disabled trikes brought up this:
http://www.trikeshop.co.uk/disabled_adaptions.htm
At the bottom of the page is a reversing gear adaptor that might be useful for you to put between the motor and transmission.
There will, no doubt be other types available but maybe searching around disability adaptations might be a good start.
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Old 11-04-2010, 10:17 AM
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Default Re: Smart Fortwo EV..... High power version!!

Quote:
Originally Posted by Woodsmith View Post
At the bottom of the page is a reversing gear adaptor that might be useful for you to put between the motor and transmission.
Add another component between motor and gearbox isn't the best way to get a simple and efficient design. Also, 11" motor can probably give to much torque for this kind of equipement.

Well there is not a lot of reply....

At this moment I have buy the 730Kg Smart Fortwo, the 1000A controller and the 1000A capable Headway battries, so now the most important thing to know is if the 11" Crown motor is capable to puch this set-up to 60 Mph under 6 or 7 second!!!

How much torque is capable this motor at 1000A and for how many RPM it can sustain this torque?
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Old 11-04-2010, 11:17 AM
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Default Re: Smart Fortwo EV..... High power version!!

For what its worth, you can use MiniBMS with any charger, so charger selection is not related to BMS, but based on what available mains circuits you will have for charging, so you don't blow their fuses.

Also, you mix 1000Amp figures in motor side and battery side calculations, but you will never actually pull 1000Amp on the battery side, especially with Kelly controllers, which are well known for their fictional ratings. You didn't mention your planned EV range, which depends on C rate you pull from the pack. More C rates means less time to drive on one charge.

I am not a motor expert, but I know the main challenges of direct drive is limited RPM/power band of DC motor and heat buildup at low RPMs. Its quite difficult to find a single sweet spot in gearing down which allows for enough power at low speed yet be within motor's RPM limit at top speed. Direct drive certainly means much less electrical efficiency, hence more motor/controller heat to deal with.

At high C rates, you may even have to deal with battery cooling, especially the cells inside the pack, where heat is not easily removed. If ignored, middle cells might fail sooner than others if they get hot on regular basis.

I have seen several attempted direct drive conversions and I can't recall one where it didn't suffer from this decision. Either poor acceleration or poor range, or poor top speed, or overheating of power components.
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Old 11-04-2010, 04:43 PM
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Default Re: Smart Fortwo EV..... High power version!!

Thanks for the answer.

Quote:
Originally Posted by dimitri View Post
you will never actually pull 1000Amp on the battery side, especially with Kelly controllers.
It's the "new" type of controller from Kelly. Supposed to be more powerfull than the old model....
Maybe it's a bad choise, but now I need to test it! Wish me luck!!


Quote:
You didn't mention your planned EV range
I expect to drive between 40 and 50 miles in normal condition.



Quote:
I know the main challenges of direct drive is limited RPM/power band of DC motor and heat buildup at low RPMs. Its quite difficult to find a single sweet spot in gearing down which allows for enough power at low speed yet be within motor's RPM limit at top speed.
I try to resolve this problem with the equation: Lightweight car + Overbuild motor and controller + 90 mph top speed = good compromise.

My direct drive motorcycle had great acceleration (0-50 Mph around 5 sec.) but the problem was the limited top speed of 68 Mph. I think this problem will be resolve with the relatively high voltage (154v) for the Smart compared to the 84v of my motorcycle.

I have calculed the weight of the Smart (with me onboard) is 3.8 time more than the weight of my motorcycle. So to have similar acceleration I need roughly 3.8 times the torque of the 6.7" D&D es-15-6 motor and that give 305 lbs-pi. The short 11" Crown motor can give this torque?


Quote:
At high C rates, you may even have to deal with battery cooling, especially the cells inside the pack, where heat is not easily removed. If ignored, middle cells might fail sooner than others if they get hot on regular basis
I know! I need to design a good managed battery pack. I work on this right now!
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Old 11-04-2010, 05:32 PM
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Default Re: Smart Fortwo EV..... High power version!!

Quote:
Originally Posted by Yabert View Post
Add another component between motor and gearbox isn't the best way to get a simple and efficient design. Also, 11" motor can probably give to much torque for this kind of equipement.
Granted, that unit might not be right.

