I’ve been messing around with motors lately, and stumbled on something that I think explains the slightly different nature of electric motors, power curves and transmissions. This time I’m not using graphs, I’m using actual performance. It works with three configurations of the same bike.
The first configuration was with the Motenergy ME0709 with gearing that gave me a top speed of around 70mph. It was capable of throttle-blip wheelies. If I geared it slightly taller, it would possibly do a higher top speed but about the fastest I think I could get it to go would be around 100mph. The top RPM is around 3200.
I then put in an ETEK. This motor is lighter, by something like 12lbs. It was using the same gearing, but with a top RPM of over 5000, the top speed looked like it was around 110mph. However, it didn’t have the low-end torque to do wheelies with this gearing. (One of the tests I want to run is to compare the performance of the two motor geared for the same top speed, to see how the power distribution goes, and see if, first, the ME0709 can pull the top speed for high mph gearing, and see if the ETEK can pull wheelies at lower mph gearing. )
So, with the ETEK, I have the top speed I want, light weight, but not enough low-speed torque. If I gear it down I may get the torque, but lose the top speed. Perfect application for a simple two-speed transmission, right? Maybe so, if there was such a beast. But here’s where it gets interesting.
Take your ETEK, keep the gearing the same (for high top speed), and add another ETEK. What do you get? You get the same top speed, but double the torque at any given RPM.
Running two motors in parallel keeps your voltage the same, but feeds twice the current (assuming you’re able to feed that kind of current). It adds the weight of a second motor, in this case, 21lbs. I suspect you couldn’t build a two-speed transmission that’s going to be lighter, so we’re even there. It also puts less strain on each motor, than you’d have for a single motor set up with a transmission, basically being asked to pull hard at low speeds, and pull hard at high speeds.
Basically, you’re adding thermal mass and torque at the same time. And that, my friends, is the secret behind getting your head around the concept of why adding motor capacity is different, and maybe better, than adding a transmission.
It gets back to the basic difference between gas motors and electric. Gas motors represent an entire system that delivers power, similar to the entire electric drive system. An electric motor is only a part of that entire system, and it’s basic function needs to be power handing capacity, rather than thinking about it as power delivering capacity. See what I’m doing here? The system delivers the power to the motor, and it has to be able to handle it without blowing up, to be a balanced part of the system. (…where gas motors ARE the system. Think in terms of the piston, if you’re a gas-head. The piston has to be able to handle the power delivery of the gas motor without melting, whether you’re running a normal motor or a turbocharged nitrous configuration. The more power in the combustion chamber, the more stress the piston’s going to get.)
So adding a transmission to a motor is going to stress it more. If the motor is big enough to handle the stress and the system can deliver the amps, then it will work. There’s parasitic loss, but that’s probably not enough to worry about. Adding another motor spreads the torque curve, by doubling it, and adds thermal mass.
Only by looking at the entire drivetrain system, can you really answer the question… will additional motor capacity work better than a transmission.
The end result is only a little more weight- roughly 42 lbs of motor, not counting a few pounds for the mounting, and twice the torque. The ME0709 is what, 34lbs? Also, twice the current handling capacity. The ME0709 handles 300amps peak. The peak current for the ETEK is hard to find, and I don’t believe some of the numbers out there, but if you take a conservative 250amp peak and double it, then you’re looking at 500amps peak. Well above the ME0709.
As I’m becoming fond of saying, it’s all about the system. As remotecontact is fond of saying, it’s all about finding the bottleneck in the system. Rather than simply adding juice to one component or another, you have to think in terms of the entire system and how all the components work together. And one more thing, maybe the most important of all. It all depends on what you’re designing the system to do.