344

ELECTRICAL APPARATUS

PIG. 165.—Type of alternating-cur-
rent commutating motor.

requiring two sets of brushes, but the advantage that their
power-factor can be controlled and above synchronism even
leading current produced. Fig. 165 shows diagrammatically such
a motor, as designed by Winter-Eichberg-Latour, the so-called
compensated repulsion motor. In this case, compensated means
compensated for power-factor.
The voltage which can be used in the motor armature is limited
by the commutator: the voltage per commutator segment is
limited by the problem of sparkless commutation, the number
of commutator segments from
brush to brush is limited by
mechanical consideration of
commutator speed and width
of segments. In those motor
types in which the supply cur-
rent traverses the armature, the
supply voltage is thus limited
to values even lower than in
the direct-current motor, while
in the repulsion motor (4 and
,5), in which the armature is the
secondary circuit, the armature voltage is independent of • the
supply voltage, so can be chosen to suit the requirements of
commutation, while the motor can be built for any supply
voltage for which the stator can economically be insulated.
Alternating-current motors as well as direct-current series
motors can be controlled by series parallel connection of two or
more motors. Further control, as in starting, with direct-current
motors is carried out by rheostat, while with alternating-current
motors potential control, that is, a change of supply voltage by
transformer or .autotransformer, offers a more efficient method
of control. By changing from one motor type to another motor
type, potential control can be used in alternating-current motors
without any change of supply voltage, by appropriately choosing
the ratio of turns of primary and secondary circuit. For in-
stance, with an armature wound for half the voltage and thus
twice the current as the compensating winding (ratio of turns
~~2 = 2j, a change of connection from type 3 to type 2, or from
• type 5 to type 4, results in doubling the field current and there-