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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


practically constanT1 irrespective of time or the position of the axis of polarity. A magnetic field, the strength of which is constant and the points of maximum intensity of which rotate with constant velocity, is called a perfect rotating field. In commercial polyphase motors with the common drum type of stator winding, as shown in Fig. 18-5, a great number of coils evenly distributed in slots aroun the inner stator bore are used. These coils are grouped in phase belts, in which all coils (two or more adjacent coils) of a belt are connected in series.2 A phase belt occupying its portion of the total peripheral space is actually used instead of a single coil per phase, as shown in the elemental diagram of Fig. 18-6 (a). The direction of rotation of the field, and hence of the motor, may be reversed by interchanging any two of the three line leads.

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.

Last Update: 2010-10-05