[ausev] VW FOX CONVERSION, GEAR RATIO QUESTION

[Erik]

Torque is a roughly a function of amps squared through the motor for
"normal" currents, and getting closer to linear as you get into the high
hundreds of amps through a motor.

torque = k * current^n, and using constants from the Kostov

T = .0167 * 600^1.46 = 190 ft lb (I assume the units are foot pounds)

Also, the 600 is motor amps, which is not very high. If you have a Zilla you
can dial in a maximum current, but the higher power Curtis controllers can
put out 550 amps for a little while.

As for the EV calculator, it is a solver of sorts and you have to be careful
when you choose a number that it spits out. For instance, the "ft-lbs" value
it displays is the torque required to maintain the vehicle at that speed,
not the amount your motor can produce. If you let your motor have all a
Zilla can dish out something will probably give, hopefully the tires =)

Erik



On Mon, Jun 9, 2008 at 12:23 PM, loopcat <loopcat at yahoo.com> wrote:

> Hey Chris,
>  I fiqured in the final drive ratio to the EV calculator, and now the fox
> transmission looks pretty normal. 1st-14.17, 2nd-7.35, 3rd-4.64, 4th-3.41,
> 5th-2.79. I did notice that the RPM's in 1st gear were too high, for the
> GE-11". Looks like the RPM's in 2nd and 3rd gear would work though.
>
> The scenario on the calculator is a 1995 Mazda Protege, (about same size
> car but the fox lighter at 2100 lbs), Kostov motor with 3800 RPM's at 156v
> and 210 lbs, Zilla LV 156v at 1000amps, Trojan J-150 12v =156v. The torque
> limit seems never to go passed 85 lbs. The fox is set up to take 93 lbs of
> torque. From the calculator, the torque never reaches 87 lbs, regardless of
> amps or gear selection.
>
> Curiously, the torque changes very little between most of the motors on the
> list at the same volts and amps. Always somewhere between 84 to 86 lbs max.
> Why is that?
>
> Is torque more a product of the batteries and the controller more than the
> motor?
>
> Thanks for the reply Chris,
> Stuart in SA..
>
>
>
>
> --- On Mon, 6/9/08, Chris Robison <chris at chrisrobison.org> wrote:
>
> > From: Chris Robison <chris at chrisrobison.org>
> > Subject: Re: [ausev] VW FOX CONVERSION, GEAR RATIO QUESTION
> > To: "AustinEV News Announcements and General Discussion" <
> ausev at austinev.org>
> > Date: Monday, June 9, 2008, 8:28 AM
> > loopcat wrote:
> > > Your right Chris, The motor is OVERSIZED for the car.
> > The weight difference between this GE-11 and a ADC-9 is
> > about 90 lbs. I can live with that. But I am very concerned
> > about the torque effects on the fox tranny.
> >
> > I think a 9" motor is a bit much for a Fox. I'd
> > say an 8" is about
> > right. Also be aware that a pound of rotating mass (larger,
> > heavier
> > armature) is worth potentially several pounds of mass on
> > the body or
> > chassis, in terms of the energy required to overcome
> > inertia and the
> > energy subsequently lost during braking.
> >
> > Weight is also not the only consideration. The motor's
> > additional volume
> > will take up space that could otherwise be used for
> > batteries or other
> > components. This actually ended up being a challenge with
> > Brian's
> > Saturn; choosing a 9" instead of an 8" motor
> > forced some change in plans
> > for fitting things in the front. Just that extra 1/2"
> > of height over the
> > top of the motor affected the design significantly.
> >
> >
> > > If I were to keep the motor, I could make adjustments
> > to the Zilla LV. The GE tag states 1822 rmps at 70volts, So
> > at 156v it should bring it up to about 4000 rpms. Right? The
> > fox engine produced 81 hp and 91 lbs of torque. So what is a
> > safe current to run it at?  I'm guessing that my battery
> > selection would also play a part.
> >
> > Series-wound motors don't seek to a particular RPM for
> > a given voltage,
> > like shunt-wound and permanent magnet motors do. The RPM on
> > the tag is
> > just a point at which the motor's other performance
> > values are measured
> > and rated. For example if you powered the motor without a
> > load at 70
> > volts, it would NOT spin up to 1822rpm; it would quite
> > likely spin up
> > well past its redline and the commutator would explode.
> >
> > I don't know how much torque per amp this motor will
> > make. I also don't
> > know how fast back-EMF will climb with RPM (which
> > determines how
> > controller output voltage must rise to maintain the same
> > current flow).
> > It's pretty hard to say what will be "safe"
> > but bear in mind that at
> > 1000A, your motor will quite possibly produce 3+ times the
> > amount of
> > torque that the stock engine did. I've read that the
> > Fox's 4-speed was
> > actually a durable, well-made transmission, but this is so
> > far beyond
> > it's intended operating area that it's hard to
> > predict how long it will
> > stand up to the abuse. Turn the current down to 500 or 600A
> > or so and
> > you may solve this problem, but then you'll have a lot
> > of unused
> > capacity in both your motor and your controller.
> >
> > Generally speaking, larger motors are capable of producing
> > more
> > continuous mechanical horsepower than small ones. However,
> > this higher
> > output power is in the form of higher torque, not higher
> > RPM (a larger
> > motor tends to have a lower redline). If your vehicle
> > cannot take
> > advantage of the higher torque, then the larger motor is
> > dead weight.
> >
> >    --chris
> >
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> > AusEV at austinev.org
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>
>
>
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