Rotating magnetic field

[3] In 1879, English physicist Walter Baily replaced the horseshoe magnets with four electromagnets and, by manually turning switches on and off, demonstrated a primitive induction motor.

[9] Ferraris, who did research about the theory and design of alternating-current machinery, built a working model for a classroom demonstration in 1885, but did not describe it publicly until 1888.

The rotor or armature consists of coils wound in slots, which are short circuited and in which the changing flux generated by the field poles induce a current.

The result of adding three 120-degree phased sine waves on the axis of the motor is a single rotating vector that always remains constant in magnitude.

This magnetomechanical attraction creates a force that will drive the rotor to follow the rotating magnetic field in a synchronous manner.

Three similar coils having mutual geometrical angles of 120 degrees will create the rotating magnetic field in this case.

These types of motors are not usually synchronous, but instead necessarily involve a degree of 'slip' in order that the current may be produced due to the relative movement of the field and the rotor.

Oscillating magnetic fields. Sine wave current in each of the three stationary coils produces three sine varying magnetic fields perpendicular to the rotation axis. The three magnetic fields add as vectors to produce a single rotating magnetic field.
U.S. Patent 381968 : Mode and plan of operating electric motors by progressive shifting; Field Magnet; Armature; Electrical conversion; Economical; Transmission of energy; Simple construction; Easier construction; Rotating magnetic field principles.
Rotating fields. As the direction of the current through the windings changes, the polarity of the windings changes as well. Since there are two windings acting in conjunction with each other, the polarity of the main field will depend upon the polarity of each winding. The arrow or vector below each diagram indicates the direction of the magnetic field in each case. [ 16 ]
Rotating three-phase magnetic field, as indicated by the rotating black arrow