Single
phase induction motor or fractional kilowatt motor is a motor built in a frame
having a continuous rating of smaller than 1KW.It is constructionally more or
less similar to a three phase induction motor with the difference that its
stator is provided with single phase winding.When single phase AC voltage is
supplied to the single phase stator winding,it will not produce a rotating magnetic
field like in three phase induction motor,rather it will produce an alternating
magnetic field which magnitude varies with respect to time,and its direction
changes by 180° at a particular instant at a particular time interval.
With
the connection of single phase AC supply to the single phase induction
motor,alternating current will flow in its stator winding and the polarity of
stator pole would alternatively be N and S.The field so produced will be
pulsating i.e. polarities will be alternating with flux rising and falling in
strength.The current induced in the rotor will tend to rotate in both
directions alternatively and thus the
rotor will be at standstill due to inertia.If a rotor is given a push by hand
or by another means in any direction,it will pick up speed and continue to
rotate in same direction developing operating torque,Thus,the single phase
induction motor is not self starting and requires special starting means.
fig: 1.b
Fig 1.a shows the
basic construction of single phase induction motor and Fig 1.b shows the nature
of air gap flux produced by single phase winding.
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