# 4-Wheel drive



## whelmed (Jun 29, 2009)

Not sure if my last thread got posted, if so sorry bout that please delete.

Was wondering if anyone had experience with using 4 small DC motors rather than 1 large one - basically functioning either off a voltage rail through a controller or 4 separate smaller controllers? What about something like: http://www.alibaba.com/product-gs/235149755/Brushless_Dc_Electric_Car_Hub_Motor.html ? If I had 4x8kW motors I'd be rocking it but I wonder how much cost would be involved with that...

Thoughts?


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## 80N541 (Jan 11, 2009)

I think that 4 motors is less efficient than a big one


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## Drew (Jul 26, 2009)

4 motors might be a problem with respect to having 4 motor controllers, but in terms of drive efficiency its a winner, because the drive doesn't have to go through a differential or similar and from what I've seen motor efficiency is independent of size.


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## whelmed (Jun 29, 2009)

Well I could just run all 4 off the same controller. I've got one of those home-brewed ones from Paul and Sabrina (500A @ 144V). Just waiting for the last few parts to arrive.


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## Drew (Jul 26, 2009)

they'd have to be 36v motors to do that, because if you tried to run them in parallel you couldn't get any current control of any of the motors


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## whelmed (Jun 29, 2009)

Why would they not be able to get any current? They each have a resistance, running in parallel would lower the thevenin resistance and thus take more power at the same voltage.


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## Drew (Jul 26, 2009)

If theres a variation in resistance or inductance between the motors I would imagine that the one with the lowest value would soak up all the juice. I'm not so good with the electrical side of thing, so I'm hoping that if I'm wrong that somebody will jump in and correct me, but as I understand it variation in individual properties of motors may lead to one blowing up.


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## whelmed (Jun 29, 2009)

I guess if they have a different Volt / RPM then one could be trying to spin faster than the others. But then again under load none of them will actually get to their supposed Volt/RPM... maybe?

Hrm, anyone else care to chime in about this idea? If this is the case, how do the duel motor setups with the chain connecting them work - are they parallel or serial power?


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## thetod (Oct 27, 2008)

Cornering in low traction situations could be a little difficult as you wouldn't really have an open differential. If all wheels are trying to turn at the same speed (driven by the same controller) then the vehicle would tend to "push" or understeer through the corner (like if you have a locked differential in a 4x4 in loose gravel or snow). 
Might be something to consider anyways. I've never tried anything like this with electric motors so I could be wrong.


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## whelmed (Jun 29, 2009)

For traction around corners, I could always do a kill switch on the right motors when going around a right hand turn, same with the left? Maybe get a 500A solenoid? 

Good heads up at the very least though.


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## thetod (Oct 27, 2008)

or maybe run only 2 motors during "low load" and only kick in the 4 under hard acceleration? (like getting up to speed on the highway or off a red light)
Even front to rear you would ideally want some sort of "differential" when cornering hard. All wheel drive cars have a center differential as well as left to right. If you have ever left a 4 wheel truck locked in 4 wheel on the pavement and turned a sharp corner you can see what happens. The slip in electric motors may provide enough of a diff front to rear though. It would be something to play with.


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## Drew (Jul 26, 2009)

thetod said:


> Cornering in low traction situations could be a little difficult as you wouldn't really have an open differential. If all wheels are trying to turn at the same speed (driven by the same controller) then the vehicle would tend to "push" or understeer through the corner (like if you have a locked differential in a 4x4 in loose gravel or snow).
> Might be something to consider anyways. I've never tried anything like this with electric motors so I could be wrong.


The wheels aren't actually linked, the motors aren't synchronous and the current throughput is only related to torque, so it will actually conform quite well to corners and won't cause torque steer or path variation, in fact quite the opposite, the four motors will actually behave as an idealised differential and deliver a quarter of the torque to each wheel at all times.


