(ET) Re: surging, motor

["Steven Naugler" <snaugler earthlink net>]

    A message for anyone who might want to add dynamic braking to their GE
accessories that didn't come from the GE factory with braking:  (Inspired 
by
Michael Neverdosky.)

General discussion of dynamic braking:
    Michael Neverdosky is correct in that any DC motor can be set up to 
have
dynamic braking.  However, a significant magnetic field must be present in
the field portion of the motor in order to stop the motor.  There is a
problem in doing this on most of the GE accessories.  (See below.)
    Michael also talked about using a resistor wired to short the armature
leads instead of shorting them directly.  Doing that is normal practice in
industry.  What happens in dynamic braking is that all of the energy of the
rotating loads is turned into heat.  And when there is no resistor, all of
that heat energy ends up in the motor.  Repeated stops can seriously
overheat a motor.  When a resistor is used, much of the heat ends up in the
resistor instead of the motor.  Also, resistors can be adjusted in wattage
to enable more or fewer stops in a given time.  These resistors do have to
be large; your Radio Shack resistors will not survive.
    There is one other beneficial effect of using a resistor.  The torque 
of
a DC motor is approximately proportional to current.  As you increase the
value of a dynamic braking resistor, the braking current drops, hence the
stopping torque drops.  This enables you to eliminate an extremely high
torque stopping situation where things could rapidly disassemble or alter
geometry.  It also means that the load will take longer to stop.
    What this all means is that any dynamic braking application is a battle
between the energy to be dissipated during the stops and the frequency of
stops versus the ability of the motor and resistor, if used, to dissipate
heat.  How fast you need to stop a load also comes into play.  Industrial 
DC
drives with dynamic braking are rated in the number of stops per time or
time between stops, load inertia, and load speed.  I have seen blown up
resistors and smoked motors where those guidelines were ignored.

Dynamic braking on our GE's:
    The problem in retrofitting dynamic braking to many (most?) of the GE
accessories is that many (most?) of the accessory motors have a wound 
field,
that is to say that the field must be energized by the battery pack.  This
must occur even when the armature leads are shorted if one were to
accomplish dynamic braking.  In order to accomplish that one would have to
run additional wires so that the field could be powered independently of 
the
armature.  This is doable, but I'm too lazy to do it.
    Any of the permanent magnet field motors can easily be modified to
dynamic brake by using the third conductor in the PTO cord (dryer plug).
Just duplicate the wiring as used in the mower decks.  If there was reason
to slow the braking, which I would recommend on all motors bigger than the
mower motors, the resistor could be spliced into the third wire at the
accessory.  That way each accessory would have a dynamic braking resistor
matched to that specific application.  The tractor itself would need no
modification, and the accessory could freely be moved between tractors.
    If someone seriously wanted to add dynamic braking, sizing the resistor
for resistance and wattage should be simple.  Should be, but I do not have
sizing guidelines.  Sizing guidelines should be the same for both permag 
and
wound field DC motors.  If anyone has information on dynamic braking
resistor sizing, please let us all know.

Steve Naugler