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.
