Design and Voltage Supply of High-Speed Induction - Aaltodoc

and eddy-currents traveling on the rotor surface see increased impedance. The same effect can be
achieved by using different materials. Pyrhönen and Kurronen (1994) used aluminum-iron alloys to
coat a solid rotor. The surface impedance became high and eddy-current loss was reduced. The
result was even better when a copper layer was put between the aluminum-iron layer and the solid
core. With the copper layer the resistance seen by fundamental currents was reduced. This yields a
lower slip relative to the torque, which is essential in a high-speed machine in order to have a nonsaturated
and efficient operation. This result was also attained by Sharma et al. (1996), when they
tested copper, iron-cobalt and iron-nickel layers on a solid steel rotor.
In conclusion, laminated squirrel cage rotors for high-speed induction motors are preferred because
of better electromagnetic characteristics. With solid rotors, the efficiency tends to be poor and the
machines would need to be larger or increased cooling would be required. However, in high-speed
applications, the mechanical properties of solid materials are well in place. In fact, at a very high
power vs. speed range the historical concept of a solid rotor motor may be the only possible
solution. The mechanical constraints of the rotor are discussed next and the literature review in
Table 3.1 will show that solid steel rotors are adopted widely in high power high speed applications.
3.1 Constraints for a high-speed rotor
In the following, the major constraints for a high-speed rotor are presented on a general level. An
accurate modeling of these phenomena requires specialization in the field not presented here. Some
references are given for the models used in this research project.
The constraints limiting the design of a high-speed rotor could be divided into mechanical and
thermal constraints. These constraints affect the choice of materials used in rotors as well as the
right length/diameter ratio. The thermal constraints relate to the loss components inside the rotor
and in the air gap. They also relate to the choice of the cooling system. The mechanical constraints
often dominate the design. They give limits to the size of the rotor, affect the choice of materials
and set requirements for the quality of manufacture and manufacture techniques. Mechanical as
well as thermal constraints are affected by the application. A good compressor or a spindle tool
design may look different because of different boundary conditions given. The different, often
contradictory constraints for a rotor affect each other so they have to be considered simultaneously
in the design process. Fig. 3.1 illustrates how the different constraints put pressure on the topology
of the rotor.
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