文档介绍：Massachusetts Institute of Technology
Department of Electrical Engineering puter Science
Electric Machines
Class Notes 3: Eddy Currents, Surface Impedances and Loss Mechanisms February 11,
2004
c 2003 James L. Kirtley Jr.
1 Introduction
Losses in electric machines arise from conduction and ic hysteresis. Conduction losses are
attributed to straightforward transport conduction and to eddy currents. Transport losses are
relatively easy to calculate so we will not pay them much attention. Eddy currents are more
interesting and result in frequency dependent conduction losses in machines.
Eddy currents in linear materials can often be handled rigorously, but eddy currents in saturat-
ing material are more difficult and are often handled in a heuristic fashion. We present here both
analytical and semi-emiprical ways of dealing with such losses.
We start with surface impedance: the ratio of electric field to surface current. This is important
not just in calculating machine losses, but also in describing how some machines operate.
2 Surface Impedance of Uniform Conductors
The objective of this section is to describe the calculation of the surface impedance presented by a
layer of conductive material. Two problems are considered here. The first considers a layer of linear
material backed up by an infinitely permeable surface. This is approximately the situation presented
by, for example, surface mounted permanent s and is probably a decent approximation to
the conduction mechanism that would be responsible for loss due to asynchronous harmonics in
these machines. It is also appropriate for use in estimating losses in solid rotor induction machines
and in the poles of turbogenerators. The second problem, which we do not work here but simply
present the previously worked solution, concerns saturating ic material.
Linear Case
The situation and coordinate system are shown in Figure 1. The conductive layer is of thicknes T
and has conductivity