i'j

|.',,'3jF

96 ELECTRICAL APPARATUS

In a quarter-phase motor:

F'=2F(1-.

Z'Q = ZQ,

(10)

Thus the characteristic constant, #', of the single-phase motor
is higher, that is, the motor inferior in its performance than the
polyphase motor; but the quarter-phase motor makes just as
good—or poor—a single-phase motor as the three-phase motor.
62. The calculation of the performance curves of the single-
phase motor from its constants, then, is the same as that of the
polyphase motor, except that:
In the expression of torque and of power, the term (1 — s)
is added, which results from the decreasing quadrature flux, and
it thus is:
Torque:
V = T (1 - s)
= (!-*) «i62. (11)
Power:
Pf = P (I - s)
= (1 - s)*aie*. (12)
However, these expressions are approximate only, as they
assume a variation of the quadrature flux proportional to the
speed.
63. As the single-phase induction motor is not inherently
self-starting, starting devices are required. Such are:
(a) Mechanical starting.
As in starting a single-phase induction motor it is not neces-
sary, as in a synchronous motor, to bring it up to full speed, but
the motor begins to develop appreciable torque already at low
speed, it is quite feasible to start small induction motors by hand,
by a pull on the belt, etc., especially at light-load and if of high-
resistance armature.
(&) By cbnverting the motor in starting into a shunt or series
motor.
' This has the great objection of requiring a commutator, and a
commutating-machine rotor winding instead of the common
induction-motor squirrel-cage winding. Also, as series motor,
the liability exists in the starting connection, of running away;