SINGLE-PHASE INDUCTION MOTOR 173
points of the commutator, C. These brushes, B, are arranged so
that they can be revolved.
With the brushes, B, at standstill on the stationary commutator,
C, the rotor, S, has no torque, and the current in the stator, P, is
the usual large standstill current of the induction motor. If now
the brushes, B, are revolved at synchronous speed, /, in the direc-
tion shown by the arrow, the rotor, S, again has no torque, but
the stator, P, carries only the small exciting current of the motor,
and the electrical conditions in the motor are the same, as would
be with stationary brushes, B, at synchronous speed of the rotor,
S. If now the brushes, B, are slowed down below synchronism,
/, to speed, /i, the rotor, 5, begins to turn, in reverse direction, as
shown by the arrow, at a speed, /2, and a torque corresponding
to the slip, s = / - (/i +/2).
Thus, if the load on the motor is such as to require the torque
given at the slip, s, this load is started and brought up to full
"speed, / — s, by speeding the brushes, B, up to or near synchronous
speed, and then allowing them gradually to come to rest: at brush
speed, fi — f — s, the rotor starts, and at decreasing, /i, accelr-
ates with the speed /2 = / — s — jfi, until, when the brushes
come to rest: /i = 0, the rotor speed is/2 = / — s.
As seen, the brushes revolve on the commutator only in start-
ing and at intermediate speeds, but are stationary at full speed.
If the brushes, B, are rotated at oversynchronous speed: /i>f,
the motor torque is reversed, and the rotor turns in the same
direction as the brushes. In general, it is:
/I +/3 + «=//
where
fi = brush speed,
/2 = motor speed,
s = slip required to give the desired torque,
/ = supply frequency.
102. An application of this, type of motor for starting larger
motors under power, by means of a small auxiliary motor, is
shown diagrammatically, in section, in Fig. 61.
PQ is the stationary primary or stator, SQ the revolving squirrel-
cage secondary of the power motor. The stator coils of P0
connect to the segments of the stationary commutator, 6Yo,
which receives the single-phase power current through the
brushes, B0.