Re: (ET) Electronic controller

["David Roden (Akron OH USA)" <roden ald net>]

On 9 Jan 2002 at 22:48, Bob Murcek wrote:

> I haven't tried it, but I suspect that without the
> shunt field the motor speed would be very unstable, like a sewing 
> machine.
> 

I've tried it accidentally (when my reversing relay went south).  With 
the shunt field unpowered, the armature tries to draw gargantuan currents 
but doesn't run.  

I'm not an engineer either, but I think Rhett George is.  I hope he'll 
forgive me if I quote some excerpts from messages he posted in March of 
2000, along with one from Steve Naugler. The gist appears to be that the 
ET motors (at least in the E15 and E20) are differential compound types, 
and GE made that design decision to stabilize their speed under load.

= = = = =

Rhett George wrote:

The E-20 [is] compound and [reverses] the armature rather than the field.

Regarding what goes where, the factory schematics show a series winding
internally connected to the armature.  ...

The motor may be cumulative compound in the forward direction.  If so,
this will give extra torque in cases where the load torque demand is
increasing.

Let's review the fields that may be found in a wound-field, D-C, brush-
type machine. There may be a shunt field or a series field, giving the 
definition to shunt-wound motor and series-wound motor.  If the motor has 
both shunt and series and they add to produce the total field, it is a 
cumnulatively-compound motor.  If the series field subtracts from the 
shunt field, it is a differentially-compound motor.  [This one may not be 
very stable over a wide torque range.]  The shunt and the series fields 
are wound in the same locations in the motor.  
 
= = = = =

Here is a short description of the two types.

The cumulative compound has two field windings which add to produce the 
total field.  The connection and typical magnetic pole contributions
follow in this ASCII drawing.

        __________________________________________
      +                    |                     |
      |         - North -  (          - North -  (
      |     shunt winding   )    series winding   )
      |     many turns of  (       few turns of  (
      |       small wire,   )       large wire,   )
      |   high resistance  (     low resistance  (
      |                     )         - South -   )
      |                    (                     |
      |                     )                    |
      |                    (                    ***
      |                     )                 *     *
      D C                  (                 *       *
     supply                 )                *       *
      |                    (        armature  *     *
      |         - South -   )                   *** 
      _                     |                    |
        ------------------------------------------


The differential compound motor will be built the same way but the series 
field will have the connections reversed so that the South pole is on 
top, decreasing the effect of the North pole of the shunt field.  Then 
the North pole of the series field is at the bottom.

As torque demand on the cumulatively-compound motor increases, the series 
field adds to the total field strength.  Torque increases a bit more than 
linearly with armature current.

As torque demand on the differentially-compound motor increases, armature 
current goes up, net field strength drops, back EMF drops, and armature 
current goes up more, further decreasing net field strength.  To avoid 
having the armature current increase to the point of no net magnetic  
field, the series winding for a differentially-compound motor should have 
no more than a few turns.

= = = = =

Steve Naugler added:

    I have rebuilt two E 15 traction motors and they both had 
stabilization windings.  The E20 motor on my garage floor looks like it 
is built the same as the E15 motor except slightly longer.  I suspect it 
has stabilization windings as well.  I would expect that they are wound 
as a differentially compound motor in forward and as a cumulatively 
compound motor in reverse. 

    My home owner's service manual shows the E15 reversing the motor via
field reversal and the E20 reversing the motor via armature reversal.  
This manual's schematics also show both the E15 and E20 motors as having
stabilization windings.

= = = = =

     When your shunt wound motor is heavily loaded it tends to slow down.
This is because not all of the armature voltage is used to generate the 
armature magnetic field, but rather some of the voltage is lost 
parasitically as resistive losses.  As the armature current goes up, a 
larger percentage of the voltage is lost ...

    The full armature current then goes through the stabilization 
windings opposite to the field current, thereby weakening the field's  
magnetic field.  When the field's magnetic field becomes weaker, the 
motor tends to speed faster.  

Now you have two competing and opposite effects at high motor loads, one 
of which slows the motor, one of which speeds it up.  By adjusting the 
sizing of these opposing magnetic fields by sizing the windings when the 
motor is designed, you can actually get the motor to slow down, not 
change speed, or even speed up under load.

= = = = =

Note:  the discussion above is taken from writings of Rhett George and 
Steve Naugler.  I'm not an engineer, and although I have some 
understandings of the basic ideas here, please don't ask me to explain 
further!


David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1979 General Engines ElectroPed 24vdc
1970 GE Elec-trak E15 36vdc
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