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## thetod (Oct 27, 2008)

I'm new to the whole electric motor world so I am learning so I might be totally out to lunch but if two motors are getting the same voltage and one is turning slower than the other (ie the inside wheel in a corner) wouldn't the torque be higher on the slower moving wheel (it is my understanding that the torque would go up if a motor is "loaded" or slowed down at a given voltage). Maybe I am thinking of the wrong type of motor or something else. 

Thanks for starting a good discussion!


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## Drew (Jul 26, 2009)

TBH I don't know, one of the electrical guys would have to tell you, if they're in series they all have to be running at the same current, but apart from that I've got no idea about how the motors would behave.


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## goglahey (Jul 8, 2009)

Im going to test this, but i'm pretty sure that it will go sideways if there is not a controller, controlling the speed of each wheel, even at 2wd.

There is something called software differential car manufacturers are looking in to, but whats its all about, i don't know yet.

But i'm pretty sure that the resistance in cabels, different batterypack voltage, and different wheel surface, would make it go sideways.

Imagine one wheel on ice and one on dry road 

/Allan


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## Woodsmith (Jun 5, 2008)

If you are looking at four motors to improve traction then that isn't good really.

If you lost traction on one wheel then you have also lost 25% of your available power.

Imagine a single axle driven by a motor through a conventional diff. If you loose traction on one wheel then all the power is lost. So you get a LSD. Then when one wheel looses traction the diff locks and all the power is available to the wheel with traction.

However, if you have the same axle with two motors then you will have 100% power if both wheels have traction. But when one wheel looses traction that motor's power is lost and the other wheel will only have 50% of the power the axle had.

With four wheel drive there would only be 25% of the power to each wheel with traction regardless of the other wheels.

I have built motorised model trucks with multiple motors, one for each wheel. They pull fine when all the wheels are gripping but sometimes can't move at all if one or more wheels loose grip.

I revertered back to conventional transmissions with multiple motors coupled together to power the transmission input and there were no problems.

As to motor control, Jozza was telling me about the difference between current controllers and speed controllers.
This is what my logic tells me based on what I think I understood from him.

Current controllers will feed current to maintain the rpm of the motor to match a throttle position. This would make the motors want to at like the diffs are locked, all turning at the same speed the throttle is set for.
A speed controller will feed the same power to the motor so the motor will change speed to match the load, the inner wheel slowing as the load increases and the outer one speeding up as the load decreases. This would make the motors act like an open diff.


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## Qer (May 7, 2008)

Ok, let's try to salvage this derailed thread.

If we're speaking brushed DC-motors they can share the same controller and as has already been mentioned turning the vehicle won't be a problem since the motors aren't synchronized. BUT (and this is a big but) if you put the motors in series they will get the same current which means that if one wheel loose traction RPM will increase and with that the voltage (since back-EMF is proportional to RPM) over the motor which means it will get MORE power than the other three, spin even faster and quickly over-rev until it blows up.

If you put them in parallel they will always get the same voltage and if one wheel loose traction RPM will of course still increase but since back-EMF goes up, current will drop and the power over that motor will decrease. HOWEVER, if this is series wound motors the risk is still great that it will over-rev and commit Hara-kiri, they're kinda known for easily over-rev and break if they're running without sufficient load. Other kinds of brushed DC-motors might behave better, but I'm no motor expert.

If we're speaking brushless motors (AC, BLDC whatever) they need individual controllers and this is not a problem.


