Concept of Rotating Magnetic Field

CONCEPT OF ROTATING MAGNETIC FIELD

If an balanced 3 phase voltage is applied to a balanced 3 phase winding, it produces rotating magnetic field of constant amplitude.

The speed of the rotating magnetic field is given by:

N_S = 120f / P

where,

f is the frequency of the supply.

n is the number of stator poles.

The stator of synchronous and induction motor may be star (or) delta connected. The three phase windings are displaced from each other by an angle 120°.

The current through the three phase windings are displaced from each other by an angle of 120° electrical. The expression for instantaneous values of three three flux are given by:

Let us consider the following cases:

Case 1: Phasor Addition of Fluxes

The Figure 3.7 shows the phasor addition of the above three fluxes from the figure,

we get

Therefore, the magnitude of φ_Res is 1.5 m and its position is vertically upward.

Case 2: Phasor Addition of Flux

Figure 3.8 shows the phasor addition of the above three flux, from the figure, we get

Therefore, the magnitude of φ_Res is 1.5 φ_m and it is rotated through 60° in space in clockwise direction compared to its previous position in case 1.

Case 3:

Figure 3.9 shows the phasor diagram addition of the above three flux, we get,

Therefore, the magnitude of φ_Res is 1.5 φ_m and its rotated through 120° in space in clockwise direction compared to its previous position in case 1.

Case 4:

Figure 3.10 shows the phasor addition of the above three fluxes, from the figure, we get

Therefore, the magnitude of φ_Res is 1.5 φ_m and its is rotated through 180° space in clockwise direction compared to its previous in case 1.