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1 INTRODUCTION

The conventional transfer of electric power to a moving vehicle is based on the principle of current rails and brushes or on the principle of cable-systems. The applications of these principles are cranes, ground floor transportation systems, mono-rails, vehicles, elevators, battery charging systems and other transportation systems. These principles are well known and proven. A new contactless tech-nology for the transfer of electric power to a moving vehicle is based on the use of alternating magnetic field which is generated by a distributed primary conductor loop (power source) and a secondary coil which is called pick-up (power sink). The primary conductor loop and the secondary pick-up are mag-netically coupled to ensure the inductive power transfer from the power source to the power sink. The total system acts similar to a single phase trans-former which is characterized by large stray induc-tivities (especially on the primary side) and a large

magnetization current due to the large air gab be-tween the primary side and the secondary side of the inductive power transfer system.

Possible applications of contactless inductive power transfer are • material transportation systems (e.g. automotive

industry), • ground floor transportation systems, • elevators, • storage systems, • cranes.

The use of a alternating magnetically field means the use of high frequency with regard to the trans-former design. With a given volume of the trans-former the transferred power increases in accordance to the frequency. In practice frequencies in range of 10 kHz to 100 kHz are used.

The different power transfer systems are charac-terized by the criteria shown in Tab.1.

CONTACTLESS INDUCTIVE POWER SUPPLY

Jürgen Meins Institut für Elektrische Maschinen, Antriebe und Bahnen, TU-Braunschweig, Braunschweig, Germany j.meins@tu-bs.de

Günther Bühler, Robert Czainski, Faisal Turki B. Author Institut für Elektrische Maschinen, Antriebe und Bahnen, TU-Braunschweig, Braunschweig, Germany info@imab.de

ABSTRACT: Inductive Power Supply is a new technology which is advantageous in comparison to current-rails, cables and battery supply systems. The advantages of this new developed technology are based on main-tenance-free operation, no sparkling effects due to contact problems, complete isolation of primary and sec-ondary conductors and ruggedness against dust and environmental conditions. The basis for this new devel-oped technology is the use of a secondary coil, located on the moving transportation system inductively coupled to a primary winding which is located and fixed along the track of the transportation system. There are a number of technical problems to be solved to make this contactless inductive power supply work-ing with high efficiency and to become an economically power supply system. The use of high frequency leads to acceptable dimensions of the active secondary coil. The distributed winding on the primary leads to a large impedance and therefor corresponding apparent power is required. The effect of specific compensating technologies is that only real power has to be supplied by the high frequency power supply on the primary. In addition special attention has to be given to the design of the secondary coil arrangement to provide the mag-netization of the air gap between primary winding and secondary coil. The different design aspects of contact-less inductive power supply will be presented.

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Tab.1: Power Supply Technologies (Electrical Power) Criteria Current

Rail/Brushes Cable Contactless, Induc-

tive Technology complex simple very complex

Voltage, Current DC, AC low Frequency DC, AC low Frequency DC, AC high Frequency no of Phases 1, 3 1, 3 1

Power Conversion no no yes Power Range up to MW up to MW up to MW

Efficiency low-high high medium Wear and Tear yes (rail, brushes) yes, cable no Maintenance yes yes no Reliability medium medium high

Sensitivity Environment Dust, Ice no no Safety Aspects not isolated/hazards isolated isolated

EMV, EMI Effects low low medium Isolation Prim./Sec. Side no no yes

Pollution yes, brushes no no Expenditure medium low high

Cost (Install./Maint.) medium/medium low/low high/low Regarding the characteristics shown in Tab.1

there are some disadvantageous and some advanta-geous of contactless power transfer who are pointed as follows: Advantages • No Wear and Tear • High Reliability • No sensitivity against Environment Conditions • Fully Isolated System • No Pollution • No Maintenance Requirements. Disadvantages • Complex Technology • Multiple Power Conversion • High Investment Costs.

Based on the large number of advantages the use of contactless inductive power supply becomes more and more importance during the last 5 years in the automotive industry, storage systems and transporta-tion applications.

Fig. 1a: Container Crane with CPS Power Transfer

Fig. 1b: Pick-Up (2x25kW)

Fig. 1 shows the first application of the CPS-system at a container crane in Nor-folk/Virginia/USA. Data (CPS Container Crane Virginia): • Beam Length: l = 70m • Transferred Power: P = 50 kW • No of Pick-Up: 2 (connected in parallel) • Frequency: f = 20 kHz • Output (480VAC, 50Hz, 3-Phase, 230VAC,

50Hz, single phase).

