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Different from the traditional transformer, the rotary transformer proposed by authors has a rotary secondary winding which was called rotor. In the transformer, the vibration noise sources were more complex and electromagnetic vibration noise is particularly prominent. Because of the special structure, the radial electromagnetic force inside the transformer was much larger than the tangential electromagnetic force, which is the main factor affecting the electromagnetic vibration noise. The effect of the air gap lengths and air eccentricity on the radial electromagnetic force density of the rotary transformer was investigated on the finite element model and suggestions were given for optimum design.