Study of a Contactless Power Transmission System Chun-Hung Hu
Chia-Hui Lai
Wen-Ren Yang
Ching-Feng Chang
Tsair-Rong Chen
No.2, Shi-Da Road, Changhua City, Taiwan Department of Electrical Engineering National Changhua University of Education E-mail:
[email protected] Tel: 886-4-7232105#7273 Fax: 886-4-7129180
Abstract–This paper discusses the contactless power transmission system. The magnetic coupling model and structure required by contactless power transforming system with high efficiency are set up according to the analysis results. Then, advantages and disadvantages of various power converter models are investigated with circuits of compensation and efficiency promotion to design the best main circuit structure of converter. However, by simulation experimental results, the contactless transmission system is confirmed with high safety and efficiency. Finally, the contactless power transmission system with output voltage of 31V and output current of 11A is confirmed. Keywords: contactless power transmission system, converter
I.
INTRODUCTION
Contactless power transmission technique and conventional contact power transmission both are quite different because contactless power transmission technique does not require a contact point to supply energy. Although contact power transmission has been widely used, however, in some certain special environments such as oil drill place, sparkles produced by contact point and so on are dangerous. Also, conventional contact power transmission in water is potentially hazardous due to leakage and electric shock. In this paper, we develop and design a contactless power transmission system. Papers [1-3] discuss the contactless power transmission system. Papers [4-6] depict about the power transmission efficiency enhancement. Therefore, In terms of primary side, various converter design structures are proposed to reduce switch loss and electromagnetic interference. In the secondary side, we focus on coupling coefficient enhancement and change loading application. Moreover, the design for contactless transmission converter is shown in Fig. 1-2. Fig. 1 shows the push-pull resonant converter in current [7-8]. Parallel resonance is used for the primary side and the secondary side. In general, the current provided in push pull converter has less loss on the primary side of the coil. Also, power is transmitted to the secondary side more efficient. So, the two power MOSFETs are more likely
978-1-4244-1666-0/08/$25.00 '2008 IEEE
approach to 0V Switching. In Fig. 1, La1 is much larger than La2. Hence, ZVS is easier to be approached. Also, the behaviors of mutual inductance, output power, output voltage, and resonant frequency can be predicted. Fig. 2 shows the LCL load resonant converter [9-10]. The switch-type power supply uses a full-bridge converter that the input power is DC. Lp represents the inductance of primary side of the coil. Parallel compensation of the primary side uses the coil winding of the primary side with the compensation capacitor, Cp. Therefore, the circuit is composed of the three components called “LCL resonant circuit”. Impedance on LCL resonant circuit reflected by load is the magnetic coupling coefficient of the secondary side passing through the primary side, and the secondary side represented as M. Ls is the inductance value of the coil of the secondary side. The compensatory method is facilitated with compensation capacitor Cs of the secondary side. The switch controller and load are equivalent to one load impedance R. La2
La1
Id
CP
Q1
LP
LS
CS
RO
Q2
Fig. 1 Structure of push-pull resonant converter in current Lr Q1
Q2 CP
Vd
CS R
Q3
Q4
LP
LS
Fig. 2 Structure of LCL load resonant converter
Since pickup is used to couple with litz wire of the primary side to induce voltage, after voltage is induced, it passes through a rectifier/filter to be the power source for the load. So, size and
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shape of pickup iron core tend to affect the winding coupling of the primary side. It is essential that pickup iron core is selected in the contactless power transmission system.
