Re: (ET) Bye bye brakes

["Christopher Zach" <czach computer org>]

Well, personal anecdote:

A number of years ago my mom was bounced hard on the Craftsman 16hp tractor
they owned. The engine seat kill switch disabled the engine, however she
landed half on the seat/half off and the motor happily restarted and drove
forward till she fell completely off.

She was ok, but very shaken. This is part of the reason I like the
Elec-Trak's interlock system: If you fall off and grab the seat on the way
down the tractor will stay in "failed" mode.

That said, what I think we need here is a circuit that instead of just
shutting down the motor it brings the field current to full, then shorts 
the
armature dead to itself along with the rest of the normal fault functions
(ie: killing all the attachments). This way the motor will be off, yet
shorted for maximum stopping power. After 30 seconds of this, the custom
circuit can cut out, returning the tractor's logic to a default "faulted"
condition.

Now the first set of questions are:

1) With the field current at full and the armature shorted, will the motor
behave the same way as a PM motor? If the field current is dead then it
doesn't matter what happens to the armature, so the field needs to be
powered somehow.

2) Where do you get max regen power on a shunt wound motor? With the field
current at max or min (I would guess max)

3) What is the effect of having field current at 100% but armature current
at zero (or less than zero)? Will the field windings burn up? How long can
they go with nothing in the armature, and will the back current coming from
a shorted armature blow up the field windings?

4) Will shorting the armature like this put so much braking energy on the
motor the belts will just slip rather than stop the tractor (even properly
tensioned belts must have some sort of slip point where they just go, esp 
in
high gears)?

It would be neat to build a truly fail-safe stopping system into the seat
switch. When going down a steep hill, or when towing something heavy, you
want all the "stop power" you can possibly have.

Chris


----- Original Message -----
From: "Jim (fiskfarm)" <fiskfarm mediaone net>
To: "Pieter Litchfield" <plitch attglobal net>
Cc: "Discussion List Electrac tractor" <elec-trak cosmos phy tufts edu>
Sent: Monday, February 04, 2002 5:14 PM
Subject: Re: (ET) Bye bye brakes


> Hmmm, maybe the seat switch is better off disabled. I only have one
tractor
> with it hooked up. Never missed it and I'd rather have the motor stay
engaged
> during regen braking. Sounds like a personal preference decision.
>
> Jim
>
> Pieter Litchfield wrote:
>
> > I have recently replaced the transzxle (and  rebuilt the original 
> > brake)
on
> > my e-15.  Apparently there were at least two different designs - one in
> > which the caliper was fixed but the rotor floated (was free to slide a
bit
> > on the transmission shaft) and another where the caliper floated on 
> > pins
but
> > the rotor was fixed.  Somehow I managed to combine the two designs so
that
> > both parts are now floating on mine.  It seems to work as well as can 
> > be
> > expected of two square inches of mechanical disk brake!  In addition, I
use
> > a bit of "Never-Seize" graphite/lithum grease to lightly coat all the
> > floating points, since as I am sure we ALL have experienced, the brake
rotor
> > dearly loves to weld itself to the shaft.  Lubing and checking for free
> > float should be an annual rite.  In addition the brake power cut off
switch
> > on the linkage needs to be adjusted so that the motor is not working
against
> > a fully applied brake, causing wear and ineffective braking.  I allow
for a
> > bit of overlap - the brake just begins to take effect when the power
switch
> > kills motor current.  It is, in my opinion, a design which when working
well
> > is of marginal capacity, and which almost never works "to spec"!  
> > Bill's
> > upgrade is surely worth it.  I suspect that the design was an original
part
> > from the transaxle supplier (Peerless) and is designed to be used on
lighter
> > gas engine machines.  Nobody envisioned six big batteries headed down a
> > steep slope.  If the throttle is not cut, or if you don't bounce a
activate
> > the seat cut-off switch, the tractor is capable of some effective
> > regenerative braking effort.  If however (as I have done) a bump causes
a
> > power cutoff in the middle of a hill, the mechanical brake may be 
> > barely
> > adequate to save you!  I always shift to a lower speed range (using the
> > mecahnical transmission) for steep downslopes, and then keep my foot 
> > off
the
> > brake pedal so I maintain power.  Of course, we are talking about a
brake on
> > the input shaft of a differential, so braking is limited by the
available
> > traction at either rear wheel.  If one rear wheel is unweighted (as 
> > when
> > traversing across a slope), the available braking traction is limited 
> > to
the
> > amount of force that can cause the lighter rear wheel to slide -
sometimes
> > not much.  All in all, the E series brake is a weak point!  If equipped
with
> > two drum or disk brakes at the rear wheels, the brakes would have been
far
> > more effective, and of course more expensive.  But you could have
employed
> > brake steeering and traction control as well as do "real" tractors.
This
> > would appeal to me because even with a weight box on the back and
chains, I
> > often spin rear wheels with a full bucket on the front mounted loader.
> > Being able to independently brake each rear wheel would be a big
advantage
> > for me in trasferring power when one has better traction than the 
> > other.
> > Hmmmm...... winter project?
> >
> > I used to have an old Jacobson tractor that had a transmission brake on
the
> > input shaft like the GE, but it was a very simple "outside drum" where
the
> > pedal linkage pulled tight a lined metal band looped around a drum
attached
> > to the shaft.  It was very effective, but of course suffered from the
same
> > limits on brake force generated by sliding a wheel.
> >
> > -----Original Message-----
> > From: owner-elec-trak cosmos phy tufts edu
> > [mailto:owner-elec-trak cosmos phy tufts edu]On Behalf Of Jim 
> > (fiskfarm)
> > Sent: Monday, February 04, 2002 9:43 AM
> > To: Christopher Zach
> > Cc: Elec-trak
> > Subject: Re: (ET) Bye bye brakes
> >
> > Hey Chris,
> >
> > The rotor is NOT supposed to "deform" but rather float free on it's
keyed
> > shaft. I use a combo of air chisel, Breakaway and sometimes heat to 
> > free
up
> > the
> > rotor and then remove it , clean up both shaft and sleeve, retighten 
> > the
> > rivets
> > by hammer and anvil (or replacement) and reinstall all with a gentle
coating
> > (you don't want any on the rotor and pads) of Never Seize. Here, unlike
a
> > car,
> > the caliper doesn't float, but the rotor does. Also you will want to
sand
> > the
> > rotor smooth with either a power sander or rotate the rotor in place
with
> > the
> > motor while sanding by holding the paper against it. Not for the faint
of
> > heart
> > but probably the fastest for the more experienced. DO NOT "grab" the
rotor
> > with
> > the paper , but rather sand one side at a time. New pads (if they are
shot)
> > go
> > without saying. Stainless steel rotors would avoid about 99% of the
problems
> > with these breaks. That and Never Seize.
> >
> > Hope this helps you along,
> >
> > Jim
> >
> > Christopher Zach wrote:
> >
> > > Well, I figured out why I have no brakes....
> > >
> > > No brake pad :-)
> > >
> > > Seriously, the outboard pad is gone. It appears that the brake lever
> > engages
> > > a cam which pushes on a metal dowel which pushes on a pad which 
> > > pushes
the
> > > disc into another static pad on the other side. Interesting design,
are
> > > there any specs for pad clearance on the static pad (if it wears 
> > > down,
the
> > > disc will have to deform more and more for the brake to engage, thus
> > > reducing brake effectiveness)
> > >
> > > Ah well, it looks like the pad disintegrated or something. I'll call
Mr.
> > > Gunn and see if he has a new puck.
> > >
> > > Chris
>
>
>