The nature of vibration in electric machinery

Quentin Graham (Westinghouse Electric and Manufacturing Company, East Pittsburgh, Pa.): It may be noted that there is one source of electromagnetic force that Professor Graybeal has not emphasized. If the center of the rotor travels in a small circle around the center of the stator, such as would occur with a bent shaft, there is a point of minimum air gap which travels at rotor speed. As this coincides with the maximum magnetic field at twice slip frequency, the characteristic beat wave is accounted for without the necessity of a second harmonic in the mechanical force wave. The test shown in Figure 3 of the paper is, of course, evidence that a second harmonic mechanical force did occur in the tested motor, which would be accounted for by keyways or other dissymmetries. However, it is my belief that in many cases a bent shaft alone is sufficient cause. This does not mean, necessarily, a permanent distortion of the shaft, but rather a bending within the elastic limit as a result of the centrifugal force of a slightly unbalanced weight accentuated by the increased magnetic pull on the side having the minimum air gap. I should like to ask the author whether he has experience that may confirm or disprove this point of view.