Rotating Magnetic Field

a magnetic field that arises as a resultant field during the superimposition of two or more alternating magnetic fields of identical frequency but spatially displaced in phase with respect to one another. The phenomenon of a rotating magnetic field, which was first described in strict scientific form in 1888 independently and virtually simultaneously by the Italian physicist G. Ferraris and the Serbian engineer N. Tesla, is used in electric motors, measuring instruments, and various AC regulation and control equipment. In most cases the rotating magnetic field in an electrical machine is induced by a three-phase current that feeds three coils (1, 2, and 3 in Figure 1) whose axes are displaced 120° in space. A two-phase rotating magnetic field is used mainly in small induction motors and electrical measurement devices. The direction of rotation of a rotating magnetic field depends upon the phase order of a multiphase system and the relative orientation of the coil axes in space.

Figure 1. Formation of a rotating magnetic field upon superimposition of three sinusoidal magnetic fields that are displaced 120^° spatially and with respect to phase: t denotes the pole pitch; B₁,, B₂, and B₃ are magnetic inductions created by the currents of the first, second, and third phases; and B is the total magnetic induction of the rotating field.