Inverter
II. LITERRATURE REVIEW
Power
The contactless coupling induction system is based on the electromagnetic induction coupling concept in which current and electromagnetic field transmission of primary side coil to independently isolated equipments. The theories are based on Faraday’s Law and Ampere’s Circuital Law. This type of transmission power and energy from the electromagnetic field is similar to theoretic basis as wireless microwave transmission.[1-2] The only difference in wireless microwave transmission is higher frequency and lower energy while in the contactless power transmission system. The basic structure of the contactless power transmission system is shown in Fig. 3. There are two independent systems that transmit electrical energy through mutual inductance coupling between the litz wire and the transformer. Analysis shows that contactless power transmission system related papers are circuit related applications [3]. Papers [4-7] depict the coupling relationship between primary side and secondary side of litz wire and transformer and the contactless power transmission system working efficiency analysis. Also, paper [8] shows the iron core, and relative locations of air gap and winding intended to seek best coupling coefficients. Discussions are included: transformer iron core material selection, iron core form, air gap of primary input and secondary output, and location [4-9].
Switching mode controller
D/C power
3PH input Power provision
Track cable line inductance L1
Fig. 3 Contactless power transmission system
Pick-Up
Converter
Controller
Fig. 4 Structural diagram of the contactless power transmission system
According to Ampere’s Law, a litz wire is produced in the peripheral of the conductor. If the iron core of secondary output is setup nearby the conductor and also according to Faraday’s Law, voltage will be induced by secondary output. The structural diagram of such induction is shown in Fig. 5 [10-13]. Since iron core of secondary output comes in a variety of shapes, it is speculated that the best secondary output iron model will be found by using simulation such as Maxwell litz wire, Simplorer circuit and Ansoft software and so on.
Fig.5 Structure of single-track E-type pickup used in the contactless system
In iron core model of the contactless power transmission system, different iron core shapes are simulated by using Maxwell manage tic line of force simulation software. Fig. 6 shows the distribution diagram of litz wire of E-type iron core. Fig. 7 shows the distribution diagram of litz wire of K-type iron core. Fig. 8 shows the distribution diagram of litz wire of S-type iron core. Fig. 9 shows the distribution diagram of litz wire of U-type iron core. Fig. 10 shows the distribution diagram of litz wire of Z-type iron core. Simulation results show that the litz wire of S-type of iron core has the densest distribution and the best transmission results followed by E-type of iron core.
III. SYSTEM DESIGN
The structure of the primary side and the secondary side of the contactless power transmission system is shown in Fig. 4. The primary side includes three parts: power source, controller, and converter. The converter frequency design is set as 22 kHz and it ensures maximum transmission power. The controller offers detection and protection functions during abnormal conditions such as over-current and circuit shortage. The current of the primary input passes through track transmission to produce litz wire and secondary side induction. Voltage induced by pickup iron core and winding is converted to DC voltage by the converter. Finally, it is added to load output.
Load
Fig. 6 Distribution diagram of litz wire of E-type iron core
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inductance under different frequencies, capacitance, resistance, and impedance value in Fig. 11-13. The diagram shows that litz wire constituted by 45 wicks is in a stable state in terms of both inductance value and capacitance value.
