Superconducting Electrical Machines
Bulk Superconductivity Group, Department of Engineering
Dr Mark Ainslie
Royal Academy of Engineering Research Fellow
4th IOP Superconductivity Summer School, Oxford, UK, July 2016
Presentation Outline
• Electrical machines
• Three-phase & rotating fields
• Types of machines
• Synchronous, induction machines
• Superconducting electrical machines
• Case studies/examples with technical challenges & results
• Use of high temperature superconducting (HTS) wire
• Use of bulk HTS materials
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Superconducting Electrical Machines
• Approx. one third of electricity consumed by industry [1]
• Approx. two thirds of this consumed by electric motors [2]
• Using superconductors can increase electric / magnetic loading of an electric machine
• Torque proportional to these + active volume
• Higher current density, higher magnetic field increased torque/power density reduced size & weight
• Lower wire resistance lower losses & higher efficiency / better performance
• Bulk superconductors >> permanent magnets
[1] International Energy Agency, “2013 Key World Energy Statistics”, http://www.iea.org/publications/freepublications/publication/KeyWorld2013.pdf [2] ABB, “ABB drives and motors for improving energy efficiency,” 2010
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Electrical Machines
• Electrical machines = motors & generators
• Motors: Convert electrical power mechanical power
• Generators: Mechanical power electrical power
• Huge range of sizes, power ranges & applications
• Sizes: Nanometres up to 10s of metres
• Power ranges: µW up to GW
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Electrical Machines - Applications
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Washing machine
Hydroelectric generator
Extractor fan
Wind power generation
Computer fan
Synchronous generator Hobby motors
4th IOP Superconductivity Summer School 2016
Electrical Machines
• Generally operate via interaction of current-carrying conductors & magnetic fields
• Various classifications based on how this interaction occurs:
• Alternating Current (AC) / Direct Current (DC)
• Synchronous machines
• Induction machines
• Hysteresis machines
• Switched reluctance machines
B S G 4th IOP Superconductivity Summer School 2016
Electrical Machines
• Generally operate via interaction of current-carrying conductors & magnetic fields
• Various classifications based on how this interaction occurs:
• Alternating Current (AC) / Direct Current (DC)
• Synchronous machines
• Induction machines
• Hysteresis machines
• Switched reluctance machines
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Most widely used machines Usually “three-phase AC”
4th IOP Superconductivity Summer School 2016
Three-Phase Systems & Rotating Fields
• Three-phase is a common method of AC electric power generation, transmission & distribution
• Economical & efficient polyphase system
• Good compromise between complexity, no. of conductors, power transmitted & cost
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Three-Phase Systems & Rotating Fields
• Allows generation of a rotating magnetic field
• Basis for three-phase generators & motors
• Balanced three-phase currents flowing in a balanced three-phase winding produce a rotating field of constant magnitude!
B S G 4th IOP Superconductivity Summer School 2016
Three-Phase Systems & Rotating Fields
• Allows generation of a rotating magnetic field
• Basis for three-phase generators & motors
• Balanced three-phase currents flowing in a balanced three-phase winding produce a rotating field of constant magnitude!
Phase Angle (α) Current (I) A 0° I cos ωt
B –120° I cos (ωt – 120°)
C –240° I cos (ωt – 240°)
B S G 4th IOP Superconductivity Summer School 2016
Three-Phase Systems & Rotating Fields
• Allows generation of a rotating magnetic field
• Basis for three-phase generators & motors
• Balanced three-phase currents flowing in a balanced three-phase winding produce a rotating field of constant magnitude!
Phase Angle (α) Current (I) A 0° I cos ωt
B 120° I cos (ωt – 120°)
C –120° I cos (ωt – 120°)
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Three-Phase Systems & Rotating Fields
B S G 4th IOP Superconductivity Summer School 2016
Three-Phase Systems & Rotating Fields
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Electrical Machines – Synchronous Machines
• Synchronous machines
• MWs – 100s of MW, used in most large scale power generation, fixed speed applications in mills, factories, etc.
• Rotor rotates in synchronism with line frequency/stator rotating field
• ns [rpm] = 60 f / p f = line frequency, p = no. of pole pairs
e.g., n = 60*50/1 = 3000 rpm
• Frequency of induced generator voltage ↔ rotor/machine rpm
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Electrical Machines – Synchronous Machines
• Synchronous machines
• Rotating rotor winding can be
• DC field winding = direct current (“excitation”) fed into winding
• Requires slip rings/brushes
• Can also use permanent magnets
• More costly (economics), but no slip rings/brushes required (maintenance)
• Varying excitation for reactive power compensation not possible
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Electrical Machines – Synchronous Condensers
• Synchronous condensers
• Most industrial loads are inductive (“lagging”) by nature
• Draws excess current larger capacity equipment, more line losses, penalties from electricity supply companies
• Varying excitation of rotor windings (under- or over-excited) absorbs or supplies reactive power (VARs)
• Can drive the mechanical load & improve power factor*
*Power factor = ratio of real power to apparent power. A low power factor draws more current for same amount of useful (real) power transferred.
