Calculation of electrical machine magnetic stray fields

Most new ships are electrically driven. However, some military or oceanographic ships require low magnetic stray fields. Thus, electrical machines for propulsion have to be magnetically silent. The calculation of magnetic stray fields far from an electromagnetic device by standard finite-element method (Flux3d) may be inaccurate. Numerical noise may appear that can be as high as the weak magnetic induction to be computed. Consequently the calculation of low fields requires a specific model. A model has been developed in the Flux3d post processing. The principle is to use the magnetisation of the ferromagnetic parts of the device. The magnetisation is well computed by Flux3d (high field values). This magnetisation allows one to analytically calculate the induction outside the device by using the magnetic moment equivalence. Finally, the magnetic noise is eradicated and the model is able to calculate low field values. To validate the authors´ model, a comparison with measurements on a synchronous permanent magnet machine has been made. Experiments give good results and the validation is satisfactory. The effects of each magnetic source (rotor magnets and stator coils) of the machine are well calculated. The model is mainly static. Its application to the computation of the stray fields while the motor is rotating is presented.