Suddenly, this whole question seems to make sense to me. Obviously, I'm tragically mistaken about something... but bear with me.
The implications of adding a second, or bigger motor rather than a transmission started falling into place when I starting thinking about adding a two-motor configuration to the R5e. Here's the basic jist of it.
When you add a second motor, in parallel, then you're doubling the torque (and load) at any given RPM. You're also doubling the current handling capacity- a 300amp system becomes a 600amp system.
It's all about the overall system. The ideal system has current delivery and motor load that's balanced. If you have high current delivery but your motor is not big enough, you need to add motor (either by adding a motor, or by substituting a bigger motor). If your motor is not running close to it's capacity, you need to add current delivery, or, if you have maxed out your current delivery you can add load to the motor- through gearing- either just adding taller gearing if the motor has the torque, or adding a transmission to spread the torque out on the curve. Or the motor is simply too big for your system.
Even as I type this (as per Jack Rickard "typing yourself smart") the transmission starts sounding like a bandaid for a system that's poorly balanced...
Anyway, more here: http://evmc2.wordpress.com/2012/01/0...s-dual-motors/
The nice thing about transmissions in an electric vehicle is that now you can start an oil thread!
Happy New Year Ted! Your take on this ongoing transmission issue is right on! Additionally, your charactarization of thinking in terms of the "overall system" can also be thought of in terms of horsepower. A dual motor system not only only produces twice the torque, twice the current handling and thermal mass capacity, but most importantly, produces twice the HP. HP, the measure of work, is the only thing that matter ultimately.
Thanks, and Happy New Year back atcha!
Right, as much as HP is watts, (which is current x voltage), and what the motor is rated as maximum, yeah. You have two 10kw motors in parallel, you've got the same voltage and twice the amperage, so you now have 20kw capability. You "get" 20kw assuming that your system (batteries and controller) can supply that kind of power.
Being the only die hard transmission supporter here I am going to say you summed it all up pretty well with the last sentence in your article "It all depends on what you’re designing the system to do." You touched back on the Weight issue and as I stated before a 5 speed trans only weighs 35lbs wet. Also you are not factoring in extra battery cost and the extra electronics to power and control a dual motor system. I am pretty sure between the electronics and higher battery requirements plus an additional motor the 600 bucks a trans costs to broaden the useable power band is the more economical route for a reliable long lasting powertrain. At any rate in the next few weeks you will see what a transmission can really do when set up on lipo and using what I feel is some of the best electronics in the EV industry.
Happy New Years Ted!
Let me see if I can say what Ted said using the AC-20 as an example. Let's assume that if I can fit an AC-20 + transmition, then I could fit an AC-31. They use the same controller, have the same voltage and current ratings.
Motor Weight Torque
AC-20 53 lb 72 ft-lb
AC-31 85 lb 112 ft-lb
So, roughly, the AC-31 gives me similar final torque to an AC-20 + 30 lb transmission with a 1.5:1 low gear.
That does start to make sense...but to really make the comparison in performance you'd have to plot torque and HP curves vs speed for both setups.
What the AC-31 won't give you is the equivalent of a 2:1 or 5:1 drive ratio. I think this is what Brutus is getting at - you can only make the motor so big before you have to mount your batteries in the passenger seat. If you want that kind of torque at the rear wheel, a transmission will get you there.
I think it comes down to "want" vs "need".
Well, holy crap. We're starting out the New Year basically in agreement about transmissions? Just what the hell is going on here?
Brutus, can't wait to see your results, for sure. As far as batteries and electronics, I'm just basing this on the same battery pack (about 20ah of 20C lipo) and running it in parallel off a single controller. So there's nothing extra, just the bracket assembly for the motor, which is lighter than your transmission - at least in my case.
At any rate, the R5e- it's just by way of a very simple illustration. But yeah. It's all about what you want out of it. As long as you understand the notion that the entire system needs to be a balance of the individual component characteristics, then you can design a system, transmission or no, that takes the most advantage of all the parts.
Documentarians of the obvious
Wait... What are you telling me that two motors (or one twice the size) are twice as good as one? And that balance system is better than an unbalanced system? And that a transmission does not make HP?
There goes my plan for 2012. I was going to outdo the geniuses at Brammo by building a 12 speed transmission and go twice as fast as them- Damn... foiled again!
As stated before- look to the torque curves, they hold all the answers! (everything else is a waste of time)
I keep thinking about this from the basis of my experience- building gas motors, and I keep coming back to it all being about finding the bottleneck, as RC says. On a gas motor, the first thing you do is get the thing breathing- reduce the intake and exhaust restrictions, which gives you more mixture with more potential energy, and more efficiency through the cycle.
As I've said before, the motor is almost like the piston- it's function is to transform the energy from one form (either the explosion of the gas/air mix, in the ICE model or the current in the electric model) to mechanical energy- watts, HP, whatever you want to call it. Putting in a bigger motor, or dual motors, is almost like boring, porting and polishing. You're putting in more energy handling capacity... trying to take advantage of the increased breathing- again, removing bottlenecks.
For us math-challenged guys, the curves only tell me so much... I need to hold it in my hand, or feel it kick me in the ass at some point. Then it becomes obvious...
Last edited by teddillard; 01 January 2012 at 1420.
Originally Posted by teddillard
I agree with Noah about the butt dyno. The whole point of all of this is fun.