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54 ELECTRICAL APPARATUS
primary, that is, which receives electric power and converts it
into mechanical power, and the primary or stator of the induc-
tion machine thus corresponds to the armature of. the synchro-
nous or commutating machine. In the secondary or rotor of the
induction machine, low-frequency currents—of the frequency
of slip—are induced by the primary, but the magnetic field flux
is produced by the exciting current which traverses the primary
or armature or stator. Thus the induction machine may be
considered as a machine in which the magnetic field is produced
by the armature reaction, and corresponds to a synchronous
machine, in which the field coils are short-circuited and the
field produced by armature reaction by lagging currents in the
armature.
As the rotor or secondary of the induction machine corresponds
structurally to the field of the synchronous or commutating
machine, field excitation thus can be given to the induction
machine by passing a current through the rotor or secondary and
thereby more or less relieving the primary of its function of giv-
ing the field excitation.
Thus in a slow-speed induction motor, of very high exciting
current and correspondingly poor constants, by passing an
exciting current of suitable value through the rotor or secondary,
the primary can be made non-inductive, or even leading current
produced, or—with a lesser exciting current in the rotor—at
least the power-factor increased.
Various such methods of secondary excitation have been pro-
posed, and to some extent used.
1. Passing a direct current through the rotor for excitation.
In this case, as the frequency of the secondary currents is the
frequency of slip, with a direct current, the frequency is zero,
that is, the motor becomes a synchronous motor.
2. Excitation through commutator, by the alternating supply
current, either in shunt or in series to the armature.
At the supply frequency, /, and slip, s, the frequency of rotation
and thus of commutation is (1 — s) /, and the full frequency cur-
rents supplied to the commutator thus give in the rotor the
effective frequency,/ — (1 — s) / = sf, that is, the frequency of
slip, thus are suitable as exciting currents.
3. Concatenation with a synchronous motor.
If a low-frequency synchronous machine is mounted on the
induction-motor shaft, and its armature connected into the indue-