N2 Electrical Trade Theory|The Easy Way! 59
At time 1, fig. 4.2 (b), the currents in phases A and C are positive and the current in phase B is negative. Terefore, current is flowing toward the motor or with the arrows, in lines A and C, and away from the motor or against the arrow in line B.
Fig. 4.2 (c) shows the current directions in the stator conductors and the resulting field at each of the six different instants in time chosen in fig. 4.2 (b). At time 1, the current is outward in the three coil sides on the upper half of the stator and is inward in the three coil sides on the lower half of the stator.
Te resulting field is established toward the right, creating a south pole on the stator surface at the right and a north pole at the leſt as shown. (Note: the flux is from north to south through the air gaps and rotor). At time 2, the current is still in the same direction in phases A and B but has reversed in phase C.
Tis establishes a field of the same strength as at time 1 but the position of the field is 600 in a clockwise direction from time 1. At time 3, the current in phase B has reversed, causing the resulting field to be moved another 600 in a clockwise direction. Te remaining diagrams in fig. 4.2 (c) show the resultant field at times 4, 5 and 6.
It is evident from fig. 4.2, that the field resulting from the currents flowing in the three stator windings, shiſts around the stator surface, moving through a definite distance in each of the time intervals considered.
4.4 Principle of operation
When the stator winding (primary) is energised from a three-phase supply, a rotating magnetic field, as described above, is established, which rotates at synchronous speed. As this rotating field sweeps across the rotor conductors
Fig. 4.3 (a) Squirrel-cage rotor winding
(b) Section of rotor and stator showing the magnetic field in the air gap
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