Design and Voltage Supply of High-Speed Induction - Aaltodoc

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Fig. 3.5 Cross sections of the copper coated and the squirrel cage solid steel rotors.
The idea behind the design of the squirrel cage solid steel rotors is to combine the mechanical
strength of a solid rotor and the electromagnetic performance of a squirrel cage rotor. A drilling of
slots to the solid steel was considered difficult so the squirrel cage was constructed at the surface of
the rotors, i.e., the rotors had open slots. The down side of the open slot design is the permeance
harmonics and additional loss in the stator teeth. A minor advantage is a lower leakage inductance
of a rotor bar.
Mounting of copper as bars is more difficult than the making of a coating, however. A good contact
between steel and copper is hard to achieve, especially at the sharp corners of the slots. As a
consequence of the problem, the shape of the slot and the amount of the copper are limited. Deep
bars are denied and increasing the number of bars will decrease the total amount of copper. This is a
problem since a fair number of deep bars would be likely to produce an electromagnetically optimal
construction. This problem can be seen also in Fig. 3.5, where a 26 bar rotor could only have 60 %
of the copper compared to that in the other rotors. The amount of copper was even further reduced
because of a bit of careless machine tooling. This reduction of copper had an unfortunate effect on
motor performance, as can be seen later on.
The methods used to measure and to calculate the bending critical speeds are the same as for the
other case presented in the previous section. Mechanical characteristics of the squirrel cage solid
steel rotors are almost identical to the copper coated rotor. The bending stiffness is about the same,
as is seen from Table 3.3. Fig 3.6 shows the output of the calculation software. The mode shapes of
the first two free-free natural frequencies are plotted together with the rotor geometry for the coated
rotor.
The picture shows how the bending modes split into a forward and backward mode. The frequency
of the forward mode is a bit higher than that of the zero speed mode. The forward mode rotates in