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Electrical Machines – Induction Machines
• Induction (asynchronous) machines
• Induction motor is known as the workhorse of industry
• Simple construction, low cost, robust, variable speed possible, long operating lifetime
• Induction generators less utilised (lower efficiency)
• Used in wind turbines, small hydro power generation
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Electrical Machines – Induction Machines
• Rotor rotates asynchronously with stator rotating field
• Rotor can be wound or “squirrel cage”
• Slip, s = (ns – nr) / ns
• No load, nr ≈ ns
• Slip increases with load/torque
• Variable speed operation with electronic control
• Torque-speed curves
B S G 4th IOP Superconductivity Summer School 2016
Electrical Machines – Induction Machines
• Rotor rotates asynchronously with stator rotating field
• Rotor can be wound or “squirrel cage”
• Slip, s = (ns – nr) / ns
• No load, nr ≈ ns
• Slip increases with load/torque
• Variable speed operation with electronic control
• Torque-speed curves
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Electrical Machines – Induction Machines
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X1: stator leakage reactance R1: stator winding resistance RI: iron loss resistance Xm: magnetising reactance X2: rotor leakage reactance (referred) R2/s: rotor winding resistance (referred)
Electromagnetic torque:
Maximise power dissipated in R2/s component
Torque-speed curves:
4th IOP Superconductivity Summer School 2016
Superconducting Electrical Machines
• Over many decades, various superconducting machines shown to be technically feasible over wide range of power ratings
• First attempted in the 1960s, replacing copper windings with LTS
• Although improved efficiency (about 1%) was expected, the main rationale was the size/weight reduction
• Operated at liquid helium temperature (4 K)
• Complexity & cost of 4 K cryogenics prohibitive
• Large AC losses in armature winding unacceptable heat load
• Only DC field winding feasible
• Stationary room-temperature armature + rotating SC field winding (cooled) + electromagnetic shield to attenuate AC field
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Superconducting Electrical Machines
• Discovery of HTS materials in 1987 renewed enthusiasm
• Expectation that materials could be exploited at higher temperatures, e.g., 77 K
• Reduced capital costs for & complexity of refrigeration system
• Refrigeration efficiency ~100 times greater than at 4.2 K
• Carnot efficiency (ideal): 1 W heat = 2.8 W @ 77 K 68 W @ 4.2 K
B S G 4th IOP Superconductivity Summer School 2016
Superconducting Electrical Machines
• Discovery of HTS materials in 1987 renewed enthusiasm
• Expectation that materials could be exploited at higher temperatures, e.g., 77 K
• Reduced capital costs for & complexity of refrigeration system
• Refrigeration efficiency ~100 times greater than at 4.2 K
• Carnot efficiency (ideal): 1 W heat = 2.8 W @ 77 K 68 W @ 4.2 K
B S G 4th IOP Superconductivity Summer School 2016
Superconducting Electrical Machines
• Discovery of HTS materials in 1987 renewed enthusiasm
• Expectation that materials could be exploited at higher temperatures, e.g., 77 K
• Reduced capital costs for & complexity of refrigeration system
• Refrigeration efficiency up to 100 times greater than at 4.2 K
• Carnot efficiency (ideal): 1 W heat = 2.8 W @ 77 K 68 W @ 4.2 K
• Multiplication factor incl. cryocooler inefficiency: 20-50 @ 77 K
B S G 4th IOP Superconductivity Summer School 2016
Superconducting Electrical Machines
• Discovery of HTS materials in 1987 renewed enthusiasm
• Liquid nitrogen is inexpensive (cheaper than milk!), inert, easy to use & store, & readily available
• Improved thermal properties:
• Assuming Cu stabiliser, specific heat increases
• Critical heat flux (nucleate film boiling) much higher for 77 K than 4 K
• For 2G HTS (YBCO), improved in-field critical current density
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In-Field Performance of Various Materials
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Data from NHFML: http://fs.magnet.fsu.edu/~lee/plot/plot.htm
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Superconducting Electrical Machines – AMSC
• American Superconductor (now AMSC) HTS ship propulsion motors
• U.S. Navy moving towards all-electric ship systems incl. propulsion
• Requirements difficult to achieve using conventional technology
• 2001-2004: 5 MW / 230 rpm [1]
• 2003-2008: 36.5 MW / 120 rpm [2]
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[1] G. Snitchler et al., IEEE Trans. Appl. Supercond. 15 (2005) 2206–2209. [2] B. Gamble et al., IEEE Trans. Appl. Supercond. 21 (2011) 1083–1088.
