Split-phase motors have two stator
windings, a main winding (also referred to as the run winding) which
we will refer to with the subscript 'main' and an auxiliary winding
(also referred to as the start winding) which we will refer to with
the subscript 'aux'.
As in a two-phase motor, the axes of
these windings are displaced 90 electrical degrees in space, and they
are connected as shown in Fig. 9.3a. The auxiliary winding has a
higher resistance-to-reactance ratio than the main winding, with the
result that the two currents will be out of phase, as indicated in
the phasor diagram of Fig. 9.3b, which is representative of
conditions at starting.
Since the auxiliary-winding current
iaux leads the main-winding current Imain, the stator field first
reaches a maximum along the axis of the auxiliary winding and then
somewhat later in time reaches a maximum along the axis of the main
winding.
The winding currents are equivalent to
unbalanced two-phase currents, and the motor is equivalent to an
unbalanced two-phase motor. The result is a rotating stator field
which causes the motor to start.
After the motor starts, the auxiliary
winding is disconnected, usually by means of a centrifugal switch
that operates at about 75 percent of synchronous speed. The simple
way to obtain the high resistance to-reactance ratio for the
auxiliary winding is to wind it with smaller wire than the main
winding, a permissible procedure because this winding operates only
during starting.
Its reactance can be reduced somewhat
by placing it in the tops of the slots. A typical torque-speed
characteristic for such a motor is shown in Fig. 9.3c.
Split-phase motors have moderate
starting torque with low starting current. Typical applications
include fans, blowers, centrifugal pumps, and office equipment.
Typical ratings are 50 to 500 watts; in this range they are the
lowest-cost motors available.
Figure 9.3 Split-phase motor: (a)
connections, (b) phasor diagram at starting, and (c) typical torque
speed characteristic.
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