Superconducting Rotating Machines · 2018-11-21 · rotating magnetic field • Basis for...

Superconducting Rotating Machines

Bulk Superconductivity Group, Department of Engineering

Dr Mark Ainslie

EPSRC Early Career Fellow

EUCAS 2017 Short Course: Superconducting Power Applications

Geneva, 17 September 2017

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) conductors

• Use of bulk HTS materials

B S GEUCAS 2017 Short Course: Superconducting Rotating Machinery

Electrical Machines

EUCAS 2017 Short Course: Superconducting Rotating Machinery B S G

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


Electrical Machines

• Approx. one third of electricity consumed by industry [1]

• Approx. two thirds of this consumed by electric motors [2]

• Electric motors & systems they drive are the single largest

electricity end-use [3]

• 43-46% of global electricity consumption

• About 6 Gt of CO2 emissions

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[1] International Energy Agency, “2013 Key World Energy Statistics”, 2013

[2] ABB, “ABB drives and motors for improving energy efficiency,” 2010

[3] International Energy Agency, “Energy-Efficiency Policy Opportunities for Electric Motor Driven Systems,” 2011

[1] & [3] available from http://www.iea.org/publications/freepublications/

EUCAS 2017 Short Course: Superconducting Rotating Machinery

http://www.iea.org/publications/freepublications/

Electrical Machines – Applications

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Washing machine

Hydroelectric generator

Extractor fan

Wind power generation

Computer fan

Synchronous generator Hobby motors


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


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)


• Induction machines

• Hysteresis machines

• Switched reluctance machines

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Most widely used machines

Usually “three-phase AC”


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



• 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!



• 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°)


Electrical 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


Electrical 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



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



• 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 power electronic control

• Torque-speed curves



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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:


Superconducting Electrical Machines



• Using superconductors can increase magnetic / electric 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] S. Huang et al., “A General Approach to Sizing and Power Density Equations for Comparison of Electrical

Machines,” IEEE Trans. Ind. Appl. 34-1 (1998) 92-7


𝑃 =1

1 + 𝐾Φ

𝑚

𝑚1

𝜋

2𝐾𝑒𝐾𝑖𝐾𝑝𝜆0

2𝐵𝐴𝑓𝑚𝑒𝑐ℎ𝐷²𝐿𝜂

𝑃 =𝜋

2ത𝐵 ҧ𝐽𝐷2𝐿𝜔Synchronous (radial) machine

General machine


• 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

• Improved efficiency (about 1%) was expected, but 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



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ROTOR

Superconducting dc field winding (rotating)

External excitation current supply

Liquid helium coolant

High vacuum cryogenic insulation

Normally-conducting eddy current shielding system (conducting shell)

STATOR

Normally-conducting armature winding (stationary)

Cooling system rejecting heat at ambient temperature

Laminated back-iron flux shield

Optimal configuration for ac superconducting machines

from studies using LTS conductors in the 1960s & 70s



• 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




• 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



• Potential savings in weight / volume increased power / torque density

• Particularly attractive for applications where size / weight is a significant cost driver:

• Wind power generation, electric aviation, electric ship propulsion

• Advantages only appear over critical, “break-even” size or rating due to cryogenic system(s)

• I.e., where the refrigeration penalty (size, weight, cost) becomes negligible

• Critical / “break-even” size reduces with increasing operating temperature




• Liquid nitrogen is inexpensive, 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


In-Field Performance of Various Materials

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Data from NHFML: http://fs.magnet.fsu.edu/~lee/plot/plot.htm



– HTS Conductors –


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.



• 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



• 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



• 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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1/2 size

1/3 weight

of traditional copper-based

propulsion systems


Superconducting Electrical Machines – Other Efforts

• Japanese Super-GM program, 70 MW-class superconducting generators (LTS)

• AMSC developed a 8 MVAR (1800 rpm) synchronous condenser

• Siemens 380 kW (1500 rpm) motor, extended to a 4 MVA (3600 rpm) generator

• Sumitomo Electric 30 kW motor for electric passenger car

• Converteam 1.7 MW (214 rpm) hydroelectric power generator

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SIEMENS SUMITOMO CONVERTEAM


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



• 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



• 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



• 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



– Bulk Superconductors –


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



• 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 GBarnes et al. Supercond. Sci. Technol. 13 (2000) 875-878

d axis

q axis



• 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

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Kovalev et al. Supercond. Sci. Technol. 15 (2002) 817-822



