# Planning an Electric car conversion, am I going wrong any where?



## EVEngineeer (Apr 11, 2012)

mapexmbirch said:


> Hi, I am going to do a Electric Car conversion in 2-4 years after I finish my Degree in Electrical Engineering and I want to know if my ideas would work.
> 
> I want to have no gearbox to limit the losses and either have to motor connected to the differential or have two individual motors one for each wheel, then have independent control over both wheels to improve steering.
> 
> ...


I guess I'll start. what type of vehicle do you want? how far do you want to drive on one full charge? What is your estimated budget? You may need a lot of batteries or very few batteries depending on what you answer in those questions. How fast do you need to drive? 

"I hear most people take the Engine out and attach a motor to the clutch and gearbox. I don't want to do that because I want to limit the losses, I want the most efficiency possible for a reasonable amount of cost." I'm confused by that statement, so maybe someone else can answer this. The reason why the engine is taken out is, because you no longer need it in an "EV". it reduces the weight of the car by a few hundred pounds by removing it. it also gives you space to put the motor and other parts inside. I don't know if you have read through the wiki yet, but maybe you should. 

Here are a list of parts and tools that you'll need http://www.diyelectriccar.com/forums/showthread.php?t=73869 
http://www.diyelectriccar.com/forums/showthread.php?p=304549


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## Woodsmith (Jun 5, 2008)

EVEngineeer said:


> "I hear most people take the Engine out and attach a motor to the clutch and gearbox. I don't want to do that because I want to limit the losses, I want the most efficiency possible for a reasonable amount of cost."
> 
> I'm confused by that statement, so maybe someone else can answer this. The reason why the engine is taken out is, because you no longer need it in an "EV". it reduces the weight of the car by a few hundred pounds by removing it. it also gives you space to put the motor and other parts inside. I don't know if you have read through the wiki yet, but maybe you should.
> 
> ...


I think the OP is meaning he doesn't want to replace the ICE with the motor directly as he want to get rid of the ICE and the transmission and run direct drive to the rear diff or use wheel motors.



Here is a good thread on using an AC motor, initially with an auto trans and then with direct drive.
http://www.diyelectriccar.com/forums/showthread.php/1930-model-roadster-build-59659.html


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## mapexmbirch (Feb 15, 2013)

Sorry I wasn't very clear.

I know you have to take the engine out to have more room for the motor etc. I didn't mean keep the engine, I meant whether or not to keep the Differential and gearbox. As an idea EV doesn't have them. Well maybe it has a differential. I was asking whether it was worth the hassle.

Anyway thanks for the links.


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## mapexmbirch (Feb 15, 2013)

Thanks Woodsmith, I'll check that out.


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## Ziggythewiz (May 16, 2010)

Nearly any EV will keep the diff. 

Losing the transmission means you have to compensate with an extra large motor.


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## Siwastaja (Aug 1, 2012)

This has been discussed numerous times. As a summary, most existing motors need a reduction gear to give enough torque with good efficiency at the wheel RPM which goes around 0-1000 in typical driving. Optimal reduction is between around 1:5 and 1:10 depending on motor max rpm. Rear differentials have integrated reduction gearing, many people just call this "direct drive". It works depending on the particular differential, some cars may have just 1:2.5 reduction, some may have near to 1:5. So it's a huge difference. Start by checking your donor car's differential.

So the point in those conversions is not the differential, it's the easiness of using the existing _reduction gear_ within the differential. Of course two separate motors with two separate controllers would perform as an electronic "differential" much better than a mechanical one, but it's hard to find the parts (reduction gears).

Less reduction gearing means you need a physically bigger and heavier motor to get more torque out of it. A high-rpm motor is more elegant and smaller, OEM's use very high RPM's. A diyer may find 5000 rpm max with 1:5 reduction a good compromise.

If you want to directly drive the wheels, you need reduction gear at each motor connected to the CV joint. It's simple in theory, you just need to find someone to build the reduction gears that can handle the torque and rpm for affordable price.

There are claims that there might be motors capable of working for non-reduction drive that are still lightweight and powerful. (Search for "emrax" on this forum.) Whether it's true or not is still to be seen. Generally, I don't tend to believe in silver bullets which seem to contradict the general consensus until I see them.


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## tomofreno (Mar 3, 2009)

Well, you are an engineer so you should be able to calculate some of these things. You might start with Build Your Own Electric Vehicle, by Bob Brant, since it has many of the basic equations in it. That will get you thinking, and you likely can then expand it further. It doesn't take a lot to relate vehicle mph to wheel rpm, and that to motor rpm with no gearing in between or for known gear reductions in a transmission and/or differential. From that you can calculate what motor rpm you will need to achieve a given top speed. Calculating the forces on the car will tell you the power required to move at constant speed as well as the tractive effort (force of the driving wheels against the road) required to accelerate the car at a desired rate. Knowing the tire radius and gearing permits you to relate this to the required wheel and motor torques.

Once you know that you can start looking at torque-speed (rpm) curves for various motors to see which might satisfy your needs. As has been stated, the less gear reduction the more torque required of the motor. Also, just because a motor may reach the required high rpm range required for direct drive doesn't mean that it has much torque at that rpm range. These are things the motor specs and torque-speed curve will tell you.

You will also find that most series DC motors are limited to less than 5500 rpm, and that higher power AC motors and controllers that would permit operation at higher rpm are prohibitively expensive. Lower power ones from HPEVS are not so bad in price, and can operate up to 8k rpm, but have a ceiling of around 80 H.P. shaft power due to the controller. You will find there are much higher power series DC motor controllers available (for example in the adds on the RHS of your screen on this site).

