92 ELECTRICAL APPARATUS
no speed changing effects, and this leaves only one degree of
freedom.
The foremost disadvantage of this method of secondary excita-
tion of an induction motor, by a commutated winding in addi-
tion to the short-circuited squirrel cage, is that secondary excita-
tion is advantageous for power-factor control especially in
slow-speed motors of very many poles, and in such, the commuta-
tor becomes very undesirable, due to the large number of poles.
With such motors, it therefore is preferable to separate the
commutator, placing it on a small commutating machine of a
few poles, and concatenating this with the induction motor. In
motors of only a small number of poles, in which a commutator
would be less objectionable, power-factor compensation is rarely
needed. This is the foremost reason that this type of motor
(the Heyland motor) has found no greater application.