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.
Engineering question
How to find unbalanced magnetic pull due to rotor eccentricity?
Answer
Set up a plane-parallel QuickField DC Magnetics problem for a rotor with eccentricity and evaluate unbalanced magnetic pull from computed field results.
Problem Type Plane-parallel problem of DC magnetics.
Geometry Axial length is 130 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:
