Re: (ET) Bye bye brakes

["Bob Murcek" <RMURCEK geisinger edu>]

Chris,
I didn't see any answers to your questions so I'll take a shot.  Answers 
in CAPS, but I'm not shouting!


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? YES  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) RIGHT, 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?  THE SHUNT FIELD IS JUST AN 
ELECTROMAGNET, SO IT CAN BE LEFT ON INDEFINITELY, BUT IT WILL SLOWLY DRAIN 
THE BATTERIES.   THE BACK CURRENT FROM THE SHORTED ARMATURE WILL ONLY FLOW 
THRU THE SERIES FIELD, AND THE SERIES FIELD IS RATED TO CARRY THE ARMATURE 
CURRENT.

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)?  ACTUALLY YOU SHOULDN'T COMPLETELY SHORT THE ARMATURE SINCE 
THAT COULD CAUSE DISTRUCTIVE CURRENT.  IF YOU WANT MORE BRAKING THAN WITH 
REGEN, YOU SHOULD PUT SOME HIGH POWER LOW VALUE RESISTOR ACROSS THE 
ARMATURE.  I'D GUESS SOMETHING TO LIMIT CURRENT TO ~200 AMPS AT MAYBE 10 
VOLTS, WHICH WOULD BE 0.05 OHMS.  HMMM, MAYBE A SHORT ISN'T SO BAD AFTER 
ALL!

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.