The use of permanent magnet DC motor field
excitation. Motor due to no separate excitation system, and thus the
small size, light weight, simple structure and high efficiency. Most of
the miniature DC motors are permanent magnet. Permanent magnet DC motor
starting and operating characteristics and excited, and excited DC motor
is basically the same. In the structure in addition to the stator field
winding, armature, brushes, commutator and other components are the
same as the current excitation DC Smaller coercivity of Alnico permanent
magnets require a longer effective length in order to generate enough
magnetomotive force, so often used a cylindrical structure.In order to
avoid armature reaction magnetic field of permanent magnet, permanent
magnet motor can be added to the soft iron pole piece when the motor
load, this structure can be generated by the armature current armature
reaction flux through the pole shoe withoutinto the permanent magnet
flux of constant work to ensure and improve the utilization of the
permanent magnet, but also conducive to the commutation. Pole shoe and
the permanent magnet should fit closely with higher technology.
Using
ferrite excitation, the tile block structure can be used to tile with a
pole piece structure and the cylindrical structure. The former applies
to anisotropic materials, magnet high utilization rate of simple
structure, ease of mass production, but the air gap flux density is low.
Tile with a pole piece structure can improve the air gap flux density,
but the structure is more complex. Cylindrical structure is simple, ease
of production, material utilization is low, is not conducive to the
commutation Rare earth permanent magnet excitation, the magnetic
structure similar to the tile type. But its coercivity pole radial
dimensions can be further reduced.