(ET) ET - DC Motors 101

["Konstanty, Walter (GE Energy)" <Walter Konstanty ge com>]

Many interesting notes recently on DC theory so figured I'd chime in.
Here's more on how a DC motor works:
- E15(1 HP)/E20(1.2HP) traction motors are actually called stabilized
shunt wound.
   They are pretty similar so I'll use the E15 as an example.
- 2 pole DC motor having 466 turns of wire on each pole and connected
opposite to create a "north" and "south" polarity.  When excited, they
produce flux which goes thru the armature and around the frame to
complete the magnetic path.  You vary flux by varying field amps.  36
volts max and 15 ohms with a heating factor is about 1.8 amps.  All the
shunt field does is make flux - simple coil.

- The Armature has windings that basically connect each segment in
series all the way around and each coil makes a 90 degree mechanical
loop to the next segment.

Torque = Force x Radius
Force = Field Flux x Armature core length x Armature amps

  So, if you apply field to the motor it won't turn.  But, if you rotate
the armature by external means, you are moving conductors thru a
magnetic field and it will produce voltage (remember the old "C" magnet
and wire experiment in school?) proportional to RPM and field flux
(that's how a generator works).

  If you have no field and apply voltage to the armature, the armature
windings in series are very low resistance and it will cause high
current to flow (like when the meter pegs and the tractor doesn't move
if the field relay is open, fields are open or grounded).

  But, if you have field flux and apply armature voltage (current), the
motor will now turn due to one of those "dead guy right hand rules"
(Faraday's Law - flux moving perpendicular to current in a wire creates
force on the wire).  The force is exerted on the conductors under the
main poles and since they are held in the core and supported by
bearings, the armature rotates.  Force creates Torque by above equations
which is what a DC motor produces.  Because conductors are passing under
the poles, they produce a voltage in them and make voltage internally
(called counter electro motive force (CEMF)) which is actually under the
brushes in the rotor.
  CEMF = Field Flux x Armature core length x Velocity(RPM)

   If you have the motor running along at 36 volts and you could measure
voltage between a brush and the commutator, it will increase from 0 to
36 volts as you reach the other brush 180 degrees away.

   So, with field flux, CEMF is produced basically with field amps and
RPM.  If you rearrange this relationship, RPM is proportional to
Armature Volts/Field Amps as CEMF is just less than applied armature
volts.  When started, a DC motor has full field amps so speed is
proportional to armature volts which is why the motor is stepped thru
the batteries by the relays or increasing voltage by a drive.  Once at
full armature voltage, reducing the field amps allows the speed to
increase BUT you lose Torque as this also depends on field flux (amps).
You will notice the ammeter increase too if you have higher load.

  With 466 turns of shunt field times 1.8 amps, you have 839 ampere
turns.  A pure series motor has no separate field and needs armature
amps to power it as it is in series with it.  This could be made into a
series motor with only about 33 turns of heavy wire but you lose a lot
of speed control as you would have to run at full load or the motor
would overspeed.  Series motors are great for high starting torque (no
flux means lots of amps until it rotates - like a starter motor in a
car).  You get good speed control ability with a shunt motor which is
why it is used here.

  Unfortunately, at higher loads, current in the armature conductors
make their own flux (dead guy rule) and it subtracts from the main field
flux and the motor speed increases (called armature reaction)..... so, a
stabilized shunt winding is used here to add 1 turn around the main
field of heavy wire to carry armature current and help boost the main
field flux (cumulative).  That helps keep the speed down and more
consistent with load.  If many turns were used it would be called a
compound motor since it needs more help - like the tiller and snowblower
motors have because of sudden high loads.  More flux, more torque.

  When you reverse rotation, you need to change the polarity of the
armature to field...... but if the series field is in there (stabilized
or compound) it needs to be reversed too or it will subtract from the
main field flux and the motor speed will really increase with load.  For
the ET's, the light series will subtract somewhat but that's why it runs
a little faster in reverse and no field weakening is needed.  All the
field weakening does is decrease field amps which allows the motor to
run faster.  In theory, if you remove the field the motor would run away
but connected load and losses keep that from happening.

  Torque is also armature diameter squared times length.  Larger
diameter makes tons more torque but speed is limited to keep things
together (and for commutation).  But, the E20 motor is 1.2HP and same
winding so it is just a little longer than the E15.

  Voltage equation for a motor      Incoming Armature volts = CEMF + IR
+ BD
     CEMF is voltage produced by armature rotation under the brushes
     IR is armature amps x resistance
     BD is brush drop less than 2 volts
  If you apply 36 volts to a motor, look at the equation   36=30 + 4 + 2
.... if you load the motor, the speed goes down (CEMF goes down as it
depends upon speed) so if 36 volts is still coming in, it has to "draw"
amps from IR to make up the equation.  Likewise if you go down a hill,
the motor speeds up, CEMF goes up, and IR has to "go negative" or
generate amps to maintain 36 volts in the equation.  This is how the
motor "motors" or "generates" basically automatically.

  One of the blogs asked about making it a generator.  Any series field
makes that "not good", but if you apply field and turn the shaft you
will generate voltage.  To charge a battery, you must be 38 volts and
higher and depending upon battery state, it may take some torque to do
that.  Otherwise it will motor.  That's why voltage regulators are used
to keep the output voltage high enough to keep things stable.

  What about the other set of coils in the motor?  As the armature
rotates, individual coil polarity has to be reversed so it has the
correct polarity as it goes under the pole flux to keep helping product
force and torque.  The brushes do that by shorting out a coil to allow
the field to immediately induce the opposite polarity of voltage in it
as it sweeps by (that's why brushes span 2 or more segments).  Shorting
out current in a coil causes sparking. Commutating coils are inserted
between the main coils and they are connected in series with the
armature so the same amps flow thru them and they create an opposite
voltage in the coil to negate (or cancel) the voltage that was in the
coil just before it needs to be shorted out.  Those coils have 5 turns
of heavy wire to carry armature amps.

In a nutshell: Main field makes flux and when armature voltage is
applied rotation occurs and torque is produced by field flux and
armature amps.

  Hope the explanation is not too confusing but it gets longer trying to
simplify it.

...Walt