“Brushless and Permanent Magnet Free Wound
Field Synchronous Motors for EV Traction”
Prof. Dan Ludois – Principle Investigator
University of Wisconsin - Madison
June 9th, 2015
Project ID: EDT065
This presentation does not contain any proprietary, confidential, or otherwise restricted information
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Overview
Timeline: 2 years Budget: $616,567
– Project start date: October 1st, 2014 DOE - $493,247
– Project end date: September 30th, 2016 FY1 $279,245
– Percent complete: 33% as of today FY2 $214,002
Partners UW & IIT - $123,320
– Prof. Dan Ludois – University of Wisconsin – Madison
– Prof. Ian Brown – Illinois Institute of Technology
Barriers
– Magnet cost (about $200) is about 75% of the 2020 motor cost target;
eliminating PMs reduces motor cost by 30%
– The back EMF of Interior PM machines requires a boost converter, which
brings the power electronics cost above the 2015 or 2020 cost targets;
eliminating the boost converter saves 20% in power electronics cost
– Poor power factors for Interior PM machines cause larger currents,
increasing size and cost of PE; improved power factor saves 15% PE cost
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Background Motivation - Relevance
• Commercial & societal detractions of permanent
magnet synchronous machines (PMSMs)
– Rare earth PMs are significant fraction of EV motor cost
– Rare earth PM market is volatile
– Rare earth PM extraction and refinement environmentally
hazardous
– Rare earth PMs are largely single source from a foreign
power
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Background Motivation - Relevance
• PMSM’s operational detractions in a traction application
‒ PMs have a fixed flux level, non variable, always “on”; safety
concerns during inverter faults.
– Interior PMSMs typically operate with negative d-axis current
(especially during field weakening operation);
• Power factor lowered because of the reactive current
• Traction inverter oversized to supply reactive current
• Increased losses in inverter and stator (ohmic)
Wound Field Synchronous Machines (WFSM) stand to overcome the
limitations of PMSMs via electromagnets
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Project Objective – Relevance
• Design, develop, and demonstrate a prototype wound field
synchronous motor (WFSM) with brushless rotor excitation
via capacitive power transfer (CPT) capable of replicating the
performance of commercially available Interior PM motors
for EV traction.
• Two WFSM prototypes have the following technical targets:
DOE USDRIVE AND WFSM PROTOTYPE TARGETS
WFSM WFSM
USDRIVE USDRIVE
Prototype 1 Prototype 2
Attribute Units 2015 2020
Target Target
Target Target
Peak Power kW 55 55 55 55
Cont. Power kW 30 30 30 30
Specific Power kW/kg 1.3 1.6 1.3 1.6
Power Density kW/l 5 5.7 4.5 5
Specific Cost $/kg 7 4.7 - -
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Budget Period 1 Milestones
Milestone Type Description
Analytical and finite element confirmation of
Initial Electrostatic Design capacitive coupler transferring average field
Technical
Complete power [≥300 W] and peak field power [≥600W]
with limited electric fields [<1.5 MV/m]
Development of Combined
Thermal and Optimization results for sample designs match
Electromagnetic WFSM detailed finite element modelling results within
Technical
Multi-objective 15% for average torque, torque ripple, phase flux
Optimization Code linkage and within 20% for stator core losses.
Complete
Candidate designs meet the following technical
Multi-objective
goals: 55 kW peak power for 18 sec., 30 kW
Optimization and Selection
Technical power continuous, specific power >1.3 kW/kg,
of Candidate Designs for
power density >4.5 kW/l in optimization
Prototyping Complete
analysis.
Construction WFSM Selected design for prototype 1 constructed and
Technical
Prototype 1 Complete ready for bench testing.
Experimentally confirm capacitive coupling
Capacitive Coupling
Go/No Go transfers average field power [≥300 W] and peak
Bench Test Complete
field power [≥600W] to dummy load.
