Why Magnetic Axis Shift Takes Place?
Now let us discuss why the axis shift occurs when current is passed through the winding and how the shading coil aids in producing the shift.The current carried by stator winding produces alternating flux. The distribution of flux through the poles is greatly influenced by the shading coils.
When the alternating current through the coil increases, it induces a current in the shading coil. The direction of current in shading coil is such as to oppose the cause producing it (from Lenz law). The cause is the alternating current. So the flux in the shading coil decreases and it opposes the main flux. Hence the flux mostly crowds or shifts towards the unshaded part of the pole. So the magnetic axis lies along the middle of the unshaded part.It is denoted as NC in the picture below.
Now consider that the alternating current has reached its peak (or) somewhere near the peak. Here the rate of change of current is low, as it has already reached the peak (or) it is very close to peak value. Since the change is current is so small, the induced current at shading ring is also small and negligible. So the shading ring does not affect the distribution of main flux. The flux is distributed uniformly and the magnetic axis lies at the center of pole face.The magnetic axis is denoted as ND in the picture.
The alternating current, after reaching the peak starts to decrease rapidly and in turn decreases the main flux. The change in current induces a current in shading coil. According to Lenz law the direction of this current is so as to oppose the cause producing it (the decreasing alternating current). So the flux in shading coil opposes the decrease in main flux and strengthens it. This increases the strength of main flux in the shaded part. SO the magnetic axis shifts itself to the middle part of shaded pole.The magnetic axis denoted as NE in the picture below.
So it is quite clear that during the positive half cycle of the alternating current, North Pole shifts from unshaded part to shaded part and during the negative half cycle, the South Pole shifts along from unshaded part to shaded part. This effect is nothing but the rotation of poles from left to right.
Thus shaded coils aids in producing the rotating flux and thus the single phase Induction motor is converted into self starting one using the Shading coil. Due to fixed of position of shading coils, the direction of rotation of such motors cannot be changed.