the vector sum of 3 phase flux in an induction motor, at any instance, comes to be 1.5 times the amplitude of the flux in any phase, that keep rotating , so an RMF(rotating magnetic field) is produced, but doesn't it happens in case of 3 phase transformer?
An induction motor has a central symmetric construction and also a central symmetric flux alignment. A central symmetric construction of a transformer is too expensive. Normally the flux in transformers is spatial scattered.
When current flow from conductor, magnetic field will developed, with D.C, constant and alternating with A.C. Same applicable to coil too, strength of total coaxial field could be increased, by increasing number of turns, or by increasing current, or by lowering reluctance of magnetic field path. If coil is placed on bar core, and A.C is applied, field developed will be A.C with same frequency of supply, and if another coil place on same core, will developed the voltage(EMF), depending on the linking of flux of the A.C field, and is called transformer(single phase). In three phase A.C supply the supply voltage wave form are 120 degree electrical, apart and are with same amplitude, and hence currents will produce. the fields in other two limbs (120 degree mechanical apart), in the same way(as individual phase, as fields are independent, not linking), and is called three phase transformer. If three identical coil(winding) placed on perifery of a circular core and are symmetrical 120 degree apart(mechanical degree equal to electrical degree), and if three phase supply is given, the resultant magnetic field produced would be of 1.5 times(amplitude), the field of individual phase field, and rotates within,with the speed of Ns= 120f/p,in core. Where 'f' the supply frequency, and 'p' the pole(120 degree electrical symmetrical placements of coil on perifery( p=2, if electrical degree equal to mechanical degree)). Hence to produce rotating field important is core design(round) and winding(placement of coils) for required number of poles(speed).
A three-phase current in three windings can produce a rotating magnetic field of constant magnitude only if the windings are *spatially* placed 120° apart. AC motors are constructed this way, as mentioned by Mr. Jacob here. However, this is not the case in three-phase transformers whose physical construction varies.
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