RE: (ET) snow blower

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

Rhett is perfectly correct.

As I said in my earlier post today, I do not know if ANY snow-throwers or 
blowers used a motor that could easily be braked.  None of the ones I have 
seen do.  Truth is, the snow-thrower plugs into the SAME PTO circuit that 
brakes the mower deck and the sickle-bar mower (yes, I have one and it 
does indeed brake), and possibly others like the power broom or power 
rake, but I have seen neither.  It does NOT brake the snow-thrower or 
tiller (both of which I have), and there may be others here too, but I 
can't imagine which.  The circuit works, but series motors don't work that 
way with it, you just get NO braking.  One could design a circuit to brake 
it, but the cost would be prohibitive and involve generating a back EMF 
and probably be pulsed out of phase as well, and would have to be switched 
or detect what sort of motor is involved.  Also, notice devices that 
needed high torque with a varying load, the snow-thrower and tiller, got 
the series wound motors because in that size !
they are cheaper.  Now, did GE ever have a motor that could brake that was 
shipped on a snow-thrower?  I don't know.  Bill might.  If you look at the 
old publicity photos and the early brochures, you see a smaller motor on 
one tiller.  It might be a really big PM, but the snow-thrower is even 
BIGGER YET (I'm not sure it has higher capacity).  The sickle-bar mower 
has a 'double-length' PM motor (same dia. as a larger deck motor) that 
probably does about 2 HP or so; but even that's not big enough.  Besides, 
it would slug badly in heavy snow.  Same story for the tiller.  I have not 
seen a bigger PM motor than the one on the sickle-bar mower in real life, 
and certainly not from GE.

Larry Elie

-----Original Message-----
From: Rhett T. George [mailto:rtg ee duke edu]
Sent: Tuesday, October 23, 2001 2:14 PM
To: elec-trak cosmos phy tufts edu
Subject: (ET) snow blower


 - 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