Abstract :
In this paper the author discusses the various forces which give rise to stresses and deflections in large electrical machine frames. The mechanical or static loading is treated, and then the forces due to magnetic loading are examined. It is shown that the latter forces are by far the more important. The tendency for the rotors of induction motors to pullover to the stator is explained by means of a graph, and it is shown how a machine can be so designed as to avoid this trouble. It is pointed out that the frame of a machine is under the action of a total magnetic pull equal in most cases to 10 times the ¿unbalanced magnetic pull¿ calculated for a rotor displacement of 10 per cent of the radial air-gap. This particular problem is examined by means of photoelastic tests taken upon celluloid models, and the effect of the spread of the frame feet is very clearly demonstrated. A comparison is made between these tests and the calculated bending-moment diagram. In an Appendix the author gives a mathematical solution of the problem, the unbalanced magnetic pull being considered as a concentrated load, and a number of curves are included in order to simplify the calculation of stresses and deflections. The points of zero bending moment are also given, so that the complete bending moment diagram can be rapidly drawn for any case. In conclusion, an actual example is worked out in detail, and it is shown that an induction motor frame has to be so designed as to limit the deflection; the stresses are very low and of the order of 2 000 lb. per sq. in. or less. Machines with large air-gaps are designed on a basis of limiting stress, which occurs long before the limiting deflectionis reached.