2 FUNDAMENTALS OF CONTACTLESS POWER SUPPLY (CPS)

Basically the CPS system consists of a distributed primary winding installed along the track which is supplied by AC-voltages or currents and a secondary pick-up coil which used to supply the moving vehi-cle or device with electric power. The magnetic coupling between primary and secondary is usually

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gained by the use of ferromagnetic material. The electric power which can be transferred by such kind of system can be calculated as follows:

ϕωϕ coscos ⋅⋅=⋅⋅= FeFePPP ABIUIP (1)

P: transferred power IP: primary current UP: generated voltage (secondary → primary) Ω: frequency BFe: flux density of ferromagnetic material AFe: area of ferromagnetic iron core cosφ: phase angle (IP, UP) There are import parameters which have to be taken into account with regard to the design of a CPS-system: Constant voltage supply versus constant current sup-ply: Due to the series connection of multiple secondary loads constant voltage supply of the primary is only possible if only one load is placed on the secondary. The decoupling of multiple secondary loads requires constant current supply of the primary. Primary current IP: The transferred power increases in proportion to the primary current. In practice it should be considered that the efficiency is mainly defined by the primary losses and depending of the length of the primary. Usually a compromise between the value of the pri-mary current and the pick-up design has to be made.

Depending on the amount of power the values of the primary current varies in the range of 30A – 200A. Iron core BFe, AFe: The use of frequencies in the range of 10kHz – 100 kHz requires special magnetic material for the pick-up iron core. Laminated iron sheets as used in con-ventional line transformers leads to unacceptable high iron losses. A good choice for the material of the pick-up iron core is softferrite. Due to the limited size of soft-ferrite cores some restrictions are given with regard to the geometric design of the pick-up coils. cos φ: Primary current IP and generated voltage UP should be in phase to set the power factor to 1. In conjunc-tion with the compensation of the primary loop and the pick-up at the ringing frequency the design of the primary inverter will be optimized to minimum size. Frequency ω: The transferred power increases in proportion to the frequency. Therefore the use of line frequency is dis-advantageous. The use of a frequency in the range of 10kHz – 100 kHz requires the conversion of the line power to a higher frequency using a single phase in-verter. The limitation of the upper frequency is mainly given by the increase of inverter switching losses. In the future new developed semiconductors might defuse this problem. The fundamental schematic of a CPS-system is shown in Fig. 2.

Fig. 2: Fundamental Schematic of a CPS-System On the left side the primary inverter is shown

with 3-phase line input and single phase high fre-quency voltage output. The compensation network converts the inverter AC-output-voltage into cur-

Primary Inverter Compensation

Line

Primary Loop (Length l)Energy-Transfer

R, L

u_WR(t)

i_WR(t)

u_p(t)

i_p(t)u_s(t)

Ud

Id

-

Load 1

Ud

-

Id

Load 2

(Pick-Up)- high Frequency

- U/I-Converter

- reactive Power- Harmonics- Voltage/ Current - magnetic Coupling

- Ferrite-Material- AC-DC-Conversion- Stabilization

- Losses- EMI

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rent (if required), reduces output harmonics and compensates the reactive power of the primary loop. The resonant frequency of the compensated primary loop is set to the value of the inverter out-put frequency. So cosφ = 1 is given for the funda-mental oscillation of the inverter output.

The distributed primary loop is installed along the track and is magnetically coupled to the secon-dary pick-up’s. The number of secondary pick-up’s is in accordance to the required power for each loud and to the number of individual loads. Each pick-up consists of an iron core with an assigned coil, a compensation network and a power con-verter for stabilization and power conversion in DC-power or line-frequency power.

3 PRIMARY INVERTER

Generally the primary inverter is a single phase voltage inverter. The task of the primary inverter is described as follows:

Converting the single or three phase line input into high and fixed frequency output. The output of the inverter can be AC-voltage or AC-current in case of a single load application. The output must be constant AC-current in case of a multiple load application to decouple the individual loads from each other.

R4

0.1

U2

R5

0.1

U30A

Primary Inverter "Push-Pull"Line Input

Compensation+ Primary Loop

C11000u

L1 1uH R1 0.1

Z1 Z2

D7 D8

G1

D9

D10 G2D11 D12

Lp2 Lp1

Ls

R7 100m

R8 100m

R9 100m

U1

Fig. 3a: Primary Inverter (Push-Pull Variant)

Time4.925ms 4.950ms 4.975msV(R3:1)

-1.0KV

0V

1.0KV

V(Z1:C) V(Z2:C) V(C1:1)

0V

0.5KV

1.0KV

1.5KVV(G1) V(G2)

-20V

0V

20V

Fig. 3b: Primary Inverter (Push-Pull Variant, Output 100%)

Time

4.900ms 4.925ms 4.950ms 4.975ms 5.000msV(R3:1)

-1.0KV

0V

1.0KV

SEL>>

V(Z1:C) V(Z2:C) V(C1:1)

0V

0.5KV

1.0KV

1.5KVV(G1) V(G2)

-20V

0V

20V

Fig. 3c: Primary Inverter (Push-Pull Variant, Output 50%)

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There are a number of concepts for the topology of the primary inverter under discussion. Usually a pulse inverter is powered by a DC-voltage link which is connect to the line via a bridge rectifier.

