Q7Electrical Machine Design

Specific magnetic loading (average flux density, Bav) is defined as the average value of flux density distributed over the entire cylindrical air-gap surface of the machine:

where Φ is the flux per pole, P is the number of poles, and πDL is the total air-gap cylindrical surface area. It is one of the two fundamental specific loadings (alongside specific electric loading, ac) used in the machine output equation to determine the principal dimensions D and L.

Choice of specific magnetic loading: selecting an appropriate value of Bav involves balancing several competing factors. A higher Bav allows a smaller machine (smaller D²L) for a given output, since the output equation shows output is directly proportional to Bav, making higher magnetic loading attractive from a material-cost and size perspective. However, increasing Bav also increases iron (core) loss, since both hysteresis loss and eddy current loss increase nonlinearly with flux density; increases the magnetizing current required (and hence reduces power factor, particularly significant for induction motors), since higher flux density requires proportionally more magnetizing MMF, especially as the core material approaches saturation; and risks driving the core material into magnetic saturation if pushed too high, causing a disproportionate increase in magnetizing current and distorted (non-sinusoidal) flux waveform with associated additional harmonic losses.

Factors influencing the choice of Bav: the specific magnetic loading actually chosen for a given design depends on the core material's saturation characteristics and loss curves (better, lower-loss materials like CRGO steel permit somewhat higher Bav for the same loss budget); the permissible core/iron loss (constrained by the machine's overall efficiency target and its thermal/cooling design); the type of machine and its power factor requirements (induction motors, needing to draw their own magnetizing current from the supply, are typically designed with somewhat lower Bav than synchronous machines, which receive their magnetizing MMF from a separately-excited DC field winding and are therefore less sensitive to the power-factor penalty of higher flux density); and the machine's frequency of operation, since core losses increase with frequency, requiring a correspondingly reduced Bav at higher operating frequencies to keep losses within acceptable limits, all of which together typically place practical Bav values in the range of roughly 0.3-0.6 Wb/m² for induction motors and slightly higher for synchronous machines, based on established design practice and empirical design charts.