Electric machine, especially small motor

Claim 1

An improved electric machine, of the type comprising an armature, a permanent-magnet field magnet system including at least two diametrically located permanent-magnet segments each of generally arcuate configuration, each permanent-magnet segment having two ends and a middle located midway between the two ends and having a radial thickness which decreases proceeding from the middle towards each of the two ends, each permanent-magnet segment defining together with the armature a radial air gap whose radial length increases proceeding from the middle of the respective segment towards each of the two ends in correspondence to the radial-thickness decrease in the segment, the improvement wherein: each permanent-magnet segment comprises a first and a second subsegment respectively made of first and second magnetic materials, the first magnetic material having a higher remanence than the second magnetic material, the second magnetic material having a higher coercive force than the first magnetic material, the second subsegment being located more downstream than the first subsegment as considered in the direction of relative rotation of the armature and permanent-magnet segments, the sum of the radial length l L of the air gap and the radial thickness l M of each permanent-magnet segment being constant, the radial thickness l M .sbsb.min of each permanent-magnet segment at each of the two ends thereof being ##EQU10## the two subsegments of each permanent-magnet segment adjoining each other at a junction location which is spaced from the middle of the respective permanent-magnet segment by an angular distance α 1 such that ##EQU11## wherein: w·I e =the armature quadrature mmf of the motor, H G .sbsb.2 =the field intensity up to which the second magnetic material can be demagnetized without undergoing irreversible demagnetization, B r .sbsb.2 =the remanent induction of the second magnetic material, β=the angular span of each permanent-magnet segment, μ o =the magnetic permeability of free space, l L .sbsb.max =the radial thickness of the air gap measured at an end of a permanent-magnet segment, l L .sbsb.o =the radial thickness of the air gap measured at the middle of a permanent-magnet segment, Δl L =the amount by which the radial thickness of the air gap measured at a segment end exceeds that measured at the middle of a segment, H G .sbsb.l =the field intensity up to which the first magnetic material can be demagnetized without undergoing irreversible demagnetization, and B r .sbsb.1 =the remanent induction of the second magnetic material,