RE: A 48 volt Freeway EV?

["Roger Stockton" <stockton@racalcanada.com>]

Hi Nathaniel,

Your wording was OK; I realize that you were suggesting
that performance would drop off upon exiting current-limit,
and I agree with this.

I think the only thing questionable with your reasoning is
assuming that Uve's calculator is accurate in its prediction
of your hp requirements, and assuming that your 7" Prestolite's
characteristics are typical of a 48V motor with similar power
and rpm ratings.

You said "torque=current"; turn this around any you'll see
that the current *demanded* by the motor depends on how much
torque is applied to the shaft resisting its rotation.  That
is, *if* your car needs 18hp to maintain 60mph, and you want to
run the motor at 4000RPM (perhaps because it is most efficient
there), then your eqns indicate that the motor needs to
produce about 26.3 ft-lbs of torque.  The power drawn from the
batteries must still be about the same as you figured, i.e.
about 350A at 40V, but it is the motor characteristics which
determine how that voltage and current map to RPM and torque.
One must choose a motor whose power and RPM ratings are a good
match to the application; what you've established is that your
7" Prestolite would not be a good choice for a 48V vehicle which
requires ~20hp at higher than ~1800RPM.

Some time ago, Martin Jackson posted a reference to a site at
<http://www.epa.gov/oms/gopher/Cert/MPG/Testcars> which contains
actual road load figures for various vehicles.  You might find
values for your particular vehicle at that site, but from the
values Martin included in his post:

86 VW   CABRIOLET  2625lb   8hp@50mph  3042RPM@60mph
86 VW   GOLF       2500lb   7hp@50mph  2556RPM@60mph

(several Honda Civic models consume less than 6hp@50mph).  You
might want to compare Uve's calculator's predictions for your
vehicle at 50mph with these values before accepting its 18hp@60mph
estimate...

Cheers,

Roger.

 -----Original Message-----
From:	ev@listproc.sjsu.edu [SMTP:ev@listproc.sjsu.edu]
Sent:	Wednesday, June 30, 1999 5:32 PM
To:	ev@listproc.sjsu.edu
Cc:	Stockton@racalcanada.com
Subject:	RE: A 48 volt Freeway EV?

Hi Roger.

[snip]

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?

[snip]

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