SINGLE-PHASE INDUCTION MOTOR 97
as shunt motor, sparking is still more severe. Thus this method
is used to a limited extent only.
(c) By shifting the axis of armature or secondary polarization
against the axis of inducing magnetism.
This requires a secondary system, which is electrically un-
symmetrical with regards to the primary system, and thus, since
the secondary is movable with regards to the primary, requires
means of changing the secondary circuit, that is, commutator
brushes short-circuiting secondary coils in the position of effective
torque, and open-circuiting them in the position of opposing torque.
Thus this method leads to the various forms of repulsion
motors, of series and of shunt characteristic.
It has the serious objection of requiring a commutator and a
corresponding armature winding; though the limitation is not
quite as great as with the series or shunt motor, since in the re-
pulsion motors the armature current is an induced secondary
current, and the armature thus independent of the primary
system regards current, voltage and number of turns.
(d) By shifting the axis of magnetism, that is producing a
magnetic flux displaced in phase and in position from that in-
ducing the armature currents, in other words, a quadrature
magnetic flux, such as at speed is being produced by the rotation.
This method does not impose any limitation on stator and
rotor design, requires no commutator and thus is the method
almost universally employed.
It thus may be considered somewhat more in detail.
The infinite variety of arrangements proposed for producing
a quadrature or starting flux can be grouped into three classes:
A. Phase-splitting Devices.—The primary system of the single-
phase induction motor is composed of two or more circuits
displaced from each - other in position around the armature
circumference, and combined with impedances of different in-
ductance factors so as to produce a phase displacement between
them.
The motor circuits may be connected in series, and shunted
by the impedance, or they may be connected in shunt with each
other, but in series with their respective impedance, or they
may be connected with each other by transformation, etc.
B. Inductive Devices.—The motor is excited by two or more
circuits which are in inductive relation with each other so as to
produce a phase displacement.
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