374

ELECTRICAL APPARATUS

mutation can be allowed in repulsion motors at very low speeds
than in series motors, since in the former the period of poor
commutation lasts only a very short time. While, therefore,
series motors can not be satisfactorily operated without resistance
leads (or commutating poles), in repulsion motors resistance
leads are not necessary and not used, and the excessive current
density under the brushes in the moment of starting permitted,
as it lasts too short a time to cause damage to the commutator.
As the transformer field of the repulsion motor is approximately
constant, while the proper commutating field should decrease
with the square of the speed, above synchronism the transformer
field is too large for commutation, and at speeds considerably
above synchronism—50 per cent, and more—the repulsion motor
becomes inoperative because of excessive sparking. At syn-
chronism, the magnetic field of the repulsion motor is a rotating
field, like that of the polyphase induction motor.
Where, therefore, speeds far above synchronism are required,
the repulsion motor can not be used; but where synchronous
speed is not much exceeded the repulsion motor is preferred be-
cause of its superior commutation. Thus-when using a commu-
tator as auxiliary device for starting single-phase induction
motors the repulsion-motor type is used. For high frequencies,
as 60 cycles, where peripheral speed forbids synchronism being
greatly exceeded, the repulsion motor is the type to be considered.
Repulsion motors also may be built with primary and sec-
ondary excitation. The latter usually gives a better commuta-
tion, because of the lesser lag of the transformer flux, and there-
with a greater in-phase component, that is, greater reversing flux,
especially at high speeds. Secondary excitation, however, gives
a slightly lower power-factor.
A combination of the repulsion-motor and series-motor types
is the series repulsion motor, 6 and 7. In this only a part of
the supply voltage is impressed upon the compensating winding
and thus transformed to the armature, while the rest of the sup-
ply voltage is impressed directly upon the armature, just as in
the series motor. As result thereof the transformer flux of the
series repulsion motor is less than that of the repulsion motor,
in the same proportion in which the voltage impressed upon the
compensating winding-is less than the total supply voltage.
Such a motor, therefore, reaches equality of the transformer flux
with the commutating flux, and gives perfect commutation at a