Yes, we’re going back to the question of drive motors.
For most EV conversions, it seems, the trend is to go with high-voltage battery packs, spinning the motor at high (5-7000 RPM), and running that high-speed, low torque drive through the standard transmission and differential (To get a geared-down wheel speed of 2000RPM). This seems to be because the only way to control motor throughput is to control the voltage, with higher voltages making faster cars. It’s my understanding that a motor will take as much current as it “wants”, which makes low-speed, high torque motors very difficult to get (at least, off-the-shelf).
Personally, I believe that simplifying the drivetrain as much as possible is definately the best way to go. My real preference would be for a pair (or two pair) of wheelmotors. This isn’t feasable (As explained in a previous post), unfortunately. So my next choice would be to mount a pair of motors inbound, driving the wheels from their driveshafts (So, replacing the differential with a pair of motors, running in parallel). This is a problem, however, as driving these motors at the typical EV’ers voltage would give a ridiculous top speed (with 15” rims and fairly standard tires, with a motor speed of 5000RPM, you’d get a top speed of 1032KPH(!!!)) with little torque.
Now, I could be wrong with all my assumptions here (Which is probably quite likely). It could be that if you run a motor with such high torque loads on it, it takes all the current it needs to drive the wheels. It does mean that I need a very high current controller (5,000 amp? Higher?) which doesn’t exist yet, and I’d need very high-draw capable batteries (Or several paralleled strings of batteries).
How would other people deal with this problem, apart from using the transmission and differential and including all the weight, friction and power loss included therein? Should I just bite the bullet and leave the tranny in, rather than having to worry about all this high-current nonsense?
Thoughts, comments, and more are always welcome!
1 comment:
I'd have to disagree with some of your statements. Most EV conversions are what I'd call moderate voltage, 96 to 156 volts, using series wound motors that generate high torque at low RPMS but lose torque as RPMS increase, usually up to around 5000-6000RPM max, but they usually are most efficient between 2000-4000 RPM so the transmission is used to keep them there. If the motor is large enough, the vehicle light enough, and with a high amp controller, 1000-2000amps, then something like a TransWarp11 can be used direct drive to a differential. High voltage 200 + volt AC systems can hold their torque longer and spin faster, 14,000 RPM in a Tesla, so they don't need a transmission but usually use a single speed gear reduction or direct drive to a high ratio rear end.
Trying to go direct drive to the wheels without any gear reduction demands so much more torque and current from the motor it's not worth doing, part of the reason that good hub motors are so hard to come by. If you really want to skip the transmission then you either need a light vehicle with a high torque DC motor or a high voltage high speed AC system. That's my take anyway.
Post a Comment