Title: Investigation on the Possible Use of Magnetic Bearings in Large Direct Drive Wind Turbines
1 Investigation on the Possible Use of Magnetic Bearings in Large Direct Drive Wind Turbines G. Shrestha H. Polinder D. Bang A.K. Jassal J.A. Ferreira 2 Content
Introduction to Direct Drive Generator
Magnetic Bearing Solution- Concepts
Single Magnetic Bearing
Flexible Rotor Structure
3 Introduction to Direct Drive Generator Enercon EESG E112
D.D PM has highest energy yield
Lower speed rotation Part count (Long life)
No gears to wear out
PM cost and converter costs are decreasing
Two bearings PT Harakosan PMSG Z72 Single bearing Vensys Leitwind Avantis Goldwind Darwind Mistubishi Siemens Source www.enercon.de www.harakosan.nl 4 Introduction to Direct Drive Generator Scaling laws Electromagnetic Cylinder Arm Structural part is a significant part of the generator weight 5 Problem Statement -Some Solution
Ironless stator Low flux density level
Large diameter bearing Availability and Cost
Wheels on rail (NewGen) Maintenance issue
Use of light structural material (Composite) Cost
Can the use of magnetic bearings reduce the weight of large direct drive generators Important Low Maintenance Large Single Unit Weight of Unit (Reduce overall cost) Magnetic Bearings is non contact type Assumption Weight reduction overweighs cost of magnetic bearings Source S. Angstrom S. Lindgren 6 Magnetic Bearing Solution- Concepts
Concept I Single Magnetic Bearing
Concept II- Flexible Rotor Ring
Concept III- Hybrid Solution
7 Magnetic Bearings What is magnetic bearing
1 dof A pair of controlled magnetic actuator
Based on principle of attraction between iron and magnet
8 Single Magnetic Bearing- Motivation
Hub rigid and relatively large diameter
Removal of shaft
Large diameter bearing problem (reliability and availability)
Mechanical bearing tolerance
Diameter of a hub 4m Source www.harakosan.nl 9 Single Magnetic Bearing- Comparison of Mechanical Magnetic Bearing Replace mech. bearing with mag. bearing Harakosan Z72 choosen as the state of art (1.5MW)
Weight of magnetic bearing is about 22 more than conventional bearing
Losses in both the system is comparable (less than 0.5 of rated power)
10 Single Magnetic Bearing -Scaling Laws Scaling law for force and moments at the hub Wind Energy Explained
Scaling laws for mass and power loss of magnetic bearings
(For constant flux density and current excluding the force due to eccentricity)
Large force at stator and rotor arms at points far from centre support. Self weight Point force Scalability of this concept a problem 11 Flexible Rotor Structure- Motivation
Flexible construction is used to reduce weight
Blades and Tower construction
D.D Generator using flexible structure NewGen Concept 30 of conv. D.D Machine Maintenance Problem Wheels on Rail Source S. Angstrom S. Lindgren 12 Flexible Rotor Structure- Multiple actuator - Reference 5MW Rotor weight 50 tons Flexible Rotor with distributed magnetic bearing 12 Act. Needed based on allowable deflection Rotor weight 10.2 tons Source McDonald et. al. 13 Flexible Rotor Structure-Bending Modes Consequence 24 actuators required on the radial side Too complex to control 14 Hybrid Solution Mechanical bearing takes the wind load Rotor ring made stiff and light using hollow structures i.e. very low deflection due to gravity and magnetic pressure Magnetic bearing keeps the airgap (5 dof) Gravity Eccentricity Torque carrier carries the torque in single direction Power producing torque. Arms are significant part of the generator weight eliminating it reduces the weight. The shaft has room for more flexibility. Use of normal mechanical bearings 15 Hybrid Solution Torque carrier designed with single directional force. (I-Beam) Analogy on axial side Cantilever Beam --------------- Simply Supported on end 23 times less deflection Roark 16 Hybrid Solution-Rotor Ring
Use of hollow and stiff structure
Flat plates to stiffen
First bending mode kept above 40Hz
Ring Radius 4.175m Magnetic Pressure 300kN/m2 Hollow structure in Enercon machine E112 Source www.vetrnaelektrarna.cz 17 Hybrid Solution -Magnetic Bearings Force due to eccentricity 600kN for 4mm eccentricity 18 Hybrid Solution
Total mass (5 MW Rotor) 28 tonnes 45 less weight than comparable rotor given in the reference.
Losses in magnetic bearing Expected to be less than 0.5 of total power.
This can be reduced even further (reducing the HRS thickness)
19 Preliminary Conclusions
Direct drive machine has some good attributes but have some problem for up-scaling in the present state of art.
A hybrid concept with axial flexibility in the generator structure could be the direction to follow.
Even though the scaling laws remain the same the starting value for different concepts differs.
The rotor part has been studied and this can be extended to the stator part also.
20 Thank You for Your Attention Stiff efficient structures like hollow section are already used extensively in direct drive generator design. Further weight reduction maybe achieved by allowing flexibility to the structure
Questions Suggestions Remarks
Acknowledgement This research has been carried out in the framework of the EOS-LT programme of the Ministry of Economic Affairs Netherlands under the contract with SenterNovem.
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