Most induction motors used in VSD applications including Web Handling are constructed with a “Squirrel Cage” rotor. The conductors in the rotor have the shape of the rotating hamster cage exercise machine as in “The wheel is turning, but the hamster is dead."
The rotor conductor geometry is determined by cutouts or slots in the steel laminations which are stacked on the motor shaft to form the rotor. Once the laminations are accurately stacked and pressed, the rotor conductors are cast in place. Usually, aluminum is cast into the rotor. High-efficiency motors may cast copper. The casting includes rings connecting all the conductors at each end. Casting conductors is a very cost-effective way of forming them in specific geometries. See the attached diagram showing a slotted rotor lamination and the resulting squirrel cage.
The steel laminations are designed for high resistance and have little effect on the electrical conduction of the rotor.
The shape and cross-sectional area of the conductors in the rotor determine the torque available to the motor at various levels of rotor slip. This affects starting and steady state torque. Once the conductor design is settled, the characteristics of the motor are determined. That includes the important variables including cost, starting torque, inductance, losses, and difficulty of manufacturing.
Rotor slots have a great deal of variety.
The link to the Wolfram Demonstrations Program AC Induction Motor Rotor Design shows the torque curve with changes made to the rotor resistance, reactance and blocked rotor voltage. Watch the Web Preview.