4-13
phase difference between the two windings is obtained by connecting the auxiliary winding in series with
a capacitor and starting switch. When the motor is first energized, the starting switch is closed. This
places the capacitor in series with the auxiliary winding. The capacitor is of such value that the auxiliary
circuit is effectively a resistive-capacitive circuit (referred to as capacitive reactance and expressed as
XC). In this circuit the current leads the line voltage by about 45º (because X
C
about equals R). The main
winding has enough resistance-inductance (referred to as inductive reactance and expressed as XL) to
cause the current to lag the line voltage by about 45º (because X
L
about equals R). The currents in each
winding are therefore 90º out of phase - so are the magnetic fields that are generated. The effect is that
the two windings act like a two-phase stator and produce the rotating field required to start the motor.
Figure 4-11.—Capacitor-start, ac induction motor.
When nearly full speed is obtained, a centrifugal device (the starting switch) cuts out the starting
winding. The motor then runs as a plain single-phase induction motor. Since the auxiliary winding is only
a light winding, the motor does not develop sufficient torque to start heavy loads. Split-phase motors,
therefore, come only in small sizes.
RESISTANCE-START.—Another type of split-phase induction motor is the resistance-start motor.
This motor also has a starting winding (shown in fig. 4-12) in addition to the main winding. It is switched
in and out of the circuit just as it was in the capacitor-start motor. The starting winding is positioned at
right angles to the main winding. The electrical phase shift between the currents in the two windings is
obtained by making the impedance of the windings unequal. The main winding has a high inductance and
a low resistance. The current, therefore, lags the voltage by a large angle. The starting winding is
designed to have a fairly low inductance and a high resistance. Here the current lags the voltage by a
smaller angle. For example, suppose the current in the main winding lags the voltage by 70º. The current
in the auxiliary winding lags the voltage by 40º. The currents are, therefore, out of phase by 30º. The
magnetic fields are out of phase by the same amount. Although the ideal angular phase difference is 90º
for maximum starting torque, the 30-degree phase difference still generates a rotating field. This supplies
enough torque to start the motor. When the motor comes up to speed, a speed-controlled switch
disconnects the starting winding from the line, and the motor continues to run as an induction motor. The
starting torque is not as great as it is in the capacitor-start.
