SINGLE-PHASE COMMUTATOR MOTORS 377

Switch 1 closed gives the inductively compensated series
motor.

Switch 2 closed gives the inductively compensated series
motor with secondary excitation, or inverted repulsion motor.

Switch 3 closed gives the repulsion motor with primary
excitation.

Switches 4 to 7 give the different speed steps of the series re-
pulsion motor with primary excitation.

Opening the connection at x and closing at y (as shown in
dotted line), the steps 3 to 7 give respectively the repulsion motor
with secondary excitation and the successive steps of the series
repulsion motor with armature excitation.

Still further combinations can be. produced in this manner, as
for instance, in Fig. 181, by closing 2 and 4, but leaving 0 open,
the field, F, is connected across a constant-potential supply, in
series with resistance, E, while the armature also receives con-
stant voltage, and the motor then approaches a finite speed, that
is, has shunt motor characteristic, and in starting, the main
field, F, and the quadrature field, AC, are displaced in phase, so
give a rotating or polyphase field (unsymmetrical).

To discuss all these motor types with their in some instances
very interesting characteristics obviously is not feasible. In
general, they can all be classified under series motor, repulsion
motor, shunt motor, and polyphase induction motor, and com-
binations thereof.

IX. Other Commutator Motor

210. Single-phase commutator motors have been developed as
varyirig-speed motors for railway service. In other directions
commutators have been applied to alternating-current motors
and such motors developed:

(a) For limited speed, or of the shunt-motor type, that is,
motors of similar characteristic as the single-phase railway
motor, except that the speed does not indefinitely increase with
decreasing load but approaches a finite no-load value. Several
types of such motors have been developed, as stationary motors
for elevators, variable-speed machinery, etc., usually of the
single-phase type.

By impressing constant voltage upon the field the magnetic
field flux is constant, and the speed thus reaches a finite limiting
value at which the e.m.f. of rotation of the armature through