RE: (ET) Tractor Pulls

["Elie, Larry (L.D.)" <lelie ford com>]

Actually, as a farm-boy, I used differential braking extensively before
differential locks became common.
They work.  Well.  Even when drawing large torque, for example plowing.
That's a very severe case as one 
can run a brake for a quarter mile straight at only 4 mph... 4 minutes at a
time!  If you do that for more 
than a couple of rounds the brakes overheat.  If the ET had two breaks, 
this
would be trivial.  

As far as traction control in cars, the loss of power is very small; power
loss is the product of torque 
transmitted and RPM, and the duration is pretty short.  Worst case; one
wheel on sheet ice and about 100% 
of the torque needed by the good wheel has to come through the brake,
multiplied by the difference in RPM.  
As soon as the RPM's match, all the break has to do is produce torque at
essentially zero RPM... no 
appreciable power, and the duration of the event is small.  If it were not
short, the brakes would quickly 
burn out.  But that's really because cars never need much torque except for
launch; you don't plow fields 
with cars.


On the subject of over currenting the motor in a pull...

The resistance of copper wire changes with temperature.  The formula is the
change in resistance due to 
temperature divided by the room temperature resistance is equal to the
temperature coefficient of copper 
times the change in temperature, in Celsius this is:

Delta R(t)/((R.T. R=4.07 E -3/deg. C)* Delta t)

The torque the motor puts out is directly proportional to the number of
turns of wire and the current through 
each turn.  Since the motor is warming up, the resistance is INCREASING 
with
temperature.  Since the control 
is basically voltage control, and voltage=current*resistance, the available
current (and torque) DECREASES 
with motor temperature.  In a perfectly temperature protected motor, you
CAN'T overheat as the resistance 
changes too quickly.  The GE instead is perfectly happy to over current for
brief periods, but resistance is 
trying to stop you.  The limit is the varnish temperature of HML wire is 
190
deg. C; beyond this temperature 
for extended periods and the coils will short.  Even at this temperature 
the
bearings can begin to boil dry.  
In addition, as you load the batteries, the voltage drops, lowering the
available current and torque.  The 
bottom line is; KEEP THE BATTERY COOL!  People who race electric vehicles
often chill the motor before a 
heat with dry ice; assuming your connections are OK this would help.  A 
full
charge is a necessity.

Larry Elie



-----Original Message-----
From: Dave Reuter [mailto:david reuter iavinc com]
Sent: Thursday, March 25, 2004 8:52 AM
To: Elec-Trak Tractor
Subject: Re: (ET) Tractor Pulls



As long as the coefficient of friction is the same under both wheels you
really don't need to weld up the diff. If it is sweeped concrete or asphalt
surface you're pulling on then you would be fine. If it is dirt or sand and
stone on hard surface then the welded gear set would have a huge advantage.

Brake biasing is a good traction control which is used by the auto
companies now days but it is at the cost of power to the wheels. Granted
you are transferring power to the non-spinning wheel but you're also
absorbing power from the drive unit in to the braking wheel brake as well.
I would weld the gear set before modifying the brakes if it is allowed. Or
find an axle that was manufactured with out a differential set. I notice a
lot of old 3 wheeler and some of the 4 wheeler ATV don't have differentials
in the rear axle, so maybe there is a cheap lawn tractor that doesn't have
a diff too.

Again this is just my personal feelings, I do not mean to knock anyone's
opinions. I have designed 4WD and AWD systems for a few years in my very
distant past before my enlightenment of HEV's, FCV, and EV's

Dave Reuter



                                                                           
             "Pieter                                                       
             Litchfield"                                                   
             <plitch@attglobal                                          To 
             .net>                     "Elec-Trak Tractor"                 
             Sent by:                  <elec-trak cosmos phy tufts edu>    
             elec-trak-bounces                                          cc 
             @cosmos.phy.tufts                                             
             .edu                                                  Subject 
                                       (ET) Tractor Pulls                  
                                                                           
             03/25/2004 08:03                                              
             AM                                                            
                                                                           
                                                                           
                                                                           




Here's a tip based on some hill climbing and racing experience.  If you are
thinking about dedicating a tractor to pulling, open up the rear end and
weld the differential spider in place.  You now have a solid rear axle and
two wheels driving rather than one.

If there is a rule against this or you want to preserve the differential
for lawn work, it might be possible to mount a brake drum on each rear
wheel and at least be able to transfer power manually from the spinning
wheel by applting the brake to it.

Just a thought.....
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