What is Unbalanced Magnetic Pull (UMP) or Unbalanced Magnetic Force (UMF)?


Unbalanced Magnetic Pull (UMP) is the net electromagnetic force acting along X or Y axis in the lamination plane (for radial flux machines) . Similarly to cogging torque, it is characterized by an average / DC value and a ripple / time harmonic component. Unbalanced Magnetic Force (UMF) term is generally reserved to axial magnetic forces in radial flux machines, but some authors used it instead of UMP.

UMP magnitude corresponds the magnitude of the Maxwell stress harmonic component of wavenumber r=1. In magnetically-balanced electrical machines without eccentricities or uneven magnetizatio, UMP is null.

Example of a harmonic force wave of wavenumber r=1 (unbalance magnetic pull)
Example of a harmonic force wave of wavenumber r=1 (unbalance magnetic pull)

Application to e-NVH

UMP can create a bending motion of an outer stator electrical machine (especially in clamped-free conditions), resulting in air-borne acoustic noise. UMP can also excite the inner rotor shaft bending mode, resulting in structure-borne acoustic noise.

EV HEV NVH is generally driven by pulsating forces. Eccentricities modulate all pulsating forces, resulting in additional UMP harmonics. Static eccentricity creates UMP that cannot be seen in spectrogram but that can be seen in spatiogram.

Application to MANATEE

MANATEE software can calculate the effect of UMP both rotor and stator and the resulting structure-born or air-borne acoustic noise. Noise mitigation actions such as notching can be easily implemented in MANATEE. The net magnetic forces in X and Y directions can be plot using plot_FxFy command of MANATEE:

Global magnetic forces
Global magnetic forces

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