Eddy current losses in permanent magnets of the BLDC machine

Purpose - This study aims to calculate eddy current losses in permanent magnets of BLDC machine in the generator mode of operation with no-load. Design/methodology/approach - Stator slot openings and special design of the stator poles cause changes in the magnetic flux density changes in permanent magnets. The stator windings are not connected to an outer source and no currents flow in them. The induced eddy currents in permanent magnets are dependent solely on the stator geometry. Analytical approach to calculate the eddy current density distribution in permanent magnets is based on known distribution of magnetic flux density in the air-gap of BLDC. The magnetic flux density distribution is obtained from magneto-static finite element model of BLDC. For verification of analytical approach the eddy current density distribution in permanent magnets is also calculated by magneto-transient finite element model of BLDC. Findings - The eddy current losses in PM obtained with the FEM indicate additional heating of the BLDC machine at high rotational speeds even when it operates at no load. When some special stator designs (the side of the air gap) are needed, the losses in PMs and their heating increase. Research limitations/implications - To get more precise results, the proposed analytical method for eddy current losses calculation in PM should be further analyzed. More geometric parameters of the BLDC design should be introduced to analytical formulations, especially those which affect variations in reluctance. Practical implications - When some special stator designs (the side of the air gap) are needed, the losses in PMs should be observed. This is particularly recommended at higher rotation velocities. Any kind of magnetic flux density change induces eddy currents and together with them also power losses. These losses give rise to additional heating of PM. With this, the temperature-dependent working characteristic of PM (second quadrant of the B-H curve) moves toward the coordinate origin point. The overall machine performance is reduced. The presented work gives the view about happenings in permanent magnets regarding induced eddy current losses. It is a useful tool for fast estimation and reduction of eddy current losses in PM due to stator geometry. Originality/value - The value of the paper is the closed view about happenings in permanent magnets regarding induced eddy currents and the calculation of eddy current losses in rotor permanent magnets of BLDC due to stator design. The originality is in the analytical approach to calculate the eddy current losses based only on known magneto-static flux density distribution in air-gap of BLDC.