Wireless Charging of Electric Vehicles Using

Strongly‐Coupled Resonance

Morris KeslerWiTricity Corporation

National Electric Transportation Infrastructure Working Council

Outline

• Motivations

• Introduction to WiTricity’s Technology– Resonance

– Coupling and Quality Factor

• Application to EV/PHEV Charging– System description

– Performance

– Issues

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In the Middle of the Night…

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From the Wall Plug to the DeviceApproaches to Wireless Energy Transfer

Omni-directional

Directed

Radiative techniques Induction

Add Resonance to the Picture

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• Resonator:– Stores Energy– Energy oscillates between two modes (spatial, temporal, form, etc.)

– Examples: Pendulum, Quartz crystal, LC Circuit

• Coupled Resonators:– Coupling mediates energy exchange between resonators

– Efficient and selective energy transfer can be achieved

– Examples: Coupled pendulums, coupled waveguides (filters, switches)

Coupled Resonators

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• Described using coupled‐mode theory– Parameters: Coupling rate (κ), loss rate (Γ), resonant frequency

• Conditions for efficient energy transfer– “Similar” resonant frequencies– Coupling rate greater than loss rate

• Figure of Merit for system– U = κ/sqrt(Γ1Γ2) = k*sqrt(Q1Q2)– Optimum efficiency only a function of U

Efficiency of Energy Transfer

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( )2

221 1

U

Uη =

+ +

Optimum efficiency only a function of the figure-of-merit U

1 21 2

U k Q Qκ= =

Γ Γ

where

Resonators with High Quality factor enable efficient energy transfer over distance.

50% at U=3

90% at U=20

Coupling and Q are important factors

Using Magnetic Resonance

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• Magnetic resonator– Simple example is a loop and capacitor

B

E

Coupled Magnetic Resonators

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A Multitude of Applications

Consumer Electronics

Medical Devices

LightingRobotics

Electric Vehicles

New applications are limited only by one’s imagination

Solar Power

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Residential Use Case

Requirements for Wireless Charging of EV

• Power levels up to 3.3 kW (initially)

• High‐efficiency (90% end‐to‐end)

• Tolerant to parking variations

• Tolerant to variations in vehicle ground clearance (vehicle loading, tire pressure, etc.)

• Safely operate with people in and around vehicle

• Satisfy EMC/EMI requirements

• Safe, unattended operation

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Resonators designed for high Q and coupling, efficient power electronics

Efficiently operate over a range of magnetic coupling

EM fields below ICNIRP limits where accessible

Low radiated EM fields, Choice of frequency

Detection of foreign objects, Built-in temperature sensing

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System Components for Wireless EV Charging

RF AMP(DC‐RF)

AC/DC(PFC)

SourceResonator

DeviceResonator

AC/DC(Rect.)

Battery

MainsPower

Source Electronics

DeviceElectronics

BMS

Source Efficiency> 95%

Wireless Efficiency90 – 98.5%

Rectifier Efficiency> 99%

Control

AC Mains to Battery Efficiency of greater than 90% possible

Tolerance to Offsets

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SourceResonator

DeviceResonator

Direction of Travel

Parking Tolerance

Δx

Δy

Δz

Source Resonator

Device Resonator

Air-Gap Variations

Δx up to +/- 20 cmΔy up to +/- 10 cmΔz up to +/- 2.5 cm

Typical ranges:

Systems must operate at high efficiency over this range of offset.

Magnetic Field Strengths

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• Zone 1: Energy Transfer Region– Largest B field– No prolonged human exposure

• Zone 2: Under Vehicle Region– B rapidly decreasing– No prolonged human exposure

• Zone 3: Exterior Region– B < ICNIRP MPE– Unlimited human exposure

• Zone 4: Vehicle Interior– B < ICNIRP MPE– Unlimited human exposure

Foreign Object Debris

• Magnetic field in energy transfer region (between coils) is large– Maximum field depends on coil design and size– Can cause heating of some metallic objects

• Examples of likely debris

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Detection of Foreign Object Debris

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• Two Basic Approaches• Passive techniques:

– Reduce likelihood of FOD interacting hazardously with high magnetic fields.

– Large coils to reduce peak B field– Shaped structures

• Active techniques:– Detect and react to the presence of FOD.– Reduce power or interupt charging– Scales to higher power 

• WiTricity prototype FOD detection system demonstrated– http://www.youtube.com/watch?v=my5fvOh15kg

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Standard Capture Resonator

Standard Source Resonator

Rectifier

Integrated Power Supply(Level 2)

Standard Resonator Configuration(10-15cm or 15-20cm offset)

(1)

WiTricity Prototype System

WiTricity 3.3 kW PrototypeOn‐Vehicle Installation , June 2010

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Electric Smart Car Demonstration

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Source Coil on Floor

Device Coil mounted on Car

Wireless Charging for EV/PHEV is Coming

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• High efficiency ( > 90%)

• High power rates (3.3 kW and greater)

• Power transfer over several tens of cm

• “Robust” to:  misalignment, weather, vehicle materials, building materials

• Safe operation in residential, commercial, and municipal configurations

Availability of wireless charging will increase adoption rates for EV/PHEV