2.2 Materials and Temperature

The magnetic and electric circuits essential to machines require materials of high permeability and low resistivity, respectively, and these are generally metals. Metals with good magnetic and electrical properties do not necessarily have high mechanical strength. Indeed the atomic structure of a good conductor is such that it will naturally have a low yield strength and high ductility. Yet the magnetic and electric circuits of the machine must bear the mechanical loads imposed upon them by the transfer of energy across the airgap. Furthermore, the magnetic and electrical circuits must be separated by insulating materials, such as films, fibres and resins, which have even weaker mechanical properties. Table 2.1 sets out the elastic moduli and tensile strength of materials used in electrical machines and highlights the relative weakness of electrical steel, conductor and insulating materials. Right from the outset then, there is a conflict between the electrical and mechanical requirements of the various parts of an electrical machine, which the designer must attempt to resolve.

However, there is a further complication. The transfer of energy inevitably involves the dissipation of heat, by ohmic losses in the electric circuit and by eddy current and hysteresis losses in...