# Dual motors with chain drive



## iss407 (Dec 6, 2007)

I'm gearing up to convert a Saturn SC2. I'm interested in range, not performance, my commute is mostly flat at 55 MPH. I figure I can run at about 10-12 HP continuous. 

I had an idea of pulling the transmission and using two motors. I've been looking at the Etek-R Brushed PM motor. 8 HP cont, 15 peak, 28 pounds, $470. Two of these would be around what a 6.7" series-wound DC motor would be, both in cost and performance. I think the efficiency of these motors would be better. The motors are rated at 48V and I would run them in series with a Curtis 1231C controller.

If I used the half-shafts to mate the motors to the wheels and used off-the-shelf motorcycle chains and sprockets I could gear it so the motor was at peak efficiency at about 55 MPH. The mechanical efficiency of the chain drive will be much higher than the manual transmission. The losses from the half-shafts are the same for either approach. The weight of the motors and chain drives is considerably less than a standard 6.7" motor and transmission and it would take up less space. Fabricating a frame to fit in the engine bay and hold the motors and chain drive should be straightforward. It would be easy to design it to hold batteries and/or controller/charger as well.


So, is this viable? With a different controller these motors can do regen braking as well, but my commute is mostly highway and I would gain little doing that. I already have the Curtis controller or I would do regen from the start.

Small AC induction motors or AC PM motors could be used as well, but you'd need two controllers. The high efficiency of these DC PM motors lessens the benefits of an AC system though.


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## MalcolmB (Jun 10, 2008)

The idea is good, but then I'm bound to say that as I'm planning something along the same lines. I think you'll find those motors are underpowered for the weight of the Saturn though.

Cedric Lynch, who designed the original motor on which the pancake type motors such as the Etek are based, sells a similar, slightly more efficient and more powerful version, the Agni motor. He gives the following advice about it here http://www.agnimotors.com/home/index.php?option=com_content&task=view&id=31&Itemid=43

"It is suitable for use with a fixed transmission ratio on a road vehicle whose total loaded weight in Kg multiplied by its intended top speed in Km/h come to a figure of 30000 or less, if run on 60V (or 72V, which is safe if the motor is not overdriven above the speed at which it runs without a load). If the weight multiplied by speed come to more than 30000 it will be necessary to use a gearbox (and accept a large reduction of speed when climbing hills) or to use more than one motor. With a fixed transmission ratio it will be possible to maintain near full speed when climbing hills, provided that the battery can supply the necessary power."

So for your vehicle, weighing around 1200 kg and with a top speed of 90 km/h the weight times the speed is 108,000. Since you're using two motors this figure should really be below 60,000. It looks like you need a lighter vehicle, or a gearbox, or four motors.


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## gmijackso (Sep 30, 2008)

Ok, here's what I can figure for your setup. All calculations are rough and omit losses, so you'll have to guess at losses on your own and take the conclusions from there.

Here are the stats I could dig up for your car:
1994 Saturn SC2
Curb weight 2300lbs
Guessed final weight 2500lbs?
Stock tire size 195/60/15
Drag Coefficient 0.31
Frontal area 20.9 sqft. (1997 sc2, couldn't find the 1994)

The Etek R is most efficient at about 4300 RPM at 4000 Watts which is 5.36 HP.
Running 2 motors suggest about 11HP continuous at around 4300rpm. The one thing I can't find is any torque rating so I can't help with acceleration potential accurately, I used one of my motors torque rating which should be close to your sum torgue at 44.25 ft. lbs.

Given stock tire size, and 4300 rpms, your final drive ratio would need to be about 5.6:1 Since you're not using a transmission and wish to use gears only this would mean that is the gear ratio you'd use. 
With 44.25 ft. lbs of peak torque you'd accelerate at 3.14 ft/s/s. This means it would take roughly 12 seconds to accelerate to 25 mph, 16 sec to 35, 21 sec to 45, 26 sec to 55.

Using your coefficient of drag, and frontal area, you're 11 hp continuous would get you a top speed of about 62 MPH.

So... given all of that, I think you're accelerating way too slow. I also think that running two motors in series is asking for trouble, but maybe somebody more experienced in EV will be able to help better with that idea. 

It will take me no work to adjust the formulas if you can get me more accurate numbers. The formulas requre: Coef. of drag, frontal area, tire size being used, final build weight, motor rpm (where you want to run continuous), peak motor torque (for accleration), motor HP continuous, and then I either need a gear ratio, or desired speed at motor rpm.

Final thought is that I think you need to stick with a multi-speed transmission. I'm working through a similar project using a single motor the size of 2 of these e-teks (also small for an EV) but have found with the proper setup, it's possible, but I require a multi-speed transmission to be able to accelerate quick enough to not be a problem for other drivers.


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## Zemmo (Sep 13, 2007)

The Tropica Electric Car used this same setup in their car. It was a prototype car. There is several website out there but here is a quick link to one.

http://www.nogas.org/ev/articles/Car_&_Driver.html

This page has a lot of pictures:
http://www.getmsm.com/ev/tropica/default.htm

But that car used two motors, it used a toothed belt but thats a simular setup as a chain drive system. So I don't think it is all that strange to use that type of a setup. 

About the performance, my EV isn't a speed demon and it has a large 9 inch motor on a 4 speed tranny. My acceleration is about what you described. You can see my performance info like 0-10, 0-20, 0-30, etc on my website.


