INDUCTION MOTOR

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good as the direct-current commutating machine, and thus can
be used to insert low-frequency voltage into the induction-motor
secondary.
With series excitation, the voltage of the commutating machine
is approximately proportional to the secondary current, and the
speed characteristic of the induction motor remains essentially
the same: a speed decreasing from synchronism at no-load, by a
slip, s, which increases with the load.
With shunt excitation, the voltage of the commutating machine
is approximately constant, and the concatenated couple thus
tends toward a speed differing from synchronism.
In either case, however, the slip, s, is not constant and independ-
ent of the load, and the motor couple not synchronous, as when
using a synchronous machine as second motor, but the motor
couple is asynchronous, decreasing in speed with increase of load.
The phase relation of the voltage produced by the commutating
machine, with regards to the secondary current which traverses
it, depends on the relation of the commutator brush position
with regards to the field excitation of the respective phases, and
thereby can be made anything between 0 and 2r, that is, the
voltage inserted by the commutating machine can be energy
voltage in phase—reducing the speed—or in opposition to the
induction-motor induced voltage—increasing the speed; or it
may be a reactive voltage, lagging and thereby supplying the
induction-motor magnetizing current, or leading and thereby
still further lowering the power-factor. Or the commutating
machine voltage may be partly in phase—modifying the speed—
and partly in quadrature—modifying the power-factor.
Thus the commutating machine in the induction-motor
secondary can be used for power-factor control or for speed
control or for both.
It is interesting to note that the use of the commutating ma-
chine in the induction-motor secondary gives two independent
variables: the value of the voltage, and its phase relation to the
current of its circuit, and the motor couple thus has two degrees
of freedom. With the use of a synchronous machine in the
induction-motor secondary this is not the case; only the voltage
of the synchronous machine can be controlled, but its phase
adjusts itself to the phase relation of the secondary circuit, and
the synchronous-motor couple thus has only one degree of free-
dom. The reason is: with a synchronous motor concatenated to