[ausev] VW FOX CONVERSION, GEAR RATIO QUESTION
loopcat at yahoo.com
Mon Jun 9 17:23:47 GMT 2008
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,
> 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
> 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
> 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.
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