So you can answer all the questions you posed. As far as building your own motor controller, I suggest looking at some of the threads here where people are trying to do that to gauge whether you think it is something you should undertake. Have fun!


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## mapexmbirch (Feb 15, 2013)

Thanks guys.

I understand about the gear ratios and how to calculate the speed of the car relative to the motor speed.
The main reason why I am asking this is just to check that it isn't a stupid idea, so I won't waste time.

Having a bit of a think, I think it would be easier to get a rear wheel drive car like a BMW or something and take the gearbox out and keep the diff. Maybe having two motors was being a little ambitious for my first conversion. I can always do another one later in my life.

Cheers again!


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## MalcolmB (Jun 10, 2008)

Hi Map
You might find this thread interesting: http://www.diyelectriccar.com/forums/showthread.php?t=68527

Most independent wheel drive solutions use a final gear reduction, as you suspected. It is possible to drive the wheels directly using high-torque motors like the Oxford Yasa motor, but they're pricey.
The elegance of this approach really appeals to me, though it does mean a good deal more work and expense to build the reduction drives. The simplest solution in my view is to couple the motor directly to a differential. If you use AC motors you definitely need one controller per motor.


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## mapexmbirch (Feb 15, 2013)

I would love a YASA motor, but I suspect I can't afford it. Yeah I like the idea of having independent drive to each wheel, but being realistic, I am not going to take this thing on the track or go rallying, so a standard diff will do fine.


Thanks for the link, I will give it a look.


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## Qer (May 7, 2008)

mapexmbirch said:


> And finally, in my third year of the Degree I have to do a project. I was thinking a motor controller.


Well, BUILDING it isn't too hard. Avoiding that it blows up is the tricky bit. The words "TRIACs" and "YouTube" indicates that you probably should do a bit more research before you settle for this project...


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## mapexmbirch (Feb 15, 2013)

Qer said:


> The words "TRIACs" and "YouTube" indicates that you probably should do a bit more research before you settle for this project...


Don't worry, I have over a year until I start it, I will ask my tutors and do a lot more research.


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## Siwastaja (Aug 1, 2012)

Actually doing it will be the best "tutor" you will have, of course accompanied with googling when something blows up.

I'd recommend starting with a small (< 1 kW) test motor so that you can buy smaller and cheaper IGBTs/MOSFETs because you will blow a lot of them... Start with very low voltages and bring it up looking at any voltage spikes on oscilloscope.


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## mapexmbirch (Feb 15, 2013)

Thanks for the tips. 
Does it matter if I go for an asynchronous motor or synchronous motor? The only difference I can see is asynchronous won't give me regenerative braking (which won't matter if I go for rear wheel drive) and perhaps less torque.
And where do I find < 1KW motor and the motor I will use in my conversion. Bearing in mind I live in the UK, I have found loads of website in the US that sell motors and parts but not very many in the UK.


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## Siwastaja (Aug 1, 2012)

mapexmbirch said:


> Thanks for the tips.
> Does it matter if I go for an asynchronous motor or synchronous motor? The only difference I can see is asynchronous won't give me regenerative braking (which won't matter if I go for rear wheel drive) and perhaps less torque.


What? Why wouldn't it?



> And where do I find < 1KW motor


Any scrapyard... They are used and taken out of use in hundreds every day everywhere. 99% of them work just perfectly. Or you can buy a new one for very little from any industrial machinery/electric part store. 

A 230V 3-phase motor is good for testing from about 50V at low frequencies first.


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## mapexmbirch (Feb 15, 2013)

Just thinking that YASA's choose synchronous motors because they give more torque. If I'm wrong just say.

I will have a look at my local scrapyards. Cheers.


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## cts_casemod (Aug 23, 2012)

mapexmbirch said:


> Just thinking that YASA's choose synchronous motors because they give more torque. If I'm wrong just say.
> 
> I will have a look at my local scrapyards. Cheers.


You are better off looking at all motors as the same. I you have two 2KW motors, they should be capable of about the same torque, regarless of being DC, AC, Syncronous or Asyncronous. You may have different chalenges controlling them,so you need to make sure you find something you can build or have a controller for it. Also the efficiency may differ at different RPMs, but just like you wont drive a Diesel and a Petrol at same RPMs (Altought some people do, but will have poorer mileage) you will get used to look at the instruments to be able to take advantage of it, unless of course the operating range is completely different from that found on a vehicle, like bike motors that are made to run at a few hundred RPM (200-400).

I would support the idea that has been given to find something small to try on. A Good candidate is a car alternator. You wont have to go further to find one. Take the bridge rectifier and acess all the 3 phases and get a way to control the field into the rotor slip ring. This is Syncronous motor. If you short the rotor slip rings you have an asynconous, but with a high impedance rotor, so I would advise to use as syncronous.

These little beasts are capable of 100Amps @ 100V or about 15BHP Peak. Care must be taken in regards to the bearings as they are the weakest point. Car alternators can be taken safelly to 15.000RPM. In their normal operation they run at twice the engine revs for petrols or 3* for Diesels.
Get a 24 or 36V brushless controller from an eBike and plug it into the alternator and it will run nicely for your project. You can upgrade and learn from here. Replace the mosfets on the controller with a 400/600V IGBT 6 devices power pack and you can take it to larger currents and larger voltages up to 100V with the rated amps it had as an alternator. The field can be controlled to provide either torque or speed, but care must be taken not to keep it for too long above 5V @ 2Amps otherwise it will overheat.

Both methods support regeneration. In fact any motor can have regeneration, including DC. Sometimes its just not practical to do so as on a DC for example it adds extra control and extra cost to the controller. 

Have fun


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