Magnetic noise in single-machine rotating frequency converters

The magnetic noise in rotating frequency converters with two magnetic fields in a common structure of an induction machine, each field having a different number of pole pairs corresponding to the frequency conversion ratio, is evaluated on the basis of harmonic analysis of the resultant magnetic induction in the air gap and the radial forces it generates. The rotor in small and medium-size machines is regarded as a solid cylinder so that the hollow stator cylinder remains the principal source of noise. Constraints on the numbers of stator slots and rotors slots are established so as to ensure minimum higher-frequency tooth noise and vibration, while structural requirements are stipulated for minimizing lower-frequency noise and vibration and avoiding resonances at any frequency. Typical design calculations for a three-phase motor-generator machine operating at 1:3 frequency converter are shown, including the amplitudes of design-controlled vibrations of magnetic origin.