# Two motor synchronization



## iplay1515 (Oct 2, 2013)

I'm considering a design using 2 motors, and I'm interested in locating information about controllers capable of 2 motor synchronization and the current state of the art on this topic.


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## major (Apr 4, 2008)

iplay1515 said:


> I'm considering a design using 2 motors, and I'm interested in locating information about controllers capable of 2 motor synchronization and the current state of the art on this topic.


No need. Use torque control. Motors run at any speed. Speeds do not have to be controlled so synchronization is irrelevant.


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## frodus (Apr 12, 2008)

Are they AC or DC?

Do you intend to use a single controller?


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## iplay1515 (Oct 2, 2013)

frodus said:


> Are they AC or DC?
> 
> Do you intend to use a single controller?


I haven't purchased either a motor or controller at this time.


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## frodus (Apr 12, 2008)

DC you may be able to use 1 controller for 2 motors, depending on some things.

With AC, its 2 controllers for 2 motors.


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## iplay1515 (Oct 2, 2013)

I'm only thinking two motors to avoid using a differential on the drive and it's associated inefficiencies...Is this sound reasoning or would a differential and one motor be satisfactory?

With 2 DC motors, are we talking series wound motors wired together in series operating on one controller?


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## Siwastaja (Aug 1, 2012)

A differential doesn't have much inefficiencies. It works as a reduction gearing at the same time. If you use separate motors, you also need two separate reduction boxes.

Ideal reduction ratio is more than most diffs have, though, which is why most people leave the original transmission in place, too, even if they just locked it in one gear.

Optimum reduction is around 1:4 to 1:10 depending on car and motor. Typical diffs run around 1:2.5 to 1:4. If your car has 1:4, it can be usable as is.

Two motor approach offers better traction in poor road conditions as it gives functionality similar to Limited Slip Differential (only better).


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## iplay1515 (Oct 2, 2013)

My entry into building an EV will be an over sized golf card that is street legal with a max speed of about 40-45 mph for short trips in town.

I've seen some simple drive systems that use a belt from the motor to the axle. The belt has a spring tensioner and is the cog type belt.

Is this belt drive system as efficient as using a transmission locked into one gear through a differential?

This is why I'm considering two motors using the above belt drive system.


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## dougingraham (Jul 26, 2011)

iplay1515 said:


> My entry into building an EV will be an over sized golf card that is street legal with a max speed of about 40-45 mph for short trips in town.
> 
> I've seen some simple drive systems that use a belt from the motor to the axle. The belt has a spring tensioner and is the cog type belt.
> 
> ...


I would keep it simple and use a differential. You may not be able to get enough reduction with a single stage belt. Your motor pulley should probably be no less than about 4 inches in diameter. So to get a 4:1 reduction would require a 16" diameter pulley on the wheel. And you probably want a 5:1 or even 6:1 reduction for a NEV. That might make the wheel pulley larger than the wheel.

As for efficiency it is tough to say. The losses are not too bad in a differential, probably around 3% and I would guess a belt is not too much better than this. A single larger motor into a differential is probably about the same as two smaller motors with belt drives. If you have two motors then you have three choices for how to connect them up. You use two controllers and some way to tie them together. You use a single controller and place the motors in series. You use a single controller and place the motors in parallel. Placing the motors in parallel is sort of like a limited slip diff. If a wheel slips then the other one is still driven. In series it is like a conventional diff. If one wheel slips the other one sees very little torque. In series you need twice the voltage but half the current. For an AC system you need a controller per motor.

These things are enough work that I would keep it as simple as you can if you expect to finish it. One motor, one controller, a transmission and a differential from a working vehicle is about as simple as it gets. The transmission will match the motor to the wheels. There will be at least one gear that will be close to what you want.


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## iplay1515 (Oct 2, 2013)

Wow...Thank you Doug...Very helpful information...

What is your opinion of using a differential, but instead of a transmission, using a belt/pulley arrangement to drive the differential? 

That should require less reduction and allow a smaller pulley to be used on the differential while maintaining a 4" pulley on the motor.

It's certainly nice to hear from those who have been there done that or studied the matter in detail...Thanks again.


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## dougingraham (Jul 26, 2011)

iplay1515 said:


> What is your opinion of using a differential, but instead of a transmission, using a belt/pulley arrangement to drive the differential?
> 
> That should require less reduction and allow a smaller pulley to be used on the differential while maintaining a 4" pulley on the motor.


This is a good question and I am afraid I don't know that answer. The bearing on the front of a differential clearly was not designed for that kind of side load but that doesn't mean it wouldn't work.

I also don't have any experience with using belt drive. I don't know how much power you can transfer or how that will affect belt life. But for experimentation to find your single best ratio it certainly seems like changing pulleys would be faster and less expensive than changing ring and pinion sets.

I mostly drive around town but the interstate is useful to go to the other side of town so it is nice to be able to go highway speeds. With a single speed reduction here are the issues as I see them. To go highway speeds 65mph limit across town I need something slightly lower than my third gear (5.59:1). This is because the motor RPM at 65 MPH is 5300 which is slightly over the recommended 5000. Fourth gear (3.91:1) at 65mph is 3700 RPM which is not bad but acceleration from a standstill is not stellar. Even third gear start from a standstill is just ok (alright it is better than most people do normally). The other side is that I live at the top of a grade in a residential neighborhood so the speed limit is 25 and when going 20-25 in third gear the motor is not spinning fast enough to effectively cool itself (<2000 rpm) so this would require forced air. Cools great in 2nd gear (8.66:1) at <3200 RPM.

I would have no issues at all using something between 2nd and 3rd gear ratios as a single ratio arrangement if I never went above 45mph. To feel comfortable with it and have highway speeds it would need more torque and forced air through the motor.

Having a transmission does add some complexity but it also means you don't need as powerful a motor or batteries that can provide huge currents. It makes a DIY conversion a lot easier to get close enough to right the first time out. One of those gear ratios will be right for what you are trying to do.


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