The magnetic fields pro-
duced currents flowing through these out-of-phase windings create the
rotating field and turning force required start the motor. 10. motors have
two windings which are electrically separated about 90°.19(a), which gives 90° phase difference between the cur-
rents the start and run windings;
● designing the start winding have high resistance and the run
winding high inductance, once again creating 90° phase shift
between the currents each winding, shown Fig. supply
Centrifugal switch
Rotor
Start
winding
Run winding
(b)
. Once rotation
is established, the pulsating field the run winding sufficient main-
tain rotation and the start winding disconnected centrifugal switch
which operates when the motor has reached about 80% the full load
speed.c.
A cage rotor used single-phase a. supply
Centrifugal switch
Rotor
Start
winding
Run winding
(a)
a.c.19
Circuit diagram of: (a) capacitor split-phase motors and (b) resistance split-phase motors.
a. Reversal rotation may achieved reversing the connec-
tions the start run windings, but not both. Therefore, single-phase a.Alternating current theory and electrical machines
211
require rotating field start.c. 10.
When the motor first switched on, the centrifugal switch closed and
the magnetic fields from the two coils produce the turning force required
to run the rotor full speed. The two wind-
ings are known the start and run windings. Because both windings carry currents
which are out phase with each other, the motor known ‘split-
phase’ motor. When the motor reaches about 80% full
speed, the centrifugal switch clicks open and the machine continues run
on the magnetic flux created the run winding only. The phase displacement between the currents the wind-
ings achieved one two ways:
● connecting capacitor series with the start winding, shown
in Fig.c. 10.16.
FIGURE 10.19(b). motors, the turning force being
produced the way described previously for three-phase induction
motors and shown Fig.
Split-phase motors are constant speed machines with low starting torque
and are used light loads such fans, pumps, refrigerators and washing
machines