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Budget Period 2 - Milestones
Milestone Type Description
Design for prototyping meets the following
WFSM Prototype 1 Initial
technical goals 55kW peak power for 18 sec., 30
Dynamometer Testing with Technical
kW power continuous, specific power ≥1.3 kW/kg,
Brushes Complete
power density ≥ 4.5kW/l
The measured performance of the WFSM
Dynamometer Testing of
Prototype 1 meets or exceeds the following
WFSM and Capacitive Coupler Technical
specifications: specific power density [≥1.3
Prototypes 1 Complete
kW/kg], volumetric power density [4.5≥kW/l].
The simulation demonstrates that the WFSM stator
Simulation Validation
Technical terminal voltage can be regulated with CPT without
Complete
the need for the main traction drive.
The measured performance of the WFSM meets or
WFSM Performance – exceeds the following specifications: specific
Technical
Prototype 2 Achieved power density [≥ 1.6 kW/kg], volumetric power
density [≥ 5 kW/l].
Performance in CERTS Micro- The WFSM is able to transfer real and reactive
Go/No Go
grid Achieved power to the micro-grid.
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Approach/Strategy
Potential WFSM Advantages
• Wound field synchronous machines (WFSMs) require no
PMs
• WFSM have complete control of field excitation
– Third control variable iq, id, if
– WFSM have potential for optimal field weakening and a large
constant power speed range
– Loss minimization control
– Rapidly de-energize field in the case of inverter fault
– Traction inverter downsizing and improved efficiency
• Potential for power take off (generator operation) and grid
support when used in a hybrid vehicle application
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Approach/Strategy
Inductive (IPT) vs. Capacitive (CPT) Coupling
Rotary Field
Rotary Field
Rectifier Winding
Rectifier Winding
Rotary
capacitors
Rotary
C Lf
Power Electronics
transformer
Power Electronics
Lf
Power Source
Power Source
Rf
Rf C
Stator frame Rotor frame
Stator frame Rotor frame
Basic idea: replace PMs
with electromagnets
© Brusa 2004-2010 To
Inverter
Approach to Critical Challenges
• Design of the rotor and stator for max power density
• Non-contact rotor field power, i.e. brushless Capacitive Power Transfer
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Approach/Strategy
CPT in WFSM Advantages
Dai, J.; Ludois, D., "A Survey of
Wireless Power Transfer and a
Critical Comparison of Inductive
and Capacitive Coupling for
Small Gap Applications," Power
Electronics, IEEE Transactions on
-CPT has comparable power
capability to IPT for small gaps
• CPT Advantages for WFSMs: less shaft length, high structural integrity
– No need for back iron, vs. closed magnetic path in transformers
– Electric flux lines terminate on charge, field cancels outside gap
– Metal disks naturally suited for high speed
– No composite materials or brittle materials (like ferrite)
– Air dielectric works well at high frequency
– Light weight, low cost: No magnetic grade steel, ferrite or copper windings
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Approach/Strategy
WFSM Flexible Design Environment
A combined WFSM electromagnetic and thermal design
optimization environment has been created
MATLAB(Geometry engine, program control, optimization)
ActiveX ActiveX
mFEMM
ActiveX
MOTORCAD
(Magneto-static) (Thermal)
First prototype design to be
Infolytica MagNet
completed by late spring 2015
(Transient Electromagnetic)
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Technical Accomplishments/Progress
Milestone 1: Initial Electrostatic Design
• Class E amplifier and rectifier, “class E2”
• 2.5 kW capable, 550kHz switching, 1200V SiC switches
• Requires ~10nF of coupling capacitance for C1, C2
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Technical Accomplishments/Progress
Milestone 1: Initial Power Electronic Circuit Results
• General pad implementation (prior to WFSM)
• 1100W, 92% efficient (DC to DC)
• Output: 165V and 7A
• 9nF coupling capacitance (C1, C2)
• 540kHz soft switching
• Peak device voltage ~0.85kV (1.2kV SiC parts)
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Technical Accomplishments/Progress
Milestone 1: Axial Flux Hydrodynamic Coupling Capacitors
• Spiral groove thrust bearing design, air is working fluid
• 100mm diameter, 50 micron gap, 10nF realized for C1 & C2
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Technical Accomplishments/Progress
Milestone 1: Initial Electrostatic Design, CPT Coupling
