The Rotating Magnetic Field

Author: E.E. Kimberly

The polyphase induction motor does not have a rotating field structure, as suggested in Fig. 18-1; but, instead, has a field of rotating flux density produced by a distributed winding on a frame structure without moving parts. Such a frame, together with its winding, is called a stator. The stator winding is sometimes called the primary, and the rotor cage is the secondary. A completed stator is shown in Fig. 18-5.

Fig. 18-5. Induction-Motor Stator

Fig. 18-6 (a) represents a simple three-phase, two-pole, Y-connected stator with one coil per phase. The winding terminals L₁, L₂, and L₃ connect to the three wires of the three-phase line. A convention will be used in Fig. 18-6 (b), in which instantaneous values of current above the axis of sinusoids represent currents entering the stator winding and going down in the slots and toward the Y-point of the winding. Three instants of time will be chosen, and it will be demonstrated that the point of maximum flux density moves around the inner periphery of the stator.

If an instant T₁ be chosen in Fig. 18-6 (b), I₁ is passing through zero and phase 1 contributes nothing to the magnetic field. However, the direction of I₂ is down in slot c and this current produces an mmf which would produce a flux density

(if not affected by other magnetomotive forces)

Fig. 18-6. Windings and Currents in an Elemental Induction-Motor Stator

This flux goes into the rotor along c, d, e, f, and out of the rotor along f, a, b, c. At the same instant, the direction of I₃ is up in slot e and this current produces an mmf which would produce a flux density

(if not affected by other magnetomotive forces)

If superposed, these magnetomotive forces cancel each other between f and e and between b and c, but add between f and b and between c and e. Since the superposed magnetomotive forces are equal, the total flux density (it being assumed that there is no iron saturation between f and b and between

c and e) is

The magnetic axis lies on a line through a and d, and the north pole is at the top at a, as shown in the diagram in Fig. 18-7 for time T₁.

By a similar procedure it may be shown that at a time T₂, 120° later than T₁ when I₂ is zero, the currents I₁ and I₃ produce a field whose density is

The axis of this field is on a line through / and c, and the north pole is at c, as shown in the diagram in Fig. 18-7 for time T₂.

Similarly, it may be demonstrated that at time T₃ the axis of magnetism will be as shown in the diagram in Fig. 18-7 for time 23. Also, it may be demonstrated that the value of flux density is

Fig. 18-7. Field Positions Corresponding to Times fT1, T2, T3 of Fig. 18-6 (6)

1 Principles of Alternating Current Machinery, by R. R. Lawrence. McGraw-Hill Book Co.

2 Connecting Induction Motors, by A. M. Dudley. McGraw-Hill Book Co.