Q2Electrical Machine Design

A good magnetic material for use in transformer and rotating machine cores should possess the following properties.

High permeability: allows a large flux density to be established with a relatively small magnetizing force (MMF), keeping the magnetizing current and associated core losses low, and permitting a more compact core design for a given flux requirement.

Low hysteresis loss: a narrow B-H hysteresis loop (low coercivity) minimizes the energy dissipated as heat during each cycle of magnetization reversal, which is essential since the core is cyclically magnetized at supply frequency continuously during operation.

Low eddy current loss / high electrical resistivity: since eddy currents are induced in the core by the time-varying flux, a material with high electrical resistivity (achieved by adding silicon to steel) inherently limits eddy current magnitude; additionally, laminating the core into thin, mutually insulated sheets (rather than using a solid block) further restricts eddy current paths and loss.

High saturation flux density: allows the core to carry a higher useful flux density before the material's permeability begins to drop sharply (saturation), enabling a smaller core cross-section for a given flux requirement and hence a more compact, material-efficient design.

Mechanical strength and workability: the material must be strong enough to withstand the mechanical stresses of assembly, operation, and short-circuit conditions, and should be readily workable (rollable, punchable/shearable into laminations) without excessive brittleness, since manufacturing large numbers of precisely-shaped laminations is a routine part of core construction.

Low cost and good availability: as with all engineering materials, the magnetic material must be economically viable for mass production, which is why silicon steel (rather than more exotic, higher-performance magnetic alloys) remains the dominant core material for conventional power-frequency transformers and machines despite the existence of materials with superior individual properties.