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## gor (Nov 25, 2009)

in general, electric drivetrain and multiple motors - considered to be natural ("automatic") differential function


even in perallel ("independent" motors) configuration - it's pretty adoptive: more load -slower, less -fester; steering wheel - your "differential"; for more more control - by seering angle, controller can alow one wheel to go faster/slower then another 

we really don't want diff to be too "ideal" (one wheel spins - rest stand still) - so better to have more control; ("limited slip differential" : ))))

4x4 separate motors config -ability to choose torque(power) distribution between motors - 25% (equal to each) or 40/60; 30/70 front/rear - on your demand

------------------
found 2mt unit: http://www.greenmotorsport.com//products_and_services/3,1,388,17,27462.html

DualmotorsystemunitREF : GMS-DUAL-36N GMS M1 motor 
Powerrating *36kW *Nominal. *70BHP *peak, depending on parameters.
*Torque **300NM *or 221.2 footpounds *SystemVoltage *48V
*SystemProfile *Motor, controller, Dual preset profile with electronic differential; liquid cooled
*ReductionGearing *3.75:1
*Weight **43.5KG *(without controllers)


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## Jan (Oct 5, 2009)

Theoreticaly you could calculate the relative RPM per wheel depending on the position of the steering wheel. With the current inverters you can't do anything with this, but things might change. With individual inverters per motor you could control them with a computer that does this computing. 

With the old diff's concept steering is a problem of fighting forces. With electric all wheel drive steering is going to be a synergy of combining forces.


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## goglahey (Jul 8, 2009)

Qer said:


> If we're speaking brushless motors (AC, BLDC whatever) they need individual controllers and this is not a problem.


If this is not a problem, then who made this setup DIY with four independent controllers hooked up to one throttle??


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## Qer (May 7, 2008)

goglahey said:


> If this is not a problem, then who made this setup DIY with four independent controllers hooked up to one throttle??


Pardon? I'm sorry, I don't quite understand the question.

Do you mean why noone has done it despite it's being possible? Then I guess the answer is simply that it's expensive, overly complicated and probably not something most DIY-ers could pull off. To make 4 motors/controllers follow one throttle isn't very hard, to attach 4 motors directly to each wheel, rebuild the suspension in a fitting way and, at least in EU, get it street legal is definitely more than for example I personally would ever consider doing. Bolting a motor in place instead of the ICE, building battery boxes etc seems to be enough of a challenge to me.


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## goglahey (Jul 8, 2009)

Jan said:


> Theoreticaly you could calculate the relative RPM per wheel depending on the position of the steering wheel. With the current inverters you can't do anything with this, but things might change. With individual inverters per motor you could control them with a computer that does this computing.
> 
> With the old diff's concept steering is a problem of fighting forces. With electric all wheel drive steering is going to be a synergy of combining forces.


http://ieeexplore.ieee.org/xpl/freea...isnumber=16239 
Eight pages of math gibberish (for my part) and after testing it in snow i did not work as expected, so it is unfortunately not that simple.

It's like trying to DIY a ESP program for EV's.

/Allan


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## goglahey (Jul 8, 2009)

Qer said:


> Do you mean why noone has done it despite it's being possible? Then I guess the answer is simply that it's expensive, overly complicated and probably not something most DIY-ers could pull off. To make 4 motors/controllers follow one throttle isn't very hard, to attach 4 motors directly to each wheel, rebuild the suspension in a fitting way and, at least in EU, get it street legal is definitely more than for example I personally would ever consider doing. Bolting a motor in place instead of the ICE, building battery boxes etc seems to be enough of a challenge to me.


So has anyone made it with 2WD, two independant motors, with one controller each? 
I'm looking in to it myself, but i think i'm hitting a wall here.

Even with rev control on each motor, the smallest difference in wheel speed at high speed would not be pleasant for the rider.

/Allan


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## Jan (Oct 5, 2009)

goglahey said:


> http://ieeexplore.ieee.org/xpl/freea...isnumber=16239
> Eight pages of math gibberish (for my part) and after testing it in snow i did not work as expected, so it is unfortunately not that simple.
> 
> It's like trying to DIY a ESP program for EV's.
> ...


I didn't say it would be easy. But if you're going that way, I would suggest to take the most out of it.


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## Qer (May 7, 2008)

goglahey said:


> So has anyone made it with 2WD, two independant motors, with one controller each?
> I'm looking in to it myself, but i think i'm hitting a wall here.


