This article tells you the process of production of the rotating magnetic field in an AC rotating machine. The stator of a three-phase AC motor consists of a physically distributed balanced three-phase winding. The figure below shows the conceptual view of the three-phase winding in a stator.
Determining the Direction of Magnetic Field
We can determine the direction of the flux or magnetic field for each phase by applying the right-hand grip rule.
The current in the field winding produces the magnetic field in a certain direction. The current in the opposite direction produces the magnetic field in the opposite direction.
Production of Rotating Magnetic Field
Whenever we apply a three-phase supply to the three-phase winding of the stator, the magnitude and direction of the magnetic field for each phase winding depends on the value and direction of the current through that phase winding respectively.
At ωt = 0°
For more clarity, let us draw the waveform of a three-phase supply.
From the above waveform, it is clear that at t = 0, or ωt = 0, the value of current in the red phase winding is zero.
Therefore there will be no magnetic field produced in the red phase at that instant.
At the same instant, the value of the current in the yellow phase winding is,
This value is negative therefore the direction of the magnetic field produced in the yellow phase is opposite of the direction of the yellow-phase-field when the current is positive.
At that instant the current through the blue phase winding is positive and its value is
Now draw the vector representation of these three instantaneous magnetic fields.
Here, Fm is the amplitude of mmf per phase. From the above diagram, we get the resultant magnetic field.
At ωt = 60°
Now we come to the instant when the angular displacement is 60° or π/3. At that instant the current through the red phase is positive and its value is
At that instant, the value of current in the yellow phase winding is still negative. Its value is
The direction of the magnetic field in the yellow phase will be the opposite of the same field when the current was positive.
But at that instant, the current through the blue phase is zero, hence there will be no magnetic field produced in the blue phase.
Now again we will draw the vector representation of instantaneous magnetic fields of all three phases at that instant.
From here we get the resultant magnetic field at that instant.
Now, we observe the magnitude and direction of the resultant magnetic field, in both the above-mentioned instances. Here, we see that the resultant magnetic field has just rotated 60° without changing its magnitude.
When angular displacement is 120°, we can similarly show that the resultant vector again rotates further 60° with the constant magnitude.
In this way, due to a balanced three-phase supply to the mechanically balanced three-phase distributed winding in the stator of an AC machine produces a continuously rotating magnetic field.