===Incremental Permeability AC Solution about a DC Operating Point===

David Meeker
04Jan2014

**Introduction**

The problem of computing the impedance of a system about a DC operating point arises with regularity during the study of electric machines.  Two examples are:

  * Computation of the frequency-dependent impedance of a Loudspeaker Driver about the DC operating point created by a permanent magnet;
  * Computation of frequency-dependent impedance of a radial magnetic bearing about the DC operating point created by bias currents in the coils.

A recent version of FEMM 4.3 has the capability of computing incremental permeability solutions based on the DC operating point simulated in a linked solution.  This note explains the incremental permeability formulation and demonstrates its use through a tutorial example.