(ET) snow blower

["Rhett T. George" <rtg ee duke edu>]

 - Greetings -

Bob Murcek asked a fine question regarding dynamic braking applied to
series motors.  When operating as a motor, the applied voltage, say, 
36 VDC, is dropped across the total series resistance of the motor
thru which the current flows * and * across the voltage generated in
the armature, knows as back e.m.f.  In a lightly loaded motor, the 
current may cause a 3 VDC drop across the total resistance which 
yields a 33 VDC back e.m.f.  If the motor is loaded more heavily, 
it slows down, the back e.m.f drops to 32 VDC as a result of the slow
down, and 4 VDC is dropeed across the resistance.  This yields a 
1/3 increase in current required.

If the 36 VDC is turned off and the motor terminals are shorted, it
would seem that 32 or 33 VDC is still being generated in the armature
windings and this would cause a lot of current to flow.  Well, the 
current would flow in the opposite direction because it is now the
bakc e.m.f., not an applied voltage, causing the current.  This causes 
a reversal of the magnetic field, the field necessary to generate the
back e.m.f.  - - So, the motor kills its own field and nothing in the
way of braking takes place.

Diesel-electric locomotives achieve dynamic braking by using the Diesel-
driven generator to continue to supply current to the field winding,
now no longer in series with the armature.  The voltage generated in
the traction motor armatures is connected to roof-mounted resistors
where the electrical energy is converted to heat and is blown away.

With the shunt motor used as traction motor in larger, if not all,
Elec-Trak tractors, there is no reversal of field current.  I really
appreciate this feature as I live on a hill and use the E-15 to turn
my 16' sailboat and trailer around.  The motor provides regenerative
braking going downhill and, yes, the ammeter reads downscale.

Hope this helps.

                                        Rhett George