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Main >> Applications >> Sample problems >> Unbalanced magnetic pull

Unbalanced magnetic pull

QuickField simulation example

A synchronous generator has a permanent magnet rotor and a three-phase winding stator. The total electromagnetic force is zero if the rotor is properly centered. Rotor eccentricity causes an uneven air gap and an unbalanced magnetic pull. 

Problem Type
Plane-parallel problem of nonlinear DC magnetics.

Geometry:
Axial length is 130 mm.
Unbalanced magnetic pull Rotor eccentricity makes the air gap to be uneven and causes an unbalanced magnetic pull <> Rotor Steel ε Ø 264 mm Ø 185 mm

Given
Permanent magnet coercive force is 955 kA/m, relative magnetic permeability is 1.2.
Air gap length is 2 mm, static eccentricity ε = 50% of the air gap length.
Steel BH-curve:

Task
Calculate the force acting on the rotor.

Solution
We leave the stator winding unenergized, so only the magnetic pull caused by the eccentricity remains.
The rotor is shifted down by the half of air gap, 2mm*50% = 1 mm.

Results
The unbalanced magnetic pull force is 706 N. The plot below clearly shows the asymmetries of the flux density in the air gap.

Field picture of the magnetic flux density distribution in a synchronous reluctance motor:
Unbalanced magnetic pull