I believe so, yes. There's a Swedish project where some enthusiasts are doing a fairly complicated (imo overly complicated) conversion of a SAAB 9^5 where I'm quite sure they're powering it with two asynchronous AC motors for the two front wheels.

This isn't any different from that the motors will slow down when you go uphill, they simply adapt to the situation. When you turn the car, one wheel will rotate slower which will lead to either constant torque and less power or constant power and more torque (depending on the controller) since power=torque*rpm. If the current (and thus the torque) is constant this will be more or less like driving an ICE-car, if the power is constant the car will be heavier to turn than the ICE-car but it will still be possible to turn.

Now, if you have synchronized motors the car will behave like if you have locked diffs, but why on earth would you even bother with synchronizing the motors on a street car? If you run on a paved road synchronized motors will only result in grief and problems and you might very well break something like you'll break a locked diff if you run it on pavement. There's not even any point in synchronizing the motors if you drive in terrain, the reason you do it on Jeeps etc is that since you usually have one drive train that is shared by two or four wheels it means that if one tire loose grip, it'll spin and thus stealing all the power. If you have a setup with four individual motors with individual controllers this simply won't happen and thus there's no need for locking, or synchronizing, the individual motors.

Stop worrying. It'll work. The only thing you have to worry about is what happens if a wheel loose traction. If you're using series wound motors powered by the same controller a motor that loose traction will probably over-rev until it breaks, but if you use individual controllers and tachometer feedback you won't have to worry about that and you will have superior grip compared to ICE-based 4wd.

Oh, and of course you might have to worry about other things, like cost, suspension etc, but having individual motors on the wheels won't turn your car into an uncontrollable beast causing death and destruction along it's way.


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## Qer (May 7, 2008)

Jan said:


> I didn't say it would be easy. But if you're going that way, I would suggest to take the most out of it.


But that paper seems to be more about using the individual motors for anti skidding, traction control and other "bonus items", not just getting the car to move. Traction control is definitely an interesting regulation problem but I think that would have to be considered phase 2. First you might want to get the car to run at all...


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## goglahey (Jul 8, 2009)

whelmed said:


> Not sure if my last thread got posted, if so sorry bout that please delete.
> 
> Was wondering if anyone had experience with using 4 small DC motors rather than 1 large one - basically functioning either off a voltage rail through a controller or 4 separate smaller controllers? What about something like: http://www.alibaba.com/product-gs/235149755/Brushless_Dc_Electric_Car_Hub_Motor.html ? If I had 4x8kW motors I'd be rocking it but I wonder how much cost would be involved with that...
> 
> Thoughts?


 
Btw. one thing to consider with in wheel motors is unsprung mass: http://en.wikipedia.org/wiki/Unsprung_mass , if you have a light vehicle this can have some effect on steering and road traction.

/Allan


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## goglahey (Jul 8, 2009)

Qer said:


> Oh, and of course you might have to worry about other things, like cost, suspension etc, but having individual motors on the wheels won't turn your car into an uncontrollable beast causing death and destruction along it's way.


Damn, i was hoping for death and destruction , well im looking forward to testing this out.



Qer said:


> But that paper seems to be more about using the individual motors for anti skidding, traction control and other "bonus items", not just getting the car to move. Traction control is definitely an interesting regulation problem but I think that would have to be considered phase 2. First you might want to get the car to run at all...


True true..


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## gor (Nov 25, 2009)

goglahey said:


> If this is not a problem, then who made this setup DIY with four independent controllers hooked up to one throttle??


check this - not DIY, but...: :http://www.electrictractor.com/etc_dtls.asp?prodid=8


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## piotrsko (Dec 9, 2007)

most of the GM stuff I drive dvelops traction control from the antilock BRAKE system, IE: apply brake force to the spinning wheel.

Definitely Neanderthal, but works here in the US.


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## EV-propulsion.com (Jun 1, 2009)

And don't forget the kid's Power Wheels Jeep! with multiple motors!


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