Rotating Magnetic Field Due to 3-Phase Currents
Rotating Magnetic Field Due to 3-Phase Currents:
When a 3-phase winding is energized from a 3-phase supply, a rotating magnetic field is produced. This field is such that its poles do no remain in a fixed position on the stator but go on shifting their positions around the stator. For this reason, it is called a rotating field. It can be shown that magnitude of this rotating field is constant and is equal to 1.5 Φm where Φm is the maximum flux due to any phase.
To see how rotating field is produced, consider a 2-pole, 3i-phase winding as shown in Fig. (2 (i)). The three phases X, Y and Z are energized from a 3-phase source and currents in these phases are indicated as Ix, Iy and Iz [See Fig. (2(ii))]. Referring to Fig. (2 (ii)), the fluxes produced by these currents are given by:
Here Φm is the maximum flux due to any phase. Fig. (1) shows the phasor diagram of the three fluxes. We shall now prove that this 3-phase supply produces a rotating field of constant magnitude equal to 1.5 Φm.
(i) At instant 1 [See Fig. (2 (ii)) and Fig. (2 (iii))], the current in phase X is zero and currents in phases Y and Z are equal and opposite. The currents are flowing outward in the top conductors and inward in the bottom conductors.
This establishes a resultant flux towards right. The magnitude of the resultant flux is constant and is equal to 1.5 Φm as proved under:
At instant 1, ωt = 0°. Therefore, the three fluxes are given by;
The phasor sum of - Φy and Φz is the resultant flux Φr [See Fig. (3)]. It is clear that: