# EV Conversion (electronic differential and regen braking)



## neljoshua (Oct 21, 2008)

Hello, all.
This is my first post to this forum, so while it may be a bit long, I want to make sure to include all pertinent info.

My name is Joshua Nelson. I am working on a Master's degree at Purdue University in Sustainable Energy Systems. I enjoy working on cars and with alternative fuels. Two summers ago, I converted my VW Beetle to run on waste vegetable oil (WVO). It has been an enjoyable project, but I am looking now to convert an EV.

I have been looking for a donor car, and was recently given a 1992 Civic. It has a good body, and it still runs. However, it is a 4-door and heavier than I would like (3200 lbs). So, I am going to sell it and buy something smaller. Right now I am looking at a 2-door Civic, a Golf, or a Rabbit. I have a RX-7 available locally, but it has a live rear axle, and I do not want to have to rebuild the rear suspension (more below).

Basically, I want to use this car both as a personal project and for research. I want to do a full conversion using regen braking and no differential. This is because a differential is 75-100 lbs of wasted weight (I would not call the tranny wasted, but you can go without a tranny if you get a decent motor). A differential can be represented mathematically pretty easy (still working on the numbers); if you have the width of the wheel base, the speed of the opposite wheel, and the angle of the opposite wheel (all inputs), you can calculate the difference in radius and therefore the difference in wheel speed.

I would like to use induction or dc-brushless motors, and I will need two (one at each wheel), as they are easier to accomplish regen braking (I think). At this point, I figure it would be easier to do the conversion on a front-wheel-drive car. Originally, I wanted to do the RX-7 (my favorite car--I used to have 2), but it has a live (all one piece) rear axle. If I am to remove the differential, and therefore the rear axle, I would have to rebuild the entire rear suspension. Not in my plans. So, a Civic would be great. I am going to have to work with what is available, but there are plenty of junk yards around.

A few questions.
1). Has anyone else tried to remove the differential? Thoughts?
2). Any suggestions on donor cars? I know there are many views, but I am open for ideas.
3). Am I correct in assuming that induction and dc-brushless motors are best suited to regen braking? Have others been successful in this venture?

Thanks
Josh


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## madderscience (Jun 28, 2008)

Hi Josh-

Dual motor direct drive is an interesting Idea. I initially planned that exact thing for my conversion but the DC motors that I wanted to use weren't rated for the RPM I would need, so I fell back to a more conventional setup.

As for motors that can do regen, permanent magnet DC, shunt wound DC (and maybe compound wound DC, not sure there) and of course AC can do regen. The only one that can't easily do it is series wound DC. of course series wound is the type most commonly used in home conversions.

You might have trouble finding permanent magnet DC motors that are big enough and have the RPM rating you will want to do direct drive. Note that you will still need a fixed gear ratio of around 8:1 to 10:1 from the motor to the wheels to get enough torque, meaning you will want motors with a 10,000 RPM rating or so. This is far in excess of most large DC motors, but not a problem for many AC motors.

Please note that the diff isn't necessarily wasted. You would be replacing its function with a second motor, second controller, a second independent gear reduction, and however much extra control system complexity is necessary to manage the whole mess. There is an efficiency loss there too. Maybe keep the differential, but lose the ring gear, add a large sprocket, and do a chain drive back to a single larger motor with the appropriate gear ratio?

Good Luck.


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## neljoshua (Oct 21, 2008)

So,
I would need to use AC motors, right? One of my big concerns is that the motors will be too long to fit between the wheels, especially since you need room for the cv shafts to travel.

I have really been back-and-forth about this idea. A chassis redesign may be needed, and I do not have the equipment or capital for that. I may just stick with a single AC motor. Then I could use the RX-7...


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## Qer (May 7, 2008)

madderscience said:


> The only one that can't easily do it is series wound DC. of course series wound is the type most commonly used in home conversions.


Actually, I've had a long conversation with Tesseract where he, among other things, pointed out that a series wound motor can do regen without any problem, provided it is not advanced. By advancing the brushes you gain some things, like being able to run it on higher voltage than specified, but lose others, like regen. If you have a series wound motor that is specified for the voltage you intend to use it for you don't need to advance it and thus it can regen.

See Tesseract, I do listen.


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## neljoshua (Oct 21, 2008)

So, after much thought, I have decided to forgo the electronic differential idea...for now. The donor car is going to be the RX-7 I mentioned, but it will probably be a month before I have the money to get it. I still plan to use regen braking, however.
There are a few profs here at the university that have been experimenting with evs lately. I hope to be able to tap some of their knowledge. The things that I need to figure out at this point are...
1) What kind of motor do I want?
2) How powerful should it be (before conversion the car will be less than 900 kg)
3) Are there any gear requirements? If so, should these be worked out in the differential, or separately?
4) Are li-ion batteries financially feasiable?
There is a li-ion battery company here in Indiana, called Enerdel. I am hoping that I can work with them.
More info as need warrants.
--Josh


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## TheSGC (Nov 15, 2007)

What kind of Civic weighs 3200lbs????!!!?? I am doing a 1996 Civic LX 4 Door and it is only 2300 LBs before I stripped it. 

If you want regen braking, your best bet is either AC or a shunt/sepex motor. Permanent Magnet motors will do regen, but they aren't powerful enough to reliably push a car around. At least nothing the size of a Civic or RX-7.

Now for power, you probaly want a 20HP continuous motor. Something in the 8" or 9" size if it's DC. 

And Lithiums. They are expensive and need special care and are out of the price range for most EV converters. A decent sized pack costs $15-20K USD for a pack and the BMS. If the company is nearby they might be able to give you a deal, especially if it's college related.


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## neljoshua (Oct 21, 2008)

My bad. That must be a common mistake. The GVWR is 3200 lbs.

In any event, the RX-7 will still be much lighter. And I found a relatively easy way to put rear seats in it--just like the JDM version--with second gen seats. Not very comfy, but solves the two-seater problem.

It may be the motor-head in me, but I still much prefer rear-wheel to front-wheel drive. That's another issue.

Thanks for the tips, TheSGC.


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## tormented (Jul 30, 2008)

At least three problems present themselves with using a motor for at each wheel, not having a transmission means no reverse gear, how will you back up?, DC motors do not reverse direction without brush shifting, another is when making a left or right turn the outer wheel has to travel/rotate faster then inner wheel to prevent one wheel from dragging or skidding, hence the use of the differential spider gears, one motor would have to rotate faster or slower than the other to prevent damage. Even if these two problems can be overcome and I doubt it, you still need some type of planetary gear system to match the motor speed to desired road speed. All in all a bad idea.... just my two cents worth.....


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## neljoshua (Oct 21, 2008)

I do not see the absence of a transmission as a problem. If you have the correct circuitry, you could simply reverse the direction of the motor (I plan on using a brushless dc).
As far as the difference in wheel speed, one only needs the diameter of the tires, width of the car, rotational speed of the opposite wheel (say, the inside wheel), and what I will call the "turn angle," i.e. the angle the wheel makes with respect to the vehicle. Using these inputs, one could then calculate the radius of the turn. Then add half the width of the car and you now have the radius that would correspond to the outside wheel. Then convert the circumference of this circle to a rotational speed and you would have the speed of the outside wheel with respect to the inside wheel.
The mathematical representation of a mechanical differential is, in my opinion, not a difficult calculation. I am not pursuing this route currently because I do not have the means (or desire) to rebuild suspension components in order to make room for the motors.
That being said, although this is not difficult mathematically, I imagine implementation would be a bit more challenging.


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