[ausev] High torque or High RPM?

Wed Dec 5 21:35:47 GMT 2007

I also have a hill country commute and the regen definitely helps with range
on the downhills in traffic.  I drive down (and then up) tumbleweed hill on
2222 every day. If traffic is light enough, I can coast much of the way to
work, but usually traffic slows me down and it is nice to recapture some
portion of that.

My Ranger EV has a single-speed transmission with a ratio of about 1:12.  A
couple drive AC OEMs sell transmissions with motors, but usually only the
low power variety, which won't get you anywhere near 6 second 0-60.  You can
have this custom fabbed, which I'm also looking into for my conversion
project.  That almost guarantees you're using an AC motor or a permanent
magnet DC motor because you'll need the very high RPM limits due to reach
your top speed.  These motors allow for high RPMs through lack of mechanical
commutation (brushless).  Usually though, performance-oriented
conversions/builds would use multi-ratio transmissions and DC motors (price,
simplicity and availability).

Your friend is partially right about the transmission.  You do want one that
can handle high-torque. That is one of the great things about electric
motors, but he is wrong(ish) about efficiency.  Most electric motors are
very efficient across the RPM range.  It is specific to each individual
motor, but a factor of torque at a given RPM.  You're realistically talking
about a 20% difference at most anywhere along the band beyond zero.  For AC
motors, the difference is even smaller.

According to wikipedia (caveat emptor) the "Lotus Seven has the highest drag
coefficient of any known production car--ranging from 0.65 to 0.75,
depending on bodywork".  That is unfortunate for highway speeds, and
hopefully your locost can reduce that, somewhat.

There isn't much room in a 7 for people, much less batteries.  I'm thinking
you are looking at some very expensive batteries to get that much range.
Add in the acceleration demand, and you're definitely in LiPO4 territory.
The Valence U-Charge XP series is the only drop-in battery with that
chemistry that is suitable for EVs at the moment; all others requiring a
custom BMS, pack design, cell welding, etc.  Of this series, I would suggest
the UEV, which is a 19.2V 65Ah model ($1935/module).  It's got high-voltage
and high capacity, so you're getting a lot of bang for your many, many
bucks.  How many batteries depends on what drive system you choose - AC will
be high (300+), DC will be lower (usually 96-252).  If you want to get up a
hill while maintaining speed, you'll want the lighter batteries, too.

Use the EV calculator to run some simulations:
http://www.geocities.com/hempev/EVCalculator.html?200629

-ian

On Dec 5, 2007 3:05 PM, Brian Lasseter <blasseter.cmpe01 at gtalumni.org>
wrote:

> On Dec 5, 2007 2:06 PM, Josh Handel <jhandel at ktomics.net> wrote:
> > > You mentioned a single speed transmission, and I had that thought.. If
> I understand correctly Electric motors have 100% torque at 0 RPM. Too me,
> that meant I should use a single gear that is a blance between not bogging
> down my motor off the line and reaching my ideal topspeed when the motor
> reached its max RPM. I talked it over with a friend and he seemed to think
> that electric motors were more efficent at higher RPM so your better off
> using a full blown transmission. He also seemd to think that the larger
> magnets of a high Torque electric motor were performance prohibative over a
> higher RPM motor that can run with smaller magnets. From a NON EV
> perspective that makes since, but I don't know if those costs are real
> applicable to design when you are talking about something that is already
> 1000lbs + in size...
>
> Electric motors do have 100% torque at 0 RPM... however you will be
> pulling a lot of power to run your car slowly over hills with a single
> speed transmission... See Otmar's discussion on this in his Apr 2nd
> blog post here... http://www.cafeelectric.com/blog/?cat=3
>
> Also... with so much torque... the motorcycle transmission would be
> hard pressed not to fly apart with a suitably large electric motor.
> We have enough problems destroying regular car transmissions with
> electric motors, and I would presume that the car transmissions are
> meant to handle higher torque and abuse.  Randy, at Canda EV, told me
> a story of a customer of his that floored a recently delivered
> Electric Vehicle conversion done by his shop, and the motor proceeded
> to shatter both front CV axles from the massive torque.  A regular
> car's computer will not let you apply that much power to the
> transmission... an EV will not care.   (Though you could program a
> Zilla not to allow you to accelerate faster than a given rate.)
>
> I recommend a car transmission.
>
>
> > > So all that leads back to my first question. Which Electric motors are
> more efficient, High RPM low torque or Lower RPM high torque?
>
> Well... for the most part, electric motors are both.   They have high
> torque at low RPMs, and they have low torque at high RPMs.  However
> series wound DC motors are more efficient at higher RPMs, but not by
> much.
>
> If you look at a chart for my FB1-4001 Advanced DC motor, operating at
> my voltage of 144V...
> http://www.evmotors.com.au/products/download/fb144.pdf
>
> You will notice a few things...
> - I have 160 ft/lb. of torque at 3000rpm, and I have 10 ft/lb. of
> torque at 8000rpm.  So the motor is both High RPM low torque AND Low
> RPM high torque.
> - The motor runs most efficiently at 35ft/lb of torque and about
> 5500rpm.  Normally a car shifts to a higher gear at 3000-4000rpm.  In
> this case though, I will not need to up shift until 6000-7000rpm.
> This is why an automatic transmission will not work in my electric
> car, and a manual transmission is a must.
> - If you look at the total efficiency curve, you will see that the
> motor is always 80-90% efficient running between 3000-8000rpm.  So in
> this case, what speed the motor runs best at is not a real concern.
> Ambient wind speed, body shape, and even carrying the weight of a 2nd
> person will all have a greater power impact to my batteries than what
> than the speed I'm running the motor at in relation to my
> transmission.
>
>
> --
> TTFN,
> Brian "Lasso" Lasseter
>
>  ·  (512)736-1677  ·  AIM:digininja  ·  ICQ:2238123  ·  MSN:azoreg  ·
> "No Sane man will dance."   -Cicero (106-43 B.C.)
>
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