Motor Electro-motive Force.
The counter-electro-motive force of a motor. (F. J. Sprague.)
A motor rotates in virtue of the pull of the field magnet upon the poles
of the core of its armature. In responding to this pull the windings of
the armature cuts lines of force and hence generates a
counter-electro-motive force, for which the above term was suggested.
Motor-Generator.
A combined motor and generator used to lower the potential difference in
a portion of a circuit, e. g., that part within a building.
A motor-generator is a dynamo whose armature carries two commutators,
with two separate windings, one of fine wire of many turns, the other of
coarse wire of few turns. If the potential of the system is to be
lowered, the main current is passed through the fine winding. This
causes the armature to turn motor-fashion, and a potential difference is
generated by the rotation of the large coils in the field. This
potential difference is comparatively low and by properly proportioning
the windings may be lowered to as great a degree as required.
The same apparatus may be inverted so as to raise potential difference.
It acts for continuous current systems as the induction coil transformer
does for alternating current systems.
Synonym--Continuous Current Transformer.
Motor, Multiphase.
A motor driven by multiphase currents. It is arranged in general terms
for distribution of the multi phase currents in coils symmetrically
arranged around the circle of the field. These coils are wound on cores
of soft iron. A rotating field is thus produced, and a permanent magnet
or a polarized armature pivoted in such a field will rotate with the
field, its poles following the poles of the rotatory field.
The cut, Fig. 245, illustrates the principles of action of a four phase
current motor, connected to a four phase current dynamo or generator.
The generator is shown on the left hand of the cut and the motor on the
right hand. In the generator the armature N S is supposed to be turned
by power in the direction shown by the arrow. Each one of the pair of
coils is wound in the reverse sense of the one opposite to it, and the
two are connected in series with each other, and with a corresponding
pair in the motor. The connection can be readily traced by the letters A
A', a a' for one set of coils and B B' b b' for the other set.