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Superconducting Electrical Machines – AMSC
• ROTOR (AMSC)
• HTS field winding at 32 K
• BSCCO-2223 (1G) wire
• Helium gas cooled, external cryocooler module
• EM shield
• STATOR (ALSTOM)
• Dielectric oil cooled Litz wire
• Air-gap winding, non-magnetic support structure
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Superconducting Electrical Machines – AMSC
• Rotor integrated with stator at Alstom for full factory testing (2003)
• Full load, full speed testing completed • Operated for 21 hours at
Center for Advanced Power Systems, FSU (2005)
• Achieved specified performance & power ratings under full operating conditions
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5 MW, 230 rpm HTS Motor (left) with 2.5 MW load motor (right)
at ALSTOM, UK
4th IOP Superconductivity Summer School 2016
Superconducting Electrical Machines – AMSC
• Extended to 36.5 MW HTS motor
• 14:1 increase in torque over 5 MW machine
• Passed full power tests by end of 2008
• Achieved specific target of 75 metric tonnes
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Superconducting Electrical Machines – Other Efforts
• Japanese Super-GM program, 70 MW-class superconducting generators (LTS)
• Siemens developed a 380 kW motor, extended to a 4 MVA generator
• Sumitomo Electric 30 kW motor for electric passenger car
• Converteam 1.7 MW hydroelectric power generator
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SIEMENS SUMITOMO CONVERTEAM
4th IOP Superconductivity Summer School 2016
Superconducting Induction Machine
• High temperature superconducting induction-synchronous machine (HTS-ISM)
• Developed at Kyoto University
• Target: electric vehicle drive motor
• Replace rotor windings of squirrel cage induction machine with HTS materials
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Superconducting Induction Machine
• Current induced in rotor winding at slip frequency, sf
• s = 1 (standstill) s 0 (near synchronous speed)
• Large current induced when starting flux-flow resistivity (E/J) large starting torque
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Test bench for Kyoto University’s HTS-ISM, including load
(permanent magnet) motor
4th IOP Superconductivity Summer School 2016
Superconducting Induction Machine
• Current induced in rotor winding at slip frequency, sf
• s = 1 (standstill) s 0 (near synchronous speed)
• Large current induced when starting flux-flow resistivity (E/J) large starting torque
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Superconducting Induction Machine
• As motor accelerates, s and rotor resistance reduces small resistance at low slip
• At s = 0, magnetic flux linked between rotor bars is trapped (induced persistent current) synchronous torque at zero slip
• Hence, coexistence of synchronous and slip modes robust against overload conditions, dynamic switching between modes
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Bulk High Temperature Superconductors
• Can be utilised in machines in three ways:
• Flux shielding (reluctance)
• Flux pinning (hysteresis)
• Flux trapping (trapped flux)
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A large, single grain bulk superconductor
4th IOP Superconductivity Summer School 2016
Bulk High Temperature Superconductors
• RELUCTANCE MOTOR
• Difference in permeability in direct (‘easy’ path) & quadrature (‘difficult’)
• Rotor aligns itself with direct axis
• Torque, T, proportional to difference of flux in direct & quadrature axes
• T, SC machine > conventional machine (magnetic & non-magnetic interleaving)
B S G Barnes et al. Supercond. Sci. Technol. 13 (2000) 875-878
d axis
q axis
4th IOP Superconductivity Summer School 2016
Bulk High Temperature Superconductors
• RELUCTANCE MOTOR • Based on flux shielding property
• Combines bulk SC + ferromagnetic material
• Bulk SC shields flux, reinforcing ferromagnetic material
• Can show flux pinning / hysteresis motor-like behaviour
• Disadvantages
• Increase in torque only up to + ~1/3 of conventional
• Use of iron limits flux density < 2 T; usually much less than, but up to 1 T air gap field
B S G Kovalev et al. Supercond. Sci. Technol. 15 (2002) 817-822
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Bulk High Temperature Superconductors
• HYSTERESIS MOTOR
• Based on flux pinning property
• Normal hysteresis motor has ferromagnetic rotor torque produced by interaction between stator/driving field + magnetisation of rotor by this field
• Like induction motor, ‘slippage’ (lag) between driving field & rotor
• Lag independent of speed, so constant torque from start-up to synchronous speed
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Bulk High Temperature Superconductors
• HYSTERESIS MOTOR
• Type II superconductor exhibits magnetic hysteresis = pinned flux lines
• Approaching synchronous speed/steady state, behaves like synchronous motor
• Main magnetic field must be produced by stator windings
• Low operating power factor, efficiency, torque density
