Investigation on the Possible Use of Magnetic Bearings in Large Direct Drive Wind Turbines

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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
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Content

• Introduction to Direct Drive Generator
• Problem Statement
• Magnetic Bearing Solution- Concepts
• Single Magnetic Bearing
• Flexible Rotor Structure
• Hybrid Solution
• Preliminary Conclusions

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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
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Introduction to Direct Drive Generator
Scaling laws
Electromagnetic
Cylinder
Arm
Structural part is a significant part of the
generator weight
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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
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Magnetic Bearing Solution- Concepts

• Concept I Single Magnetic Bearing
• Concept II- Flexible Rotor Ring
• Concept III- Hybrid Solution

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Magnetic Bearings
What is magnetic bearing

• 1 dof A pair of controlled magnetic actuator
• Based on principle of attraction between iron and
  magnet

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Single Magnetic Bearing- Motivation

• Hub rigid and relatively large diameter
• Removal of shaft

Magnetic bearing

• Large diameter bearing problem (reliability and
  availability)
• Mechanical bearing tolerance

Diameter of a hub 4m
Source www.harakosan.nl
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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)

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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
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Flexible Rotor Structure- Motivation

• Flexible construction is used to reduce weight
• Examples
• Blades and Tower construction
• Skyscraper

D.D Generator using flexible structure NewGen
Concept 30 of conv. D.D Machine
Maintenance Problem Wheels on Rail
Source S. Angstrom S. Lindgren
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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.
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Flexible Rotor Structure-Bending Modes
Consequence 24 actuators required on the radial
side Too complex to control
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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
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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
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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
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Hybrid Solution
-Magnetic Bearings
Force due to eccentricity 600kN for 4mm
eccentricity
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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)

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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.

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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.