59 Collectable Model Trains: Motor Design: The Universal motor in operation
Description
This article is from the Model Trains FAQ, by Christopher D Coleman with numerous contributions by others.
59 Collectable Model Trains: Motor Design: The Universal motor in operation
Essentially all model engine motors are DC machines. The difference is
in the field. Traditional Lionel, Flyer and Marx locos use an electric
field coil. When DC is applied, the armature current goes one way and
the field current goes one way. It pushes the motor in a particular
direction.
If the terminals of a universal motor are reversed, the motor still
spins the same way. The current through BOTH the field and armature
are reversed so the field RELATIVE to current in the armature is the
same. The two must change RELATIVE to one another to reverse the
motor. This is why sequencers (E-Units) are used to reverse the field
current RELATIVE TO the armature current.
Now, AC current changes direction 60 times a second (50 in Europe). It
is just like reversing the terminals 60 times a second. When AC is
applied, the field current changes direction right in step with the
armature current so that they are always flowing in the same RELATIVE
direction. The force stays in the same direction.
These motors are conceptually designed for DC but will usually work
fine on AC too, as is the case in tinplate trains. It is, however,
common practice in the hobby to call them "AC" or universal to
indicate that they CAN run on AC, unlike "can" motors. I'm not trying
to change this, but rather show how it all really works.
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