It is conventional that the current indicated by is positive when it is flowing inwards in the conductors r, y & b and outwards in r1, y1, and b1.
The current indicated by "Dots" are considered negative and flowing outwards.
The current in the three phases are represented by the curves in figure.
You can observe that the current in phase R is at maximum positivee value at instant a.
At the same instant, the currents are negative in phases Y and B, and their magnitudes are half of maximum value.
These currents produce magnetic fluxes. They are represented by the dotted lines as shown.
The resultant flux axis is indicated by the flashing arrow NS.
After some milliseconds the R phase current reaches the instant b. Now, both the currents in phrases R and B will be 0.866 times the maximum value.
The distribution of magnetic flux due to these current is shown.
The axis of the magnetic field as no in line with a coin y and y1.
It would be observed that the line of magnetic field has turned clockwise by 30 degree from that of the instant with maximum current in the first figure.
At instant c the current in phase B attains its maximum negative value.
The current in R and Y are now both positive but their magnitudes are being has the maximum value.
The magnetic flux is produced by these currents are shown.
The axis of this flux could be seen displaced clockwise by another 30 degree compared with the previous instant discussed.
so for every interval of time corresponding to any angle along the horizontal Axis axis of the magnetic flux moves forward by the same angle in space.
This means that the flux rotates through one revolution in one cycle,This is a synchronous speed of a two pole motor.
Therefore it is seen that the magnetic flux is rotating at synchronous speed inherently in a three phase induction motor.
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