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Discussion Starter #1 (Edited)
Hi all

Many of you have view my motorcycle project http://www.diyelectriccar.com/forums/showthread.php/suzuki-drz-sm-2005-electric-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...:rolleyes:). 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























 

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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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Discussion Starter #3
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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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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Discussion Starter #5
Thanks for the answer.

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!!


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



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?


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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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/wheelchair-motorcycles-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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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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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/showthread.php/dc-motor-charts-ev-performance-spreadsheet-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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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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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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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 RatioWheel Torque (Nm)Wheel Force (N)
11015.286616825760
1108.24119073106
1105.46566012058
1104.05654461527
1103.24523571223

Given your equation:
"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.
 

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Discussion Starter #12
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?



duncan said:
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.


dimitri said:
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.
 

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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!!
 

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Discussion Starter #14
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?
 

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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.
 

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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!!
 

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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/showthread.php?p=208510&highlight=parallel#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.
 

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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/showthread.php?p=208510&highlight=parallel#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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Discussion Starter #19
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.
 

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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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