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Bulk High Temperature Superconductors
• Conventional magnets (NdFeB, SmCo) limited by material properties
• Magnetisation independent of sample volume
• Bulk HTS trap magnetic flux via macroscopic electrical currents
• Magnetisation increases with sample volume
• Trapped field given by
Btrap = A µ0 Jc d
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A large, single grain bulk superconductor
4th IOP Superconductivity Summer School 2016
Bulk High Temperature Superconductors
• Demonstrated trapped fields over 17 T
• 17.24 T at 29 K 2 x 26.5 mm YBCO Tomita, Murakami Nature 2003
• 17.6 T at 26 K 2 x 25 mm GdBCO Durrell, Dennis, Jaroszynski, Ainslie et al. Supercond. Sci. Technol. 2014
• Significant potential at 77 K • Jc = up to 5 x 104 A/cm2 at 1 T • Btrap up to 1 ~ 1.5 T for YBCO • Btrap > 2 T for (RE)-BCO
• Record trapped field = 3 T at 77 K • 1 x 65 mm GdBCO
• Nariki, Sakai, Murakami Supercond. Sci. Technol. 2005
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Stack of 2 x GdBCO samples that achieved 17.6 T at 26 K
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Rotating Machines Using Bulk HTS
• Recent topical review in SUST • An overview of rotating machines
with high-temperature bulk superconductors
• D Zhou, M Izumi et al. Supercond. Sci. Technol. 25 (2012) 103001
• Overview of bulk HTS machine development • Tokyo University of Marine
Science and Technology (TUMSAT) & other groups Comparison of radial
and axial machines
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Bulk HTS Axial Flux Motor
• Axial gap, trapped flux-type motor
• Advantages:
• Higher torque/power density
• Compact ‘pancake’ shape
• Better heat removal
• Adjustable air gap
• Multi-stage machines possible
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TUMSAT prototype motor for ship propulsion
4th IOP Superconductivity Summer School 2016
Bulk HTS Axial Flux Motor
• Uses stator coils to magnetise HTS bulks with pulsed field
• Cooled using liquid nitrogen
• Dual purpose: magnetising coils, then armature winding
• Closed cycle neon thermosyphon system
• Includes cryo-rotary joint
• Cryogen from static condenser to rotating rotor plate with bulk HTS
• Allows cooling of bulks HTS down to below 40 K
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Schematic diagram of TUMSAT prototype motor
4th IOP Superconductivity Summer School 2016
Bulk HTS Axial Flux Motor
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Magnetisation of Bulk Superconductors
• Three magnetisation techniques:
• Field Cooling (FC)
• Zero Field Cooling (ZFC)
• Pulse Field Magnetisation (PFM)
• To trap Btrap, need at least Btrap or higher
• FC and ZFC require large magnetising coils
• Impractical for applications/devices
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ZFC FC
4th IOP Superconductivity Summer School 2016
Pulsed Field Magnetisation of Bulk Superconductors
• Achieving in-situ magnetisation is crucial for trapped-flux-type rotating machines
• PFM technique = compact, mobile, relatively inexpensive
• Issues = Btrap [PFM] < Btrap [FC], [ZFC]
• Temperature rise ΔT due to rapid movement of magnetic flux
• Record PFM trapped field = 5.2 T at 29 K (45 mm diameter Gd-BCO) [Fujishiro et al. Physica C 2006]
• Many considerations:
• Pulse magnitude, pulse duration, temperature, number of pulses, shape of magnetising coil(s)
• Dynamics of magnetic flux during PFM process
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Future Views & Prospects
• Significant body of work 2G HTS (RE)BCO coated conductor, MgB2
• Most designs have focused on isolated, cryogenic rotor + conventional stator
• Low ac loss conductor and/or improved winding/machine design
• All-cryogenic / all-superconducting solutions with unprecedented power densities
• Will reduce complexity, improve reliability
• Cost still a major issue as identified by large-scale projects
• Appropriate infrastructure & knowledge required for large-scale manufacture
• Superconducting materials & cryogenic/vacuum systems need to be available on an industrial level
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Future Views & Prospects
• Superconductor Science & Technology ‘Focus Issue on Superconducting Rotating Machines’
• Contributions on a variety of topics:
• Novel topologies (claw-pole, homopolar machines)
• Wind turbines (MgB2 field winding & ac loss analysis)
• Bulk-based machines (TUMSAT review)
• HTS stator (BSCCO) thermal analysis
• Brushless HTS-PM exciter for rotating DC field winding (flux pump)
• Magnetic gears (HTS conductors)
Available online: http://iopscience.iop.org/0953-2048/focus/Focus-on-Superconducting-Rotating-Machines
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Presentation Outline
• Electrical machines
• Three-phase & rotating fields
• Types of machines
• Synchronous, induction machines
• Superconducting electrical machines
• Case studies/examples with technical challenges & results
• Use of high temperature superconducting (HTS) wire
• Use of bulk HTS materials
B S G 4th IOP Superconductivity Summer School 2016