Fig. 3a shows the schematic of the push-pull variant of a pulse inverter

The inverter topology shown in Fig. 3a is char-acterized by the following features: • High switch voltage load (minimum twice the

DC-supply voltage), over volt protection re-quired,

• Only 2 power switches, • Simple driver design, • Low losses, • Isolated output, • Simple adoption of the output current in accor-

dance to the primary loop design (using the transformer windings).

Fig. 4 presents a primary inverter schematic in a half-bridge configuration. This topology does not require an output transformer. Because of the AC high frequency output the symmetrically splitted DC-voltage link can be used as the second output terminal. This inverter topology is characterized by the following features: • Low switch voltage load (according to the DC-

supply voltage), • No over volt protection required, • Only 2 power switches, • Low output voltage, • Splitted driver design, • “Hot and cold output terminals, • Non isolated output, • Constant output voltage.

U2

U3

Primary Inverter "Half-Bridge"Line Input

Compensation+ Primary Loop

Cd11000u

Z1

Z2

D7 D8

G1

D9

D10 G2D11 D12 Cd21000u

R7 100m

R8 100m

R9 100m

B A

U1

Fig. 4a: Primary Inverter (Half Bridge Variant)

Time

4.90ms 4.92ms 4.94ms 4.96ms 4.98ms 5.00msV(A)- V(B)

-400V

0V

400VV(V1:-) V1(Cd1)

0V

0.5KV

1.0KV

SEL>>

V(G2) V(G1,A)-20V

0V

20V

Fig. 4b: Primary Inverter Output Waveforms (Half Bridge Variant)

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The full-bridge inverter topology which is

shown in Fig. 5a which is similar to the known 4Q-Chopper design is advantageous with regard to symmetrical outputs and high power. Many of ap-plications which are already installed use this stan-dard topology. It is characterized by the following features: • Low switch voltage load (according to the DC-

supply voltage),

• No over volt protection required, • 4 power switches, • Complex driver design, • Low losses, • Non isolated output, • Variable output voltage possible, • Simple adoption of the output current in accor-

dance to the primary loop design (using the transformer windings).

U2

U3

Z1

G1

Z2

G2

A B

Primary Inverter "Full-Bridge"Line Input

Compensation+ Primary Loop

Cd11000u

Z3

Z4

D7 D8

G3

D9

G4D10 D11 D12

R7 100m

R8 100m

R9 100m

U1

Fig. 5a: Primary Inverter (Full Bridge Variant)

Time4.900ms 4.925ms 4.950ms 4.975msV(A)- V(B)

-1.0KV

0V

1.0KVV(V1:-) V(V3:-)

0V

0.5KV

1.0KVV(G1,A)+120 V(G2)+80 V(G3,B)+40 V(G4)

0V

50V

100V

150V

Fig. 5b: Primary Inverter Output 100%(Full Bridge Variant)

Time

4.900ms 4.925ms 4.950ms 4.975ms 5.000msV(A)- V(B)

-1.0KV

0V

1.0KVV(V1:-) V(V3:-)

-1.0KV

0V

1.0KV

SEL>>

V(G1,A)+120 V(G2)+80 V(G3,B)+40 V(G4)

0V

50V

100V

150V

Fig. 5c: Primary Inverter Output 50%(Full Bridge Variant)

Although the size and cost of all components of

the inductive power system can be reduced by in-creasing the frequency it is a good and favourable practice to set the frequency in the range of 10 kHz to 30 kHz for high power ratings. The research work indicates that the switching losses of the cur-rent state IGBT-technology are limiting the fre-quency range which can be used.

On the other side the new developed SiC-technology which is already used for low power inverters leads to advantageous inverter solutions.

Fig. 6a, 6b show the test bench of a 200kW, 20 kHz primary inverter for an inductive power sup-ply system.

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Fig. 6a: 20 kHz, 200kW Primary Inverter

Fig. 6a: 20 kHz, 200kW test bench

4 PICK UP

The pick up is magnetically coupled with the pri-mary loop, but located on the secondary side of the inductive power transfer system and usually con-sist of a ferrite iron core and a coil.

Depending on the application of the power transfer system different geometric designs of the pick-up is known.

