Study of retaining sleeve and conductive shield and their influence on rotor loss in high-speed PM BLDC motors

Rotor eddy-current loss is usually significant in high-speed permanent magnet brushless dc (PM BLDC) motors. On the other hand, the rotor is usually protected with a nonmagnetic retaining sleeve. The influence of the conductivity of the retaining sleeve on the rotor eddy-current loss is analyzed in this paper. Furthermore, utility of a conductive copper shield between the sleeve and magnets is demonstrated. Although eddy current is induced in the copper shield, causing additional loss, its magnetic field can smooth the varying field in the rotor and reduce the overall eddy-current loss dramatically. The relationship between the thickness of copper shield and the rotor eddy-current loss is also examined. Index Terms--Conductive shield, high-speed motor, permanent magnet brushless dc (PM BLDC) motor, retaining sleeve, rotor eddycurrent loss, temperature rise.