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The following description is interesting, but really not necessary! The purpose of the armature assembly (the part of a motor with the shaft, that spins inside the rest of the motor, the case) is to provide a means by which current carrying conductors can be moved through the air gap of an electrical machine. By interacting with the concentrated magnetic field in the air gap, the conductors either generate a force or an EMF, depending upon how the machine is connected electrically. The iron core armature has several important advantages. This construction provides a strong, rigid support for the windings, an important consideration in machines which develop large forces in the air gap. The iron core also provides a thermal reservoir which conducts heat away from the conductors, allowing the machine to be driven harder than might otherwise be the case. Iron core construction is also relatively inexpensive compared with other construction types. Iron core construction also has several major disadvantages. The iron core armature has relatively high inertia which limits acceleration. This construction also results in high winding inductances which not only contribute to motor EMI but also limit brush and commutator life. The ‘winding heads’ on an iron core armature are necessary for electrical connection, but they do not contribute to torque generation. They do contribute extra unwanted Ohmic resistance and mass to the coil (see Figure 15). Early in the 1950’s, applications began to appear which required small, efficient DC motors capable of high angular acceleration and appropriate for battery powered operation. Traditional iron-core motors were unacceptable due to their intrinsic high armature inertia and electrical inductance. As a means of addressing these new applications, Dr. Fritz Faulhaber developed a technique for producing motors which did not require the use of iron laminations in the armature construction. This motor type became known as the ‘coreless’ or ‘ironless core’ variety. Coreless motors
depend upon the magnet wire itself to provide structural integrity for
the armature. The coil wire has a two-component insulation, one part
thermoset plastic and the other thermoplastic. The coils are wound on a
winding fixture (or mandrel). After winding, they are baked on the
mandrel at an appropriate temperature to cause the insulation of
adjacent wires to fuse. The result is a coil that holds its shape
without requiring the use of structural members such as iron
laminations. The secret to winding such coils is to design a winding
that not only positions the appropriate conductors in the air gap of the
motor to produce torque but also has sufficient rigidity to provide a
self-supporting structure. |
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01/09/2006 |
