Basic understanding as to how split-phase motors start

1. Split-Phase Motors
All single-phase motors work basically the
same. It is the way the are started that differs. In
order for a single-phase induction motor to work
the rotor has to be subjected to a rotating
magnetic field. This is produced in the stator
coils of the split-phase motor by having start
windings positioned with their axes 90° electrical
in relation to the run windings.

3. How they work
The split-phase machine has two windings from a single
phase arranged in the stator. One is the main winding and
other is starting winding, which is used only for starting
purpose. The run winding has characteristics of low
resistance but high reactance. The starting winding has
high resistance but low reactance. Accordingly, the start
windings are first to become magnetized when the
power is applied. The current flow through the start winding
begins after power is applied to the motor by 20 degrees or
so. The current in the run windings begins even later by
about 50 degrees. This timing creates a phase differential
which creates the necessary rotating magnetic field. When
the run windings become strongly magnetized, the start
windings begin to fade out. As a result the magnetic field
shifts to a new position creating a rotating magnetic field.

4. The start current lags less than the
run current.

5. The single phase induction
motor begins to work
like a two phase motor!

7. These are a net magnetic field

8. After the starter and main winding has developed
a rotating magnetic field, the rotor starts to rotate
and for further rotation of the rotor, the starter
winding is not required. So a switch is provided
that can be opened to isolate the starter winding
from the motor circuit. Usually the switch may be
of centrifugal type and thus after the motor
attaining 70 to 80% of the full load speed, the
centrifugal switch opens up, isolating the starter
winding. The centrifugal switch is connected in
series with the starter winding and it is located
inside the motor.

9. Simply constructed split phase motors are among the least
expensive.They're widely used on easy starting loads of 1/3
horsepower or less.Washing machines, tool grinders and small fans
and blowers are among the applications that use these motors.