The celing fan motor works on principle of single phaseinduction motor using capacitor.Working of capacitor start motor: The stator consists ofthe main winding and a starting winding (auxiliary). Thestarting winding is connected in parallel with the mainwinding and is placed physically at right angles to it. A90-degree electrical phase difference between the twowindings is obtained by connecting the auxiliary winding inseries with a capacitor and starting switch. When the motoris first energized, the starting switch is closed. Thisplaces the capacitor in series with the auxiliary winding.The capacitor is of such value that the auxiliary circuitis effectively a resistive-capacitive circuit (referred toas capacitive reactance and expressed as XC). In thiscircuit the current leads the line voltage by about 45�(because XC about equals R). The main winding has enoughresistance-inductance (referred to as inductive reactanceand expressed as XL) to cause the current to lag the linevoltage by about 45�(because XL about equals R). The

currents in each winding are therefore 90�out of phase -

so are the magnetic fields that are generated. The effectis that the two windings act like a two-phase stator andproduce the rotating field required to start the motor.When nearly full speed is obtained, a centrifugal device(the starting switch) cuts out the starting winding. Themotor then runs as a plain single-phase induction motor.Since the auxiliary winding is only a light winding, themotor does not develop sufficient torque to start heavyloads. Split-phase motors, therefore, come only in smallsizes.

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16 August 201415:32

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