Abstract
All a-c machines having a rotating magnetic field in the air gap are subject not only to the usual "twice-line-Frequency" magnetic vibration but also to a vibration at line frequency itself. It is shown both mathematically and by physical reasoning that this vibration arises as a subharmonic response to a twice-line-frequency variation of the unbalanced magnetic pull. When combined with a condition of mechanical resonance this vibration can become extremely severe.

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