, 4

i)

378

ELECTRICAL APPARATUS

the constant field flux consumes the impressed voltage of the
armature. By changing the voltage supply to the field different
speeds can be produced, that is, an adjustable-speed motor.
The main problem in the design of such motors is to get the
field excitation in phase with the armature current and thus pro-
duce a good power-factor.
(b) Adjustable-speed polyphase induction motors. In the
secondary of the polyphase induction motor an e.m.f. is gener-
ated which, at constant impressed e.m.f. and therefore approxi-
mately constant flux, is proportional to the slip from synchron-
ism. With short-circuited secondary the motor closely ap-
proaches synchronism. Inserting resistance into the secondary
reduces the speed by the voltage consumed in the secondary.
As this is proportional to the current and thus to the load, the
speed control of the polyphase induction motor by resistance in
the secondary gives a speed which varies with the load, just as
the speed control of a direct-current motor by resistance in the
armature circuit; hence, the speed is not constant, and the opera-
tion at lower speeds inefficient. Inserting, however, a constant
voltage into the secondary of the induction motor the speed is
decreased if this voltage is in opposition, and is increased if this
voltage is in the same direction as the secondary generated e.m.f.,
and in this manner a speed control can be produced. If c =
voltage inserted into the secondary, as fraction of the voltage
which would be induced in it at full frequency by the rotating
field, then the polyphase induction motor approaches at no-load
and runs at load near to the speed (1 — c) or (1 + c) times syn-
chronism, depending upon the direction of the inserted voltage.
Such a voltage inserted into the induction-motor secondary
must, however, have the frequency of the motor secondary cur-
rents, that is, of slip, and therefore can be derived from the full-
frequency supply circuit only by a commutator revolving with
the secondary. If cf is the frequency of slip, then (1 — c)/ is
the frequency of rotation, and thus the frequency of commuta-
tion, and at frequency, /, impressed upon the commutator the
effective frequency of the commutated current is / — (1 — c) / =
cf, or the frequency of slip, as required.
Thus the commutator affords a means of inserting voltage
into the secondary of induction motors and thus varying its
speed.
However, while these commutated currents in their resultant