[ausev] High torque or High RPM?

[Brian Lasseter]

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

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