- Split
phase induction motor has two windings its stator, a main winding and an
auxiliary winding or starting winding. The starting winding is located 90o
electrical from the main winding and operates for a short time when the
motor starts up.
- The
two windings are so designed that the starting winding has a high
resistance and relatively small reactance while the main winding has
relatively low resistance and large reactance. This can be done by having
main winding of large diameter and number of turns while starting winding
of a fine wire of a small number of turns. The currents taken by these two
windings are out of phase (25o to 30o). Schematic
diagram and phasor diagram of split phase induction motor are shown in
figure
- Operation: When the stator windings are
energised from a single phase supply, the main winding carries current Im
and the starting winding carries current Is. These two currents are out of
phase (25o to 30o) and produce a weak rotating field
which starts the motor. The starting torque is given as,
Ts = K Im Is
Sin Ф
Where
K = constant whose magnitude depends
upon the design of the motor
When
the motor reaches about 75% of synchronous speed, then the centrifugal switch
or relay opens the circuit of the starting winding. The motor then operates as
a single-phase induction motor and continues to accelerate till it reaches the
normal speed. The normal speed is below the synchronous speed and depends upon
the load on the motor.
Characteristics
of Split-Phase I / M:
- The
starting torque is 1.5 to 2 times the full load torque and the starting
current is 6 to 8 times the full-load current.
- The
power ratings of such motors are from 60 Watts to 250 Watts.
- They
have efficiency of 50 – 70 % and overload of 50%.
- As
they have moderate starting torque and low starting current, so they are
commonly used to drive fans, blowers, centrifugal pumps, domestic
refrigerators, washing machines, oil burners, small machine tools, etc.
- These
motors are essentially constant-speed motors. The speed variation is 2 to
5 % from no-load to full-load.
- The
speed range of such motors is from 2875 to 700 r.p.m.
- Such a
motor can be operated with a power factor of 0.55 – 0.65.
- The
percent slip for such motors is about 4 – 6 percent.
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Hi can I know why centrifugal switch need to be open at 75-80% of synchronous speed?
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