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EWTEC Conference

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on 10 April 2014

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Transcript of EWTEC Conference

Unbalanced Forces in Electrical Generators for Wave and Tidal Devices
Global Exchange Project
Institute for Energy Systems, School of Engineering, The University of Edinburgh, United Kingdom

Department of Systems and Naval Mechatronic Engineering, National Cheng Kung University, Taiwan

School of Electrical, Mechanical and Mechatronic Systems, University of Technology Sydney, Australia
Kaswar Mostafa, Markus A. Mueller and Jonathan K.H. Shek
David G. Dorrell
Minfu Hsieh, I-Hsien Lin and Yu-Han Yeh
Chair of Electrical Machines
Institute for Energy Systems,
School of Engineering,
The University of Edinburgh,
King's Buildings,
Mayfield Rd,
Edinburgh EH9 3JL
Prof. Markus A Mueller
Kaswar Mostafa
PhD Student
Institute for Energy Systems,
School of Engineering,
The University of Edinburgh,
King's Buildings,
Mayfield Rd,
Edinburgh EH9 3JL
Marine energy is one of the fastest growing sectors in the energy market and continues to grow annually despite recent global financial decline.
The cost of installation and operation of wind farms offshore has been expensive.
This is due, in part, to the early-life failures related to the drive-train in wind turbines
Downtime for different components.
Gearbox, drive-train and generator are the most common components to cause failure.
Drive train related failure is typically linked to bearing wear, caused by misalignment in the drive train.
That leads to lost revenue and increases the cost of electricity produced.
Diagram showing static and dynamic eccentricity. The solid line shows the centre of the stator bore and the dotted line shows the rotor axis of rotation.
Rotor eccentricity is a non-uniform air gap between the rotor and stator of an electrical machine due to the displacement of the rotor in the radial direction.
There are two types of eccentricity: static eccentricity, where the rotor rotates on its own axis but not in the centre of the stator bore, and dynamic eccentricity, where the rotor is centred in the stator bore but does not rotate on its own axis.
Schematic draw for static rotor eccentricity
Schematic draw for dynamic rotor eccentricity
Rotor Eccentricity
External radial force

Design tolerances
Inaccurate assembly of the generator’s components
Asymmetric magnetizing of the magnet rings
Unbalanced Magnetic Pull (UMP)
UMP is produced due to an asymmetric air gap in the electrical generator
UMP is a radial force pulling the rotor even further off the center because of the flux concentration where the air gap is narrowest
UMP can be classified into two types: Extrinsic UMP & Intrinsic UMP
Mechanical Wear
Block diagram of the cause and effect of rotor eccentricity in relation to bearing wear
Permanent Magnet Machines
Cross-section of an 11kW permanent magnet machine with concentric rotor
No-load case
Loaded case
No-load cases for permanent magnet machines are simple to model and simulate.
Loading cases are more complex to simulate and required additional instruction using a scripting language.
Two-dimensional finite element method (FEM) open-source software was used.
The meshing size is controllable and 30° minimum meshing angle has been chosen for accurate analysis.
Graph of UMP versus rotor eccentricity for the modelled generator
Flux density distribution in the air gap with 40% eccentricity
The relationship between static rotor eccentricity and UMP for the studied generator using FEM software is linear.
The slotting effect on the flux density distribution is clear.
The effect of armature reaction on UMP, with different load cases and different rotor eccentricities.
Spatial UMP variations for one electrical cycle with 10% eccentricity and 5A stator current.
The effect of armature reaction on UMP in both parallel and series winding connection cases with both static and dynamic eccentricities seems to be very small and can be neglected.
The direct current amplitude for each coil has been calculated and applied automatically for each rotor rotation step using the LUA scripting language.
The spatial UMP variation is mainly due to the slotting effect.
Block diagram of the experimental test rig for generator force measurement
The test generator is driven by a motor with a torque transducer interfacing the two machines to measure the input mechanical power.
The output of the generator is connected to a 3-phase load to control the power output from the generator.
Measurement data from the torque transducer, force sensors, and power analyzer all feed into a data acquisition unit.
Creating Eccentricity
For experimental purposes this can be created by physically moving the rotor in the radial direction relative to the stator.
Electrical machines typically have a rotor which is held in position by end caps mounted to the main body of the machine.
Bearings in each end cap allow the rotor to rotate freely inside the machine.
The rotor should be mounted independently of the main body by removing the end caps and mounting the rotor on external bearings.
Eccentricity can be created by adjusting the stator position and remain the rotor aligned to the rest of the drive-train.
That can be easily achieved by inserting shims between the main body of the generator and the platform to which it is secured.
Force Measurement
The most widely used force transducers use either strain gauge or piezoelectric-based technology.
Piezoelectric force transducers were chosen due to their superior stiffness and characteristics under dynamic loading.
Actual Test Rig
Diagram showing the rotor mounted on external bearings and force sensors beneath the bearing units
Diagram showing the rotor mounted on external bearings and force sensors beneath the bearing units
Multi-axis transducers were chosen because it would have been a more cost-effective option.
Transducers can be mounted under the rotor or the stator, supporting the full weight of either in order to measure the unbalanced force.
This paper has presented an FEM model of a permanent magnet generator to investigate UMP due to stator rotor eccentricity.
It also has also presented a design for an experimental test rig to measure unbalanced forces due to eccentricity and verify simulation results.
Most of the work presented in the paper is transitional but nonetheless a necessary step towards the understanding of the effect of unbalanced forces on bearings in wave and tidal devices and how wear and failure can be mitigated through design and control.
Finally, The authors would like to thank the Engineering and Physical Sciences Research Council in the UK and the National Science Council in Taiwan for funding this work.
The IEC standard requires the minimum acceptable calculated lifetime for main shaft bearings at 90% reliability to be 175000 hours or approximately 20 million revolutions
UMP forms extra force on the bearing casing a decrease in the life time
Full transcript