Single Phase Induction Motor


Single phase induction motor

A single-phase induction motor comprises a single-phase distributed winding on the stator and cage winding on rotor

There are two important methods of analyzing this motor, viz. cross-field theory and rotating field theory

The schematic diagram of a single-phase induction motor with one stator winding and a squirrel-cage rotor. The winding is distributed in space so that the space fundamental of mmf is the most dominant component of the actual mmf distribution.
When the winding carries a sinusoidal current, it produces a sinusoidally space-distributed mmf whose peak value pulsates with time. As seen from the axis of the


winding, the mmf at any angle Φ is
                                    F = Fpeak cos Φ    

Where Φ  is the angle measured from the winding axis. Now
                        Fpeak = Fmax cos wt

 so that the mmf has both space and time distribution expressed as
                     F = Fmax cos Φ cos wt

This equation can be manipulated into the form

F = .5*Fmax cos(Φ – wt) + .5Fmax cos (Φ + wt)

This eqn. tells us that a pulsating single-phase field can be considered as superposition of two rotating fields rotating at synchronous speed (w = 2p f ) in opposite directions:

F=0.5Fmax cos (Φ – wt); the forward rotating field

F=0.5Fmax cos (Φ+ wt); the backward rotating field

Both these fields have an amplitude equal to (1/2) Fmax where Fmax
is the maximum value of the pulsating mmf along the axis of the
winding.

It is therefore concluded that a stationary pulsating magnetic field can be resolved into two rotating magnetic field, both of equal magnitude and moving at a synchronous speed in opposite direction at the same frequency as the stationary magnetic field alternates. When the rotor is stationary, the induced voltages are equal and opposite. Consequently, two torques are equal and opposite. 

Hence at the standstill the net torque are equal and opposite. Hence at standstill the net torque is zero. In other word it has zero starting torque.
However if the rotor id started by some auxiliary means in either directions the torque due to rotating field in the directions of initial rotation will be more and developed a net torque in positive direction. The motor will keep running in initial direction.

Comments

  1. UnknownJuly 14, 2019 at 11:18 AM

    You are doing good sir

    ReplyDelete
  2. UnknownJuly 14, 2019 at 11:38 AM

    Sir please start measuring instruments subject , your concept are so helpful

    ReplyDelete

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