THEORY OF SINGLE-PHASE INDUCTION-MOTOR OPERATION (Electric Motors)

6.2
One of the most important concepts in the analysis and design of all electrical rotating machinery, whether it be a generator or motor, is to obtain a circuit model for performance calculations. Single-phase induction motors are no exception.
An equally important concept, which is necessary for induction or synchronous motor rotation, is the existence of a revolving magnetic field in the air gap of the motor. Without this there can be no torque development for producing rotation. Ideally, a constant-magnitude rotating mmf is preferred. This latter concept of a rotat-

TABLE 6.2 Sinusoidal Winding Distribution—Odd Tooth Spans

Slots per					Tooth span
pole	3	5	6	9	11	13	15	17	19 Kw
18	4.6	7.5	10.2	12.5	14.5	16.0	17.1	17.6	0.794
		7.8	10.6	13.2	15.2	16.8	17.9	18.5	0.820
			11.5	14.2	16.5	18.2	19.5	20.1	0.854
				16.1	18.6 22.2	20.6 24.6	22.0 26.2	22.7 27.0	0.892 0.927
16	5.8	9.4	12.7	15.4	17.6	19.2	19.9		0.797
		10.0	13.4	16.4	18.7	20.4	21.1		0.829
			14.9	18.2 21.4	20.8 24.5 31.1	22.6 26.5 33.8	23.5 27.6 35.1		0.868 0.910 0.946
12	10.3	16.5 18.3	21.4 24.0 29.3	25.0 27.8 34.1 48.2	26.8 29.9 36.6 51.8				0.809 0.854 0.910 0.969
9	18.5	28.3 34.7	34.7 42.6 65.3	18.5 22.7 34.7					0.821 0.893 0.961
8	27.6	33.2 45.8	39.2 54.2						0.815 0.813
6	42.3	57.7 100.0							0.856 0.965
4	100.0								0.923

ing magnetic field is important in the understanding not only of single-phase induction motors but of all rotating ac electromechanical devices.
In all multiphase symmetrically balanced motor windings, the existence of a rotating magnetic field can be mathematically demonstrated and verified rather easily. The case of the single-phase motor is a bit more difficult to show, but it can be derived mathematically.
Let us explore this rotating revolving field concept by beginning with a two-phase symmetrically balanced induction motor operating on a two-phase power system. This is a motor having at least two sinusoidally (or as near as possible) distributed stator windings, each of the windings being displaced 90° electrical apart in the stator slots. This means a four-pole motor would have four stator windings located 90° electrical apart, which is equivalent to 45° mechanical or spatial.