• 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



• 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



• 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

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A large, single grain

bulk superconductor



• 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 = k µ0 Jc a

a = radius; k = geometric constant (Biot-Savart law)

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Typical trapped magnetic

field profile above a

bulk superconductor


Bulk HTS Trapped Fields

• Trapped field measurements tell us the potential of a sample as a strong, permanent magnet

• 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


Bulk HTS Trapped Fields

• 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,

neutron-irradiated YBCO

• Record trapped field = 3 T at 77 K

• 65 mm GdBCO

• Nariki, Sakai, Murakami Supercond. Sci.

Technol. 2005


Rotating Machines Using Bulk HTS

Comparison of radial

and axial machines

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Radial-type bulk machine


Rotating Machines Using Bulk HTS

Comparison of radial

and axial machines

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Axial-type bulk machine


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


*Izumi et al. Tokyo University of Marine Science and Technology (TUMSAT)


• 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


Magnetisation of Bulk Superconductors

• Three magnetisationtechniques:

• Field Cooling (FC)

• Zero Field Cooling (ZFC)

• Pulse Field Magnetisation(PFM)

• To trap Btrap, need at least Btrapor higher

• FC and ZFC require large magnetising coils

• Impractical for applications/devices

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ZFC FC


Pulsed Field Magnetisation

• 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 @ 29 K

Top surface of 45 mm diameter Gd-Ba-Cu-O

Fujishiro et al. Physica C 2006

• Record trapped field by FC: 17.6 T @ 26 K

Centre of 2 x 25 mm diameter Gd-Ba-Cu-O

Durrell et al. Supercond. Sci. Technol. 2014


Pulsed Field Magnetisation

• Many considerations for PFM:

• Pulse magnitude, pulse duration, temperature(s), number of pulses,type of magnetising coil(s), use of ferromagnetic materials

• Dynamics of magnetic flux during PFM process

• Multi-pulse PFM: effective in increasing trapped field/flux

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Fujishiro et al. Physica C 2006 Zhou et al. Appl. Phys. Lett. 2017



– Future Outlook –


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


Future Views & Prospects – Bulk-based Machines

• Magnetisation techniques

• Improved in-situ PFM 5-7 Tesla trapped fields

• Magnetic field stability + demagnetisation

• Time-varying fields in real machine environments demagnetisation?

• Mechanical properties of bulks + mechanical design of machines

• Limiting factor for very high field applications (> 7-9 T); brittle ceramics

• Development of numerical models

• Crycooler + cryostat technology

• Efficiency, reliability, cost, redundancy; complete engineering solution required


Recommended Further Reading

• Superconductor Science & Technology‘Focus Issue on Superconducting Rotating Machines’ (2016)

• 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


http://iopscience.iop.org/0953-2048/focus/Focus-on-Superconducting-Rotating-Machines

Recommended Further Reading

• J. R. Bumby, “Superconducting rotating electric machines (monographs in electrical and electronic engineering),” Oxford: Clarendon Press (1983)

• K. S. Haran et al., “High Power Density Superconducting Machines – Development Status and Technology Roadmap,” Supercond. Sci. Technol. (accepted for publication)

• http://iopscience.iop.org/article/10.1088/1361-6668/aa833e

Bulk-based machines:

• D. Zhou et al., “An overview of rotating machine systems with high-temperature bulk superconductors,” Supercond. Sci. Technol. 25 (2012) 103001

• http://iopscience.iop.org/article/10.1088/0953-2048/25/10/103001/meta

• Y. Zhang et al., “Melt-growth bulk superconductors and application to an axial-gap-type rotating machine,” Supercond. Sci. Technol. 29 (2016) 044005

• http://iopscience.iop.org/article/10.1088/0953-2048/29/4/044005/


http://iopscience.iop.org/article/10.1088/1361-6668/aa833e

http://iopscience.iop.org/article/10.1088/0953-2048/25/10/103001/meta

http://iopscience.iop.org/article/10.1088/0953-2048/29/4/044005/

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) conductors

• Use of bulk HTS materials


Superconducting Rotating Machines · 2018-11-21 · rotating magnetic field • Basis for...

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