One main contribution from this WP has been to characterise the breakdown voltages for oil films in rolling element bearings when differences in electric potential (i.e. voltage) come to exist between a rotor and the stator in which it is supported. For ball bearings, this breakdown phenomenon depends on spin speed, axial load, radial load and system temperature. We have observed a strange “hysteresis” effect wherein the films become very vulnerable to breakdowns after some have been initiated but they actually recover again if the voltage is removed. Typically, the voltage levels required to initiate significant breakdowns of the oil-films are in the range 3V – 20V.
The figures below show the experimental setup used to investigate the breakdowns and an example of the results.
Schematic arrangement of components in the investigation of the electrical breakdown of oil-films.
EMIF Configuration #1: Investigating oil-film electrical breakdown in rolling-element bearings
Breakdown results: Voltages across the bearing on top row and currents through bearing below.
From the outset, the main thrust of WP6 was intended to be focused on the ability to generate transverse forces (forces perpendicular to the axis of rotation) within electrical machines such that those forces might assist in suppressing vibrations of the rotor. The primary purpose of every rotating electrical machine is to cause torque to exist between the rotor and the stator so that power is converted between electrical and mechanical forms. The torque comes to exist through the presence of a magnetic field and even small perturbations in that magnetic field can cause sideward forces to exist without significantly changing the torque. These net sideward forces are generically termed “UMP” (Unbalanced Magnetic Pull).
The central contribution of WP6 has been the development and application of a facility for exploring the capability of (cylindrical-airgap) electrical machines to produce transverse forces that are helpful in suppressing vibration. Such forces can be contributed either actively (driving currents that deliberately cause perturbations in the machine’s magnetic field) or by simply “allowing” currents to flow that are induced primarily by small disturbances in the symmetry of the machine. A versatile 24-tooth machine has been designed and wound such that by connecting the coils of the stator in different ways, different strategies for causing UMP to exist can be explored. When the CornerStone project began, we had in mind to examine Bridge Configured Windings and The Chiba Arrangement. During the course of the project, we discovered another very credible approach … The Symmetric Windings approach. The winding diagram for this 24-tooth (4-pole) stator is shown below:
Winding Diagram for the 24-tooth 4-pole motor for demonstrating controllable UMP.
EMIF Configuration #2. A facility for generating and measuring controllable UMP.
To complement the experimental investigations, a new electromagnetic FEA facility has been created that enables us to analyse the rotordynamics of coupled electromechanical systems very conveniently. Named KKFEA, (because the initial work on this package was undertaken by researcher Karuna Kalita), this facility uses structured meshable regions to generate a motor model with minimum work and uses a matrix representation of the winding patterns within the model. The figure below shows a model for a “6-9” machine – a fairly common electrical machine design.
KKFEA model of a specific type of 6-pole permanent-magnet electrical machine.
A key characteristic of KKFEA is that the rotor model and the stator model are each “reduced” so that the retain only those degrees of freedom (magnetic potentials) at the airgap. This enables the analyses to run extremely rapidly. A downside to this model-reduction is that the materials are treated as being magnetically linear. Although this has some small influence on the actual patterns of magnetic flux at the airgap, it does not cause strong errors in the predicted values for either torque or transverse force. KKFEA is intended only as a design-support tool. A full non-linear analysis should always be undertaken as a final design verification step. The figures below shows good agreement between models done using ANSYS and those performed using KKFEA.
Comparison between transverse force predictions from KKFEA vs. ANSYS
Comparison between torque predictions from KKFEA vs. ANSYS
COVID was as unwelcome in the CORNERSTONE project as it was everywhere else. It slowed progress in a number of ways but has not deflected us from the main objectives and neither has it undermined the value of the work. Clearly, the virus had a major negative impact on the aerospace industry in the short term and no aerospace company or airline went unscathed in this. It is a significant tribute to the far-sightedness of Rolls-Royce and to the importance of the work being done that the company continued to honour its original commitments to fund the programme notwithstanding the adverse conditions. One other positive that has at least some linkage to COVID is that society in general became accustomed that very significant changes in practice can happen when they have to and with climate change now wreaking havoc regularly around the world, there has been a realisation that the nettle of decarbonising aerospace must be grasped sooner rather than later. CORNERSTONE set out “to replace design-by-evolution with design-by-revolution” and the methods being developed within the programme have become proportionately more important and more urgently needed as a consequence.
The main negative effect arose from the need to work from home for many months and avoid visits. Two CORNERSTONE conferences proceeded notwithstanding (one fully virtual and one hybrid physical-virtual) but the ability to visit between the universities was severely hampered. Laboratory work suffered due to the direct effect of being unable to have people present and due to the indirect effect on suppliers. A secondary effect that arose in at least two cases was that researchers appointed to the project could not join/return due to travel restrictions. Notwithstanding these difficulties, a comprehensive experimental programme has been put underway in the case of most workpackages and will deliver important results in each one of those. The consortium is extremely grateful to EPSRC for the accommodation of an extension in the end date of the award to April 2023. Without this accommodation, the negative effects would have been significant.