Where does the twice-line-frequency vibration peak come from?

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Have you ever wondered where the twice-line-frequency peak (typically 120 Hz or 100 Hz) comes from in the spectrum? It can indicate a fault condition, and it can be confused with other fault conditions -- and it can mean absolutely nothing! In this presentation we hope to clarify two issues; why does the peak appear at twice the line frequency, and which faults are indicated by the appearance (or growth) in the peak at that frequency. In addition to understanding the origin of the twice-line-frequency peak, you will learn a little about magnetic fields and how three-phase induction motors work. As always we will use 3D animations and simulations to make the topic easy to understand (and interesting to view).