Motors built on the dynamo model may be series wound, shunt wound, or
compound wound, or of the magneto type, in the latter case having a
fixed field irrespective of any current sent through them. The field may
be produced by an electro-magnet separately excited and unaffected by
the current sent through the motor.
A current passed through a magneto or motor with separately excited
field will turn it in the direction opposite to that required to produce
the same current from it were it worked as a generator.
A current passed through a series wound motor acts exactly as above.
Both these facts follow from Lenz's law, q. v.
A current passed through a shunt wound motor acts oppositely to the
above. The direction of rotation is the same as that required to produce
a current of the same direction. This is because the field being in
parallel with the armature the motor current goes through the magnet
coils in the direction the reverse of that of the current produced in
the armature when it is used as a dynamo. Hence this also carries out
Lenz's law.
The compound wound motor acts one way or the other according as its
shunt or series winding preponderates. The two may exactly balance each
other, when there will be no motion at all. The series connections of a
compound wound dynamo should therefore be reversed, making both series
and shunt work in unison, if the dynamo is to be used as a motor.
The general principles of the electric motor of the dynamo, or
continuous rotation type, can only be outlined here. The current passing
through the field magnets polarizes them and creates a field. Entering
the armature by the brushes and commutators it polarizes its core, but
in such a way that the north pole is away from the south pole of the
field magnet, and the same for the south pole. Hence the armature
rotates. As it does this the brushes connect with other commutator
sections, and the poles of the armature are shifted back. This action
continues indefinitely.
Another class of motors is of the reciprocating type. These are now very
little used. (See Motor, Reciprocating.)
One valuable feature of continuous rotation electric motors is the fact
that they absorb energy, to a great extent proportional in amount to the
work they have to do. The rotation of the armature in the field of the
motor involves the cutting of lines of force by its coils. This
generates an electro-motive force contrary in direction to that
producing the actuating current. The more rapid the rotation the greater
is this counter-electro-motive force. The motor armature naturally
revolves faster with diminished resistance to the motion of the
armature. This increases the counter-electromotive force, so that less
energy is absorbed. When the motor is called on to do work, the armature
rotates more slowly, and the counter-electro-motive force diminishes, so
that the machine absorbs more energy. (See Jacobi's Law.)