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Old 04-03-2017, 03:00 AM   #40 (permalink)
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Originally Posted by freebeard View Post

The weights don't seem out of line, but putting one on a clutch and transmission sounds counterproductive to me. This single speed installation would be better. EV West just did a conversion on a Fiat 124 and immediately made the clutch disk into two parts. Gas motor drive trains don't expect the microsecond shock loads.

Tesla doesn't do it. Arcimoto doesn't do it. But, Wolftronics make a 2:1 planetary gear box that will pass 500hp.
As I mentioned in another thread, it's about RPM range. The poly-phase motors production EVs run can do 10 or 12,000 rpm. These DC motors are limited to 5 or 6k. The production EVs run ~10:1 final drive. If you did that with only 5 or 6k peak, you'd only be able to drive around town. If you cut the ratio in half, you'd kill your acceleration, or else require a hell of a lot more amps to make it acceptable. Which means a bigger motor and a bigger controller. More weight and much more $$.

So lets say you do it anyway. You're now running twice as many amps on take off. Amps = heat. And heat = inefficiency. Your efficiency goes to hell at low speeds. Compounded if you use lead acid batteries. (I believe, back in the day, the rule of thumb was that that direct drive cut your range in half.) Worse....your motor is air cooled by a built in fan that's driven off the motor itself. It's not spinning fast enough to shed regular levels of heat, let alone the extra heat being made by all those extra amps. You'll either have to add force air-cooling, or risk overheating and killing your motor. Half the reason for running a bigger motor is the sheer mass of copper...more thermal mass to absorb all that heat.

Heat in a production ev motor isn't nearly as big of a worry. They're liquid cooled, independent of motor speed. They have that 10:1 gear box, so they're not "lugging", and they only require half (or less) as much torque to get the same results. They're also more efficient at low rpm, so there's less heat to begin with. All-around better. That's what happens when things are purpose-made for their application, I've noticed...

Speaking of purpose-made...there's a 2-speed powerglide transmission out there for EVs, sans torque converter. Plenty strong enough, and gives you that gear ratio change you want to go with DC motors. Not cheap, though. Perhaps one could convert an old powerglide for a lot cheaper?
Powerglide 2 Speed Direct Drive Transmission for EV Motors - EVGlide, EV West - Electric Vehicle Parts, Components, EVSE Charging Stations, Electric Car Conversion Kits

And, before I forget, you want to also note the torque curves of series-wound motors.

Due to the self-weakening field nature of series-wound motors, you generally start to run out of torque above a two or three of thousand RPMs(vs an AC motor with a pretty flat torque curve). To get more torque, you need to cram more amps in to it, and the only way to do that is to overcome the back EMF with a higher voltage. If you don't...well, 5 or 6k rpm might be your motor limit, but it becomes useless well before that. You'll never get up to speed that way.

IMO, these Netgain/DC motors are designed to replace an ICE directly, and they work well when used as such, but for other applications, there are better suited motors.
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