High accuracy force computation in electrical machinesCopyright: © IEM
The eggshell method allows computing both local electromagnetic (nodal forces) and resultant electromagnetic forces (i.e. forces and torques acting on rigid objects) by applying different kinds of infinitesimal virtual test displacement fields. Nodal forces are obtained by means of a virtual test displacement field leaving all nodes of the mesh but one fixed. Resultant forces are obtained by means of a virtual test field describing an infinitesimal rigid-body motion of the piece under consideration and leaving all other nodes of the mesh fixed. The eggshell method can be shown equivalent with the Maxwell stress tensor method on the one hand, and with the virtual work principle on the other hand. It is however of a more general applicability and offers also a number of practical and conceptual advantages. The Figure represents the nodal force field at the upper surface of a rotor claw in a claw pole generator.