• <1/3 the axial length of a
traditional brushless exciter for
this machine rating
• 2.5 kW throughput
• Mass: 600 grams
• Mechanically stable to high
speeds
• Prototype construction
underway
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Technical Accomplishments/Progress
Milestone 2: Parametric Geometry and Structural Analysis
• Geometry (stator and rotor) is • Design of experiments structural
parameterized to allow full exploration analysis
of design space – Determine rotor geometric design
– Geometry engine allows for points to variable limitations
merge and collapse
– Single and double layer windings
Von-Mises Stress Strain
(a) Rotor Type A (b) Rotor Type B
(c) Rotor Type C (d) Rotor Type D
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Technical Accomplishments/Progress
Milestone 2: Rapid Transient Magnetic Behavior Reconstruction
• Using a series of magneto-static simulations and fully exploiting magnetic
and electric symmetries to reconstruct transient behavior rapidly
– Enables multi-objective population based optimization
– Coupled with thermal analysis
Rapid magneto-static Transient magnetic
reconstruction MagNet
FEMM
Torque versus Current Angle Torque versus Position Radial Flux Density Mid-Tooth
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Response to Previous Year Reviewers’ Comments
• This project is a new start
Partnerships/Collaborations
• Lead Institution (PI) – University of Wisconsin - Madison
• Sub-award Institution – Illinois Institution of Technology
– Weekly meeting between project institution leads (Ludois, Brown)
– Biweekly joint teleconferences between teams (includes students)
– Site visits for hands on collaboration
• C-Motive Technologies Inc. (Madison WI based startup)
– C-Motive advising UW on CPT deployment
– Lending capacitive surface coating and annealing know how
– Desires to participate in future commercialization effort if project is
successful
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Future Work & Activities
Budget Period 1 (Through 9/30/2015)
• Complete construction of WFSM Prototype 1
• Control code development and dynamometer testing of WFSM
Prototype 1
• Complete construction of Capacitive Coupler Prototype 1
• Bench testing of Capacitive Coupler Prototype 1
Budget Period 2 (10/1/2015 - 9/30/2016)
• Dynamometer testing of WFSM and Capacitive Coupler Prototypes 1
• Design of WFSM Prototype 2 from lessons learned with Prototype 1
• Design of Capacitive Coupler Prototype 2 from lesson learned
• Construction of WFSM and Capacitive Coupler Prototypes 2
• Dynamometer testing of WFSM and Capacitive Coupler Prototypes 2
• Investigation of power take-off capability and microgrid support
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Summary
• Relevance
– Develop a high performance wound field synchronous machine for EV traction
• Brushless & permanent magnet free
– Reduce EV motor and traction inverter cost
• Approach
– Capacitive power transfer for compact brushless rotor excitation
– Combined electromagnetic and thermal multi-objective optimization for WFSM
• Technical Accomplishments
– Initial capacitive coupler design complete, power electronics functionality confirmed
experimentally at >1kW and 92% efficient.
– Parametric geometry engine, rapid reconstruction of transient magnetic behavior from
static simulations, to enable population based optimization
• Future Work
– Construction and dynamometer testing of WFSM and Capacitive Coupler
– Design refinement and 2nd prototype development from 1st prototype lessons learned
– WFSM control algorithms and deployment in a microgrid environment
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Technical Back-Up Slides
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Coupling Capacitor Rotors
Φ 100mm
Φ 60mm
• 0.016in. thick 3003-O Aluminum sheets
• Hard anodized beyond flexures
• Torque transmitted through featured
I.D. and nylon 6/6 alignment pins
• 3003-O
• Resistivity – 3.649E-8 [Ohm-m]
• Yield Strength – 144.78 [Mpa]
• 6061-T6
• Resistivity – 4.066E-8 [Ohm-m]
• Yield Strength – 241.31 [Mpa]
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Coupling Capacitor Stators
Φ 113mm
Φ 60mm • 0.016in. thick 3003-O
Aluminum sheets
• Designed as outwardly
pumping spiral groove
bearing
Φ 85mm • Supported on flexure
beams at OD
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Capacitive Power Coupling Exploded View
• 2 coupling
capacitors, C1, C2
• Rectifier board
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