Re:Re: (erielack) Re: Transition

["Janet & Randy Brown" <jananran_@_mymailstation.com>]

Basic electromechanical theory (this will be hard to do briefly, but I'll try):

Moving a wire through a magnetic field generates electricity in the wire.  Moving electricity through a wire generates a magnetic field around the wire.  Winding the wire into a coil intensifies the effect.

Now, arrange one wire coil on a shaft so it can turn; this is the armature.  Form the other coil so that it is stationary and fixed around the armature; this is the field.  Run electricity through the field and it will create a magnetic . . . well, field. This is excitation.

Attach a diesel engine to the end of the armature shaft and turn it; electrcity happens, and this is a diesel-electric generator.  The degree of excitation and the speed of rotation combine to determine how much electricity it generates.  Arrange brushes to rub on a part of the armature which is wired to the coil; this is the commutator and draws the electricity off so we can use it.

Do it again, but this time put a gear on the end of the armature so you can apply a load.  Excite the fixed coil to make a magnetic field, then run that electricity through the armature to create another magnetic field.  Arrange the brushes so the two magnetic fields are always in  opposite polarity.  Opposite poles attract; like poles repel.  The armature rotates -- it's a motor!  The wound field is connected in series to the armature -- it's a wound-field, series-connected motor.

Gear the armature to wheels -- it's a vehicle.  On track, it's a locomotive.

However, there is a limitation.  As the motor speeds up, it finds that it is a coil rotating in a magnetic field and says, "OK!  I'm a generator!" And it starts producing electricity.  Even as it is using electricity to turn itself, it is trying to push electricity back into its only connection, the generator feeding it, creating a load.  This is Back Electromotive Force, or Back EMF.

But, now that the motor is spinning faster, it doesn't need all that voltage, which introduces the SHUNT.  Sending some of the excitation around the fixed field weakens it some, leaving enough to turn the motor while providing less incentive for the motor to produce back EMF.

As Mike and others (and I) have said, connecting two motors in series gives each motor full current at half voltage.  In parallel, each gets half the current at full voltage -- but they are still the same series-connected wound-field motors.  That never changes.

Transition is the procedure of changing them between series and parallel TO EACH OTHER and, at some time, adding field shunting -- all to balance the speed, load and capacity of the motors and generator.

The full process of transition has been explained earlier on this list.  Swichers usually didn't have it; early passenger locomotives and the FT used manual transition.  While it worked in the passenger units, probably because the demands didn't change as often or as drastically as in freight service, freight operation begged for and got automatic transition.  Once it was there, it migrated to the passenger business.

Randy Brown
- --------------------------------------------------------------
Basic electricity. You have CURRENT and VOLTAGE. . .

In electricity, SHUNTING means something much
different, and I can't think of it being used in the
generator-tracion motor circuit.

      Mike Spinelli


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