The E-shaped iron core pick-up is useful for monorail transportation systems, while the flat iron core pick-up is used for ground floor transportation systems. Due to the large air gap which is common for both designs high magnetization currents are required. Especially for the flat pick-up the high amount of stray flux leads to low magnetically coupling and high stray reactance. Fig. 7a, 7b show the results of 2D flux calculation for both pick-up designs.

Secondary Coil

Primary Coil

Iron Core

Magnetic Field

Fig. 7a: E-Shape Pick-Up (Flux-Simulation)

O.O.

OXOX

O. OX

O. OXO.O.

OXOX

Fig. 7b: Flat Pick-Up (Flux-Simulation)

The equivalent circuit of the pick-up is shown in Fig. 8a, 8b, 8c for different series and parallel compensation. The transferred power can be calcu-lated by the equation (2). With a given primary current ip the transferred power is in proportion to the generated voltage up. The generated voltage up is depending on the frequency, saturation of the ferrite core and the dimensions of the pick-up.

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volumelAlengthlareaAferritesaturationfluxB

frequencyfcurrentprimaryip

lABpipupiP

FeFe

Fe

:,:,:(max):ˆ

:2:

cos2

ˆˆcos

2ˆˆ

⋅

=

⋅⋅⋅⋅=

⋅=

πω

ϕω

ϕ

(2)

Fig. 8a: Equivalent Circuit Pick-Up

Fig. 8b: Series-Compensation

Fig. 8c: Parallel Compensation

With a given primary current the series com-

pensation gives a constant output voltage, while the parallel compensation gives a constant output current. The used compensation method is depend-ing on the load characteristic and or the applica-tion.

5 EFFICIENCY

The overall efficiency of the inductive power sup-ply system is of importance with regard to the ac-ceptance of this new developed technology. Fig. 9 shows the basic distribution of losses.

Assuming that the line power is set to 100%, the losses of the primary inverter will be in the range of 2% to 10% depending on the line voltage, the inverter design and the inverter control concept.

The losses of the primary loop (5% to 30%) are mainly depending on the length, the litz cable cross section and quality. The cable losses of a 200 mm²/ 200A primary loop were measured with 30W/m track length.

The transfer losses mainly depend on the pick-up design and can be calculated in the range of 3% to 10%. Because of the useful magnetic coupling to the primary loop of the E-shaped pick-up this design is characterized with high transfer effi-ciency.

Line Power (100%)

Inverter Losses(2% - 10 %)

Primary Loop

Input

(5% - 30 %)(Length, Current, Frequency,Litz Cable Quality)

Pick-Up(3% - 10 %)(Current, Frequency)

Output Stabilization(5% - 10 %)

Output

Load (60% - 80%)

Fig. 9: Distribution of Losses

Depending on the pick-up compensation con-cept it might be required to add an active stabiliza-tion unit to the system. Additional losses of 5% to 10% have to be taken into consideration.

In summery the overall efficiency of an induc-tive power supply system will be 60% to 80%.

Although the losses of the CPS-system are higher compared with a conventional current-rail system, the overall advantages have to be taken into consideration when planning new transporta-tion systems.

6 OUTLOOK

The focus of further development work on the field of contactless power supply is to improve the per-formance und to reduce the costs of the system. The development of new power electronic compo-nents (e. g. SiC diodes, soft-iron magnetic materi-als) indicates within the next years. Costs can be reduced by optimized fabrication and by larger quantities. Especially the optimized soft-iron core design is under development now.

Xs Rs

Ip/ws

Up

Is

Uhs

Us

w=1

Xh

Ip

w=wsCKs

Xs Rs

Ip/ws

Is

UhsUp

UsXh

w=wsw=1

IpCKs

Xs Rs

Is

Ip/ws

Up

Us

UhsXh

w=1 w=ws

Ip

9

7 OUTLOOK

M.Bauer, P.Becker & Q.Zeng.: Inductive Power Supply (IPS®)for the Transrapid Thyssen Krupp Transrapid GmBH Munich, Germany Proc. intern. conf., Dresden, 13-15 October 2006

Stielau, O.H. , Covic. G.A. : Design of loosely coupled in-ductive power transfer systems. Proc. of IEEE Powercon 2000

Kacprzak D., Covic. G.A. An Improved Magnetic Design for Inductively Coupled Power Transfer System Pickups. Proc of IPEC 2005 Conference

Meins, J., Czainski, R., Turki, F. : Phase control of resonant power supply inverters: Proc of EPE ’05 Conference, Dresden, 2005

Meins, J.; Turki, F.; Czainski, R.: Contactless High Power Supply. Proc. of UEES ’04 Conference, pp. 581-586, Alushta, 2004