Dynamo, Alternating Current.
A dynamo-electric machine for producing an alternating current; an
alternator. They are classified by S. P. Thompson into three classes--I.
Those with stationary field-magnet and rotating armature. II. Those with
rotating field magnet and stationary armature. III. Those with both
field magnet part and armature part stationary, the amount of magnetic
induction from the latter through the former being caused to vary or
alternate in direction by the revolution of appropriate pieces of iron,
called inductors. Another division rests on whether they give one simple
alternating current, a two phase current, or whether they give multi
phase currents. (See Current, Alternating--Currents, Multiphase.)
A great many kinds of alternators have been constructed. Only an outline
of the general theory can be given here. They are generally multipolar,
with north and south poles alternating around the field. The armature
coils, equal in number in simple current machines, to the poles, are
wound in opposite senses, so that the current shall be in one direction,
though in opposite senses, in all of them at anyone time. As the
armature rotates the coils are all approaching their poles at one time
and a current in one sense is induced in every second coil, and one in
the other sense in the other coils. They are all in continuous circuit
with two open terminals, each connected to its own insulated connecting
ring on the shaft. As the coils pass the poles and begin to recede from
them the direction changes, and the current goes in the other direction
until the next poles are reached and passed. Thus there are as many
changes of direction of current per rotation as there are coils in the
armature or poles in the field.
Fig. 134. ALTERNATING CURRENT DYNAMO WITH
SEPARATE EXCITER MOUNTED ON MAIN SHAFT.