The McTronX research group at the North–West University designs and develops Active Magnetic
Bearings (AMBs). The group’s focus shifted to the design and development of AMB supported drive
systems. This includes the electromagnetic and mechanical design of the electric machine, AMBs,
auxiliary bearings as well as the development of the control system.
The research group is currently developing an AMB supported high speed Induction Machine (IM) drive
system that will facilitate tests in order to verify the design capability of the group. The research
presented in this thesis describes the mechanical design and manufacturing of a high speed IM rotor
section. The design includes; selecting the IM rotor topology, material selection, detail stress analysis
and selecting appropriate manufacturing and assembly procedures.
A comprehensive literature study identifies six main design considerations during the mechanical design
of a high speed IM rotor section. These considerations include; magnetic core selection, rotor cage
design, shaft design, shaft/magnetic core connection, stress due to operation at elevated temperatures
and design for manufacture and assemble (DFMA). A critical overview of the literature leads to some
design decisions being made and is used as a starting point for the detail design. The design choices
include using a laminated cage rotor with a shrink fit for the shaft/magnetic core connection.
Throughout the detail design an iterative process was followed incorporating both electromagnetic and
mechanical considerations to deliver a good design solution. The first step of the iterative design process
was, roughly calculating the material strengths required for first iteration material selection followed by
more detailed interference fit calculations. From the detail stress analysis it became apparent that the
stress in the IM rotor section cannot be calculated accurately using analytical methods. Consequently, a
systematically verified and validated Finite Element Analysis (FEA) model was used to calculate the
interferences required for each component. The detail stress analysis of the assembly also determined
the allowable manufacturing dimensional tolerances. From the detail stress analysis it was found that
the available lamination and squirrel cage material strengths were inadequate for the design speed
specification of 27,000 r/min. The analysis showed that a maximum operating speed of 19,000 r/min can
be achieved while complying with the minimum factor of safety (FOS) of 2.
Each component was manufactured to the prescribed dimensional tolerances and the IM rotor section
was assembled. With the failure of the first assembly process, machine experts were consulted and a
revised process was implemented. The revised process entailed manufacturing five small lamination
stacks and assembling the stack and squirrel cage afterwards. The end ring/conductive bar connection
utilises interference fits due to the fact that the materials could not be welded. The process was
successful and the IM rotor section was shrink fitted onto the shaft.
However, after final machining of the rotor’s outer diameter (OD), inspections revealed axial
displacement of the end rings and a revised FEA was implemented to simulate the effect. The results
indicated a minimum FOS 0.6 at very small sections and with further analytical investigation it was
shown that the minimum FOS was reduced to only 1.34. Although the calculations indicated the FOS was below the minimum prescribed FOS ? 2, the rotor spin
tests were scheduled to continue as planned. The main reasons being that the lowest FOS is at very
small areas and is located at non critical structural positions. The fact that the rotor speed was
incrementally increased and multiple parameters were monitored, which could detect early signs of
failure, further supported the decision.
In testing the rotor was successfully spun up to 19,000 r/min and 27 rotor delevitation test were
conducted at speeds of up to 10,000 r/min. After continuous testing a secondary rotor inspection was
conducted and no visible changes could be detected.
The lessons learnt leads to mechanical design and manufacturing recommendations and the research
required to realise a 27,000 r/min rotor design. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/4940 |
| Date | January 2010 |
| Creators | Ranft, Cornelius Jacobus Gerhardus |
| Publisher | North-West University |
| Source Sets | North-West University |
| Detected Language | English |
| Type | Thesis |
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