Comparison of the Performance of Different Asynchronous Solid-Rotor Constructions in a Megawatt-Range High-Speed Induction Motor

Abstract

High-speed electrical machines have their advantages, such as higher power density than in traditional electrical machines, and disadvantages, such as very demanding and tailored design and manufacturing process. The manufacturing of the rotor for a high-speed electrical machine is extra complex. In a high-speed electrical machine, the rotor in normal operation experiences various types of loadings, such as centrifugal forces from the rotation, temperature gradients from the operational environment, losses generated in the electrical machine and loads from the process. A failure related to the mechanical integrity of a rotor leads to a halt and very probably to the complete malfunction of the machine. The aim of this study is to compare three different rotor constructions in a high-speed induction electrical machine operating at the megawatt power range. The first one is a full copper squirrel cage rotor that consists of 25 pieces (22 copper bars, two copper end rings and a steel shaft), the second is a slitted solid rotor with copper end rings that consists of three pieces and two materials, and the third is a slitted solid rotor made solely of structural steel consisting of one piece. Further, these rotors are compared in terms of their electromagnetic performance, applying the same stator structure. The manufacturing challenges are directly related to the number of individual parts needed for the construction of the full rotor. The base material for the rotor core of the studied high-speed induction machine is often structural steel that is magnetically soft, has moderate electrical resistivity, and can be combined with copper or aluminum cage winding which is non-magnetic and has high electrical conductivity. From the mechanical perspective, these materials are very different; e.g. high-strength copper (CuCrZr) has a 50 % higher thermal expansion coefficient, 13 % higher density, 600 % higher thermal conductivity, and approximately 50 % lower yield strength than common structural steel (S355). This complicates the rotor manufacturing, as the required tolerances for the assembly of the rotor components are very strict and depend on the final manufacturing method of the rotor.