Eddy current iron losses in axially laminated brushless motors

The paper presents an analytical procedure for the evaluation of iron losses produced by eddy currents in the rotor sheets of axially laminated VR motors. These losses cannot be neglected especially in inverter fed closed loop operations. To simplify the eddy currents computation, the magnetic field in iron sheets is obtained by separately considering the contribution of the stator currents with removed rotor and the one of the Amperian currents, responsible for the rotor magnetisation. With reference to a generic nonsinusoidal steady-state condition, the proposed mathematical model gives the expression of the instantaneous values of the electromagnetic field and of the current density in each point of the rotor sheets. Iron losses are then obtained by means of a proper integration of the current density distribution. Finally, the paper shows the results of a numeric example which refers to a specific motor configuration and to an assigned waveshape of the input currents.