I think the company I met, when I bought my MIG, is a UK branch of this company:
http://www.mobilityconquest.com/whee...cles-specs.php
The body mouldings, donor bike and specifications all seem to be the same.
If I am right they use, I'm told, a rally car spec limited slip diff with quick change gears and a mecanical reverse within the diff.
It might be a strong and easy way to set up a direct drive where you can have a selectable reverse and adjust the ratio to suit the motor if you need to.
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Old 11-05-2010, 01:26 AM
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Default Re: Smart Fortwo EV..... High power version!!

Hi Yabert

I have been thinking about the reverse gear issue - for a lightweight 2 seater

(1) -No reverse just push it back
(2) - Use the reversing contactor I got from an old forklift - should never be actuated under load
(3) - use two double Alderson connectors to reverse the field coils
unplug the green ones plug in the red ones!
(4) - Lotus 7 type cars with motorbike engines use a starter motor to drive the car backwards
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Old 11-05-2010, 04:40 AM
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Default Re: Smart Fortwo EV..... High power version!!

Hi Yabert

It looks like we're in a similar position: both planning to convert lightweight cars using motors we know very little about, and wanting to achieve decent performance from a single-ratio transmission. I can't provide the information you really need, which is a torque curve and maximum speed for your motor, though I did find this: http://img214.imageshack.us/i/pdfeb7.jpg/

Your suggested gear ratio of 3.5-4.0 sounds about right to me. What motor rpm would this require to get you to 90 mph?

I've found it useful just plugging different values into this calculator: http://www.diyelectriccar.com/forums...eet-41565.html (Thanks to maxvtol!)

Working with a maximum motor torque of 150 ft lbs I found that the optimum gear ratio for decent acceleration and a top speed of 80 mph is around 4.5:1. (For a 1500 lb vehicle with rpm limit of 6000 for my 7" motors).

I know a lot of people dislike the idea of fixed gear drive, and I agree that it's a big compromise. For example I also found when playing with the calculator that by using a three-speed transmission with optimised ratios I could get my 0-60 time down from just over 8 seconds to around 5 seconds and increase top speed to 90 mph, though this didn't include shift times. Even so, the advantages of using a single ratio are sufficient to make me think it worthwhile, especially in a lightweight vehicle with very limited space. One of the things I really like about my bike, which is also fixed gear, is just the simplicity of it and the way the power winds on so smoothly. It doesn't feel incredibly fast, but sails away from the lights leaving most other traffic behind. It's surprising how much time is lost with gear changes.

I'm also trying to decide how to get reverse. Costs are about the same for a good set of reversing contactors or a starter motor for reverse drive, but the reversing contactors are lighter and easier to install.

Your plans look good to me, and now you've got the controller there's no point in worrying about voltage any more. That's one less decision to make!
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Old 11-05-2010, 06:18 AM
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Default Re: Smart Fortwo EV..... High power version!!

Quote:
Originally Posted by Yabert View Post
I expect to drive between 40 and 50 miles in normal condition.
Lets say you want to drive those 40-50 miles at average speeds of 40-50 mph, just to make a simple point. This means your average battery current should be under 1C, since you expect to be driving for an hour and you don't want to reach 100% DOD. For simplicity sake lets assume that acceleration bursts and periods of coasting will cancel each other out, so average cruising current has to be under 1C.

Since you plan a battery of 80AH, that means your cruising current should be under 80Amps.

In my experience, typical acceleration current is 3-5 times more than cruising current, which puts you in the 300-400Amp range for heavy acceleration. Good news here is that you don't exceed 5C, so no need to worry about pack cooling.

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.

Of course none of this math answers the question, can your motor provide both acceleration and top speed with a single gear ratio.
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Old 11-05-2010, 03:53 PM
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Default Re: Smart Fortwo EV..... High power version!!

Hi Yabert
On the direct drive v gearbox
You may be in the same situation I was in -
I intended to use an old Ford gearbox - I even bought one!
Then I did some calculations and decided that with a 750Kg vehicle I would be able to break traction in top gear
top gear was 1:1 and top speed 80 mph
(at 5,000 rpm - I hope my forklift motor doesn't blow up)
Without a gearbox the motor fits in what would have been the transmission tunnel - leaving the "engine bay" for batteries

If you can break traction then a gear box is unnecessary!

Calcs

Motor torque x axle (diff) ratio = wheel torque
Wheel torque / tire radius = Force at tire contact patch

Vehicle weight on the driven end x tire coefficient of friction (0.8 for "inexpensive" road tires (did I mention I am a Scotsman)) = Max force the tires can use
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