| Summary: | The School of Electrical and Electronic Engineering at the North-West University is in the
process of developing an Active Magnetic Bearing (AMB) research laboratory. The aim is to
establish a knowledge base on AMBs in support of industries that make use of this
environmentally friendly technology. AMB technology is seen as one of the technology drivers
for the Pebble Bed Modular Reactor (PBMR) currently in development in South Africa and is
predicted to become largely conventional in this application.
In the process of developing an AMB laboratory some basic models are constructed to establish
infrastructure for research investigations. The aim of this project is to develop a flexible rotor
double radial AMB system. The system comprises a laminated heteropolar magnetic actuator,
eddy-current position sensors, switch-mode power amplifiers and a digital controller. Emphasis
is placed on stable suspension of a flexible rotor through the first three critical frequencies. This
project also caters for future work on high speed losses in AM6 systems.
A design process comprising aspects of modelling and analysis is developed, implemented and
verified for a flexible rotor AMB system. The design commences with a system specification
followed by an iterative process comprising electromagnetic design, detailed system modelling
and rotordynamic analysis, and is concluded with design implementation and verification.
The system design includes two interchangeable rotors; a flexible rotor for rotordynamic
analyses and a rigid rotor for high speed loss analyses. The flexible rotor system is specified to
experience the first three critical frequencies up to an operating speed of 10,000 rpm. The rigid
rotor maximum operating speed is specified as 30,000 rpm. Rotor stability at critical frequencies
places specific constraints on the equivalent stiffness and damping parameters of the AMB.
An iterative design process is then initiated by an analytical electromagnetic design of the radial
AMBs conducted in MathCAD® The magnetic actuator utilizes a 0.6 mm air gap and has a
maximum load capacity of 500 N. A force slew rate specification of 5x10~N /s is obtained from
the system's equivalent stiffness (500 N/mm) and damping (2.5 N.s/mm) parameters resulting in
a 3 kVA power amplifier requirement. These parameters are used in the detailed MATLAB®
modelling of the system. Stiffness and damping parameters as well as system dynamic
response are verified and used to design a flexible rotor. The magnetic bearing locations,
displacement sensor locations and rotordynamic response are verified using finite element
methods. The design of the rotor stands central to the iterative design process since it impacts
on the forces experienced by the AMBs as well as the critical frequencies of the AMB system.
The most important outcome of the iterative design process is a dimensioned electromagnetic
configuration and two rotor designs. The flexible rotor spans 500 mm and weighs 7.72 kg
whereas the rigid rotor has the same length and weighs 12.5 kg. A centre mass on the flexible
rotor lowers the first three critical frequencies to below the maximum operating speed.
A 3 kVA (300 V, 10 A) switch-mode, current controlled power amplifier (PA) is developed in-house
as part of the outcome of the study. The topology used is a two-quadrant controlled
H-bridge, switched at 100 kHz and controlled in current-mode. The design is thoroughly verified
through a process of prototyping and includes aspects of electromagnetic compatibility and
protection in terms of over-current and temperature. The PA exhibits a 6 kHz bandwidth and
linear characteristics and plays a critical role in the AMB system performance.
The AMB controller is realised with a dSPACE® real-time development tool (DS1104), located
inside a personal computer (PC). The rotational speed is monitored with an optical speed
sensor while the shaft is propelled via an air turbine unit.
Once constructed the actual AMB stiffness and damping parameters as well as its dynamic
response are obtained. Discrepancies between the analytically predicted, simulated and
experimentally obtained results are addressed and clarified. The sensitivity of the system to
parameter changes is obtained as a measure of marginal stability. The rotordynamic response
is characterised by measuring the rotor displacement at pre-defined locations as the rotor
traverses the critical frequencies. These results show good correlation with the predicted
rotordynamics.
This study emphasises the importance of extensive modelling and analyses in the design of
AMB systems to guarantee the required performance of the end product in terms of its dynamic
performance and stability. The most important outcome of this project is a working high speed
AMB model complete with integrated control. The system is versatile and allows for a variety of
investigations including advanced control investigations and high speed magnetic bearing loss
analyses. This project uniquely contributes to the research currently underway in the field of
AMBs in the School of Electrical and Electronic Engineering. === Thesis (M. Ing. (Electrical and Electronic Engineering))--North-West University, Potchefstroom Campus, 2005.
|
|---|