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## gmijackso (Sep 30, 2008)

Zemmo said:


> About the performance, my EV isn't a speed demon and it has a large 9 inch motor on a 4 speed tranny. My acceleration is about what you described. You can see my performance info like 0-10, 0-20, 0-30, etc on my website.


Those times seemed really slow to me. I've been shooting for about a 15 second 0-45 time. The acceleration time in traffic here in Vegas 0-45 seems to average around 15-20 seconds, that was my where I got my goal from. Perhaps in other areas people are more considerate, but here... not so much.


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## iss407 (Dec 6, 2007)

I found the Manta motor which can draw up to 450A for 45 Seconds. That give roughly 40 ft-lbs of torque. If the motors are in series both motors get 40 ft-lbs, or 80 ft-lbs total. With a 5.6:1 gear ratio that puts 448 ft-lbs to the wheels. I don't think acceleration off the line will be much of an issue. I won't smoke the tires, but it will be adequate.

The problem is that in series the voltage is divided between the motors. My Curtis 1231C can pump up to 550 Amps or up to 120V. At max that is 60V to each motor. That gives a Max RPM of 4320. Acceleration at the top end would be very poor. For top-end speed it would be best to have the motors in parallel, but that cuts the torque in half at the low end since the current would be divided between the motors.

What about wiring a DPDT contactor so that the motors can be switched between parallel or serial? I've attached a diagram of what I am thinking. My only concern would be switching at speed. If the controller was at zero throttle would you be able to "switch gears" by switching from series to parallel?

If it is possible to switch like that it would give the best of both worlds, double the torque at low speed, but full overall power still available at high speed.


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## gmijackso (Sep 30, 2008)

Two of those motors look like they'll do much better for you. With the extra HP and extra torque, you can even adjust the drive ratio a little to obtain a higher top speed if you'd like and still get a decent performance. With the 5.6 ratio your acceleration times are:
0-25 time (sec)6.4508
0-35 time (sec)9.031121
0-45 time (sec)11.61144
0-55 time (sec)14.19176
0-60 time (sec)15.48192
0-65 time (sec)16.77208

And your speed at 4320 RPM is 55 MPH. Given your new HP rating of 20HP total, you're new top speed due to wind resistance is 75.5 MPH. So, if you'd like to be able to reach your top speed of 75, you could use a gear ratio of 4:1, giving you a speed of 77.7 at 4320 RPM and the following acceleration times:
0-25 time (sec)9.031121
0-35 time (sec)12.64357
0-45 time (sec)16.25602
0-55 time (sec)19.86847
0-60 time (sec)21.67469
0-65 time (sec)23.48091

Again this is all still based on the numbers I found yesterday for coef. of friction, frontal area, and the guessed final weight. The other thing I should add is that I'm not real sure how you're SUPPOSED to deal with HP and Torque numbers when you're going to run two motors in series, but since it seems like you're planning on running them at full power even though they're in series, seems you should be able just sum the numbers.

The problem I see or the question I have for your series/parallel switch is if you're pack voltage is 120V so that when in series each motor will see 60V, how are you preventing 120V from getting to the motors when you're switched to parallel?


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## iss407 (Dec 6, 2007)

The voltage is proportional to the RPMs. It is geared for right around 1 mph/V so to reach 120V the motor would have to be spinning at 8640 RPMs which would put me well over 100 MPH. My controller would run out of Amps before that unless I was going down a steep grade, and then I'd have other issues to worry about.  The website says the motor can do 96V for 45 seconds. That's still far faster than I can do, or need to.

I was worried about how much force the chain will have, but at max controller current the motor will produce about 600 in-lbs torque. With a 2-in dia sprocket thats 600 lbf on the chain. I read that the tensile strength of an average motorcycle chain is over 5000 lbf, so I don't think that will be an issue.

Since speed is proportional to voltage and the motor is most efficient at 60 V could you setup a "cruise mode" bypassing the controller and tying directly to the battery pack? The contactor would be tied to the brake switch so the "cruise" would cut out if you tapped the brakes. In series each motor would have half of a 120V battery pack. 

It could be done with one DPDT contactor or one SPDT contactor if the motors and batteries shared a common ground. By fine-tuning the gearing you could get the speed you wanted. You could gear for 60 mph at full charge pulling about C/2 current which would give you about 55 mph at 80% Discharged, or about. The only reason to do this is efficiency. Again, what happens switching from controller to direct or back again at speed?

And while I'm on the subject of crazy ideas, here's one for you: With the two pancake motors on the sides of the engine bay there would be a huge amound of space right in the middle. What about getting an engine from a junked 125cc motorcycle and mounting it under the hood? It would be simple to add a sprocket to one of the half-shafts and run a chain to the motorcycle. Bingo, instant range-extender! 

A sinple centrifugal clutch would keep the motorcycle disengaged if it was not running. When the motorcycle gets up to the right speed for the cluch it would start powering the car. Only use it after reaching a high enough speed and you won't burn out the clutch. With regen it would even be possible to charge if you put in a larger 250cc bike.

With an electric start the engine could be started in gear. The throttle could be controlled from inside and the kill switch would turn off the bike. Only problem is that the clutch wouldn't disengage until the speed was reduced, even if the bike was turned off.

That's all of the crazy ideas I have for now. I'm sure I'll have more later. 

-- paul


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