RE: A 48 volt Freeway EV?

[Nathaniel Martin <nmartin@CellNet.com>]

Hi Roger.


At 16:56 30/06/99 -0700, you wrote:
>Many lower-voltage motors (12, 24, 36V, etc.) are
>rated for operation near 4000rpm, so 1000rpm needn't
>be an inherent limitation for a 48V motor.
>


I didn't say it wouldn't be able to get above 1000 RPM, I just said the
torque would drop off steeply after around 1000 RPM. Maybe you would have
usable torque to the 3000 range, but I don't know.



>One would want their 48V vehicle to operate near max.
>RPM at the desired road speed to minimize current and
>maximize efficiency, just as with any higher voltage
>EV.  Your suggestion of holding it just under the rpm
>where it leaves current limit will yield the highest
>power, ~40kW for a 1000A controller and a 48V pack,
>which is way more power than one needs to sustain
>highway speeds on level ground... but the controller
>would soon heat up and cut back to its lower continuous
>rating anyway.

Sorry, I didn't word that correctly. I meant that, though your could take
it above the current limit, you couldn't take it far above the current
limit. Pretty soon, because of the low voltage, the amps will fall too far
to keep the car moving at 60 mph+


>
>It is fairly common for a controller's continuous
>rating to be about 1/2 of its peak, so a 1000A
>(battery amps) controller might only sustain about
>500A, which even assuming some sag from 48V is still
>around 20kW, or at least twice the ~10hp required to
>maintain the typical small car at ~60mph.

According to Uve's EV calculator, my Rabbit at 60 weighing 2800 pounds
should take 18 HP. If the batteries have recovered to 40 volts by this
time, that would still need 350 Amps.

According to :  HP=(Torque*RPM)/5252
18=(54.5*RPM)/5252
RPM=1734

(note torque is based on Prestolite 7" motor)

So, with a 48 volt system, you would only have enough power to go highway
speeds until 1734 RPM.

I might be wrong, is there something wrong with my reasoning here?

>
>Cheers,
>
>Roger.
>

Thanks,
Nathaniel Martin
nat@lumiere.net
nmartin@cellnet.com