Re: [EVDL] Four Wheel Drive And Dual Motors With A Zilla
[quote]Brad Bowler wrote:
> I'm converting a Suzuki Samurai (4X4), and I have been toying
> around with the same idea.
> I'm installing a single Warp 9, this year and maybe a second
> next year.
> With a single motor I'm
> going through the clutch and transmission. If I go two
> motors they would be direct. The second motor weighs the
> same as the transmission and transfer case. The issue that
> I'm struggling with is the high starting amps required when
> going direct. Even if I use 5.66 gears in the differentials,
> the total ratio is kinda out of whack. The controller and
> batteries are going to take the hit when starting uphill.
The batteries are unlikely to suffer at all starting uphill; it will be the motor(s) and controller that are stressed in any direct drive system.
Without the torque multiplication offerred by a normal gearbox, the motor(s) must develop more starting torque, and this means the controller must shove more amps through them. At low speed, however, there is little voltage across the motor(s) since they are not spinning very fast; this is a high torque situation, but not a high-power one. The controller transforms the high voltage, low current at the battery side to the low voltage high current at the motor side, so the batteries really don't get hammered too badly starting out. In theory, the battery side of things will be no different with or without the tranny; it is the motor side that changes.
With direct drive, the series/parallel arrangement is going to make it easier on your controller and motors at low speed because it will take half the motor current to get the same torque with two motors in series as with a single motor.
With a direct drive (more accurately, a single ratio drive), what you need to keep in mind is how fast you need to drive, as the motor redline will set one limit on the drive ratio. It varies a bit depending on how high your pack voltage is, but very generally speaking you probably want to gear so that you will be spinning your motor(s) at about 4000RPM at your maximum cruising speed. If you have enough batteyr voltage to keep the motors from going to sleep at higher RPM you might be able to push this nearer to 5000RPM, but bear in mind that your ultimate top speed will occur with the motors spinning about 5500RPM.
You must weigh the cruising/top-speed RPM against how much time you expect to spend on steep grades or at very low speeds, since you want to gear so the motors will spin up fairly quickly to minimise the amount of time spend at high motor current and low RPM. Almost certainly you should provide forced cooling to ensure the motors receive enough cooling airflow even when spinning too slowly for the internal fans to be effective.
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