Magnetic Field Sources - Magnetocaloric Energy Conversion: From Theory to Applications - page 86

Fig. 3.44 Left Slot leakage flux, Right Skewed stator slots

Other measures in permanent-magnet motors in order to reduce the cogging
torque are related to the radial dimensions of the shoe, the length of the teeth, the
distance between the teeth, the reduction of the variation of the magnetic ux length

from the magnet to the stator, and the relationship between the number of magnet
poles and the number of stator slots. According to the last of these, if each magnet
of the rotor appears in the same position relative to the stator slots (e.g. in a four-
pole rotor with 12 slots in a stator), then the cogging torques of each of the magnets
are in a phase with each other. Therefore, the total torque will represent the addition
of each magnet.
If the motor, however, consists of, for instance, four poles and a stator with 15
slots, this will lead to different positions of the magnet poles relative to the slots.
Then the cogging torques will not fully add, since they will be out of phase with
each other.
The net change in the reluctance and, consequently, the cogging torque can also
be reduced if the slots are skewed, as shown in Fig. 3.44 b. In this case, each magnet
experiences a net reluctance that stays almost unchanged when the slots are passing
by. Therefore, changes in the axial direction are used to decrease the effect of
changes along the circumferential dimension (see Hanselman [ 57 ]). Note that if the
magnets are skewed, this effect will not have a role.
In magnet design one should also pay attention to the shape of the magnets,
since this is also related to the cost of the production, as well as the cost related to
the assembly of such magnets. Simple shapes are therefore of great interest. An
interesting solution was presented by Bouchekara et al. [ 59 ]. In this particular case
(see Fig. 3.45 ), simple bar-shaped magnets were applied. The authors performed a
comprehensive analysis based on

fl

nite elements in order to establish which con-

guration of bar magnets (the magnetic eld of 1 T) could provide the best solution.

With the rotation of the bar magnets, the ON and OFF eld operation was

provided to a pair of AMR beds (because of the need for a continuous operation of
the device). However, as an advanced solution, two such pairs of AMRs have been
considered. The dimensions of the two parts in Fig. 3.45 are identical. These should
be assembled in such a way that a 180

angle shift should be provided to magnets
with odd numbers in the second part, compared to the

°

rst part. Like this the total

torque of a device could be decreased (the torque of the rst part has a different sign

to the torque in the second part) [ 59 ].