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Fig. 7 Distribution diagram of litz wire of K-type iron core
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Fig.11 Curve diagram of wire material change V.S. inductance value frequency ႝ ߔ .ᓎ
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Fig.12 Curve diagram of wire material change V.S. resistance value frequency Fig. 8 Distribution diagram of litz wire of S-type iron core
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Fig.13 Curve diagram of wire material change V.S. capacitance value frequency
IV. CONTACTLESS POWER TRANSMISSION SYSTEM DESIGN
Contactless power transmission system design steps and processes are conducted based on the flowchart in Fig. 14. Designs of respective processes are described below [14]:
Fig.9 Distribution diagram of litz wire of U-type iron core
A. Figures and Tables System working frequency selection is the first step in contactless power transmission design. Therefore, work frequency is selected based on present power electronic technique standards, power components and related system design experiences. In terms of the presents power electronic technique standards and system costs, selection of frequency between 10 kHz ~ 100 kHz is more reasonable. Fig.10 Distribution diagram of litz wire of Z-type iron core
Conductors used in primary and secondary coil are then taken into consideration. In this paper, 6 types and speciation of conductors used in LCR impedance meters including internet line, T.V. line, instrument test line, single core wire, litz wire (strands 6AWG), and litz wire (strands 45AWG) are tested for
B. Loose coupling consideration At present, the widely adopted transformer is tight coupling model since it is likely to be limited by existing iron core material and structure. However, the contactless power transmissions system adopts loose coupling inductance. It is less likely to be limited by iron core structure. Since loose coupling inductance is selected based on related design
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experiences. After confirming the structure of loose coupling inductance device, basic parameters should be set such as primary input and secondary output coil inductance volume, coupling coefficient, mutual inductance and so on. C. Primary side current Ip selection In the contactless power transmission system, transmission electrical energy size, and primary input power source transformer structure are closely related to current Ip. Generally, Ip selection begins from smaller value of currents. After calculation, when Ip value of current fails to meet system electrical energy transmission requirements, value of current can be further increased. D. ΰVoc Iscαvalue confirmation Based on electromagnetic device selected, when using Ip selected from primary side current, the open circuit voltage Voc and short circuit Isc are tested. During experiment conduction, avoid inadequate rated current of pickup coil which might cause the instability to perform action.
G. System stability and control consideration If Qp < Qs, stability analysis should be conducted on the system. If system cannot guarantee stability control under all work conductions, the system parameters must be adjusted. Common methods include increasing primary side coil current, improving loose coupling inductance device structure or changing system work frequency etc. V.
EXPERIMENT RESULTS AND DISCUSSION
Fig.15 shows the main circuit of the contactless power transmission system. In order to increase contactless power transmission power, full-bridge circuit structure will be adopted. Resonant circuit is constituted by inductance L1, capacitor C1 and litz wireL2. Fig. 16 shows electrical energy is picked up by pickup coil using contactless method. Resonance will be produced after passing through compensation capacitor C2 and L3. After passing through rectifier/filter circuit, D/C electrical energy is supplied to load. Work frequency selection
E. Second side compensation confirmation When secondary side circuit is not compensated, load is able to attain the maximum power transmission which is equal to VocIsc/2. If load required power value is exceeded, the secondary side needs to adopt compensated circuit. The quality factor of the compensated circuit is calculated as shown below.
Qs =
P Voc I sc
Loose coupling consideration
Primary side current Ip
(1)
where P is the transmission power at the load end. The secondary side required V.A rate is shown below.
S s = P Qs2 + 1
ΰ Voc Isc αvalue confirmation
(2)
Primary side compensation confirmation System stability and control consideration Fig.14 Design flowchart of the contactless power transmission system
Vds
VL L1
D1
R3
V4
R4
V5
C1
IL 4
Q2
2
V3 3
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R1
Iac vac
F. Primary side compensation confirmation The transmission power is divided by the VΗA rated to derive the quality factor Qp of primary side coil. As mentioned, the primary coil compensated circuit is also determined by application. When primary side coil adopts longer cables, series compensation adoption is preferable; when primary side coil is in concentrated winding, parallel compensation adoption is preferable.
Q3
Q1 V2 1
If the rate of the secondary side, V A, is higher than the value in Equ. (2); the system will be able to transmit the required power. If the opposite is true, the design will not be able to transmit required power P. In that case, the design has to be adjusted to enhance the power transmission capability. After secondary side coil rated meets design requirements, the next step is to confirm secondary side compensated circuit and topology adopted. The selection of compensated circuit topology is determined by application. Parallel compensation current source features are suitable for battery charging site; series compensation voltage source features are more suitable for electrically-driven power supply sites.
Secondary side compensation confirmation
Q4
R2 VC L2
IL2
L3 C2
pick-up Fig.15 Main circuit of the contactless power transmission system
I L3
C2
Iout Vout
VC2 L2
L3 VL3
Rectifier & Filter
Load
Main
Fig.16 Circuit diagram of pickup
Figures 17~22 show the waveