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Old Sep 30th, 2020, 17:53   #20
142 Guy
Junior Member

Last Online: Today 17:21
Join Date: Jul 2020
Location: Saskatchewan

My first observation would be that grafting the Amazon body on to an existing salvaged electric vehicle platform might be the easier (and long term less expensive) option; but, in re reading the thread I see you explicitly do not want to do that. Fair game! Your car, your choice.

The choice of a series would DC motor drive without regenerative braking would also not be my choice. This is ancient technology that has been in use for a century (electric trolley cars) with the only technology development being high current DC supplies rather than a bunch of switched resistors. Having to deal with brushes does not appeal to me; but, I have not explored the cost difference between AC and DC, so again your decision.

The plan to go forward with a gearbox is the one that I would seriously question. This will depend on the selection of the actual motor and controller; but, unless you grossly undersize the motor (which will kill your top speed) when you look at the torque - speed - voltage relationships of the electric motor I think you will find that you are flying through the 1st and 2nd gears way too fast. I speculate that with a correctly sized electric motor your acceleration times will be slower with a gearbox than without because of the dead time as you shift gears. Be aware that series wound DC motors tend to have a torque curve that is the reverse of a gasoline engine. Very high torque at stall (0 RPM) with the torque dropping off as motor speed picks up - assuming constant voltage. I expect that any electric motor capable of getting the Amazon up past 80 mi/hr will probably shred an M40 transmission in first gear if you apply full throttle.

Do a little math before you select your drive and make the decision on the gearbox. Look at the B18 torque curve between 900 RPM and 5000 RPM and use the gear ratios to determine the effective torque on the output shaft of the transmission in the various gears. Now, as a first pass select an electric motor capable of matching the power output of the B18 if that is your objective. If this is a non highway car then you can downsize the motor appropriately. Have a look at what the stall torque of that motor is. Chances are that at full voltage (particularly if the motor has a short time rating for acceleration) it greatly exceeds the first gear output torque that the B18 can produce at 3000 RPM. Applying that much torque to the input shaft of your transmission is a recipe for turning your transmission into a piece of shrapnel.

Transmissions are a necessity on a B18 internal combustion that has a very positive slope torque - speed curve between 1000 and 3000 RPM. You need to completely change your thinking when dealing with the negative torque - speed curve for a series wound DC motor.

Last edited by 142 Guy; Sep 30th, 2020 at 18:01.
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