CONCLUSION

475

Most of the less-known and less-used types of apparatus have
been discussed in the preceding, and a comprehensive list of
them is given in Chapter XXIII, together with their definitions
and short characterization.
While electric machines are generally divided into induction
machines, synchronous machines and commutating machines,
this classification becomes difficult in considering all known
apparatus, as many of them fall in two or even all three classes,
or are intermediate, or their inclusion in one class depends on the
particular definition of this class.
Induction machines consist of a magnetic circuit inductively
related, that is, interlinked with two sets of electric circuits,
which are movable with regards to each other.
They thus differ from transformers or in general stationary
induction apparatus, in that the electric circuits of the latter are
stationary with regards to each other and to the magnetic circuit.
In the induction machines, the mechanical work thus is pro-
duced—or consumed, in generators—by a disappearance or
appearance of electrical energy in the transformation between
the two sets of electric circuits, which are movable with regards
to each other, and of which one may be called the primary cir-
cuit, the other the secondary circuit. The magnetic field of the
induction machine inherently must be an alternating field
(usually a polyphase rotating field) excited by alternating
currents.
Synchronous machines are machines in which the frequency of
rotation has a fixed and rigid relation to the frequency of the
supply voltage.
Usually the frequency of rotation is the same as the frequency
of the supply voltage: in the standard synchronous machine,
with direct-current field excitation.
The two frequencies, however, may be different: in the double
synchronous generator, the frequency of rotation is twice the
frequency of alternation; in the synchronous-induction machine,
it is a definite percentage thereof; so also it is in the induction*
machine concatenated to a synchronous machine, etc.
Commutating machines are machines having a distributed
armature winding connected to a segmental commutator.
They may be direct-current or alternating-current machines.
Unipolar machines are machines in which the induction is
produced by the constant rotation of the conductor through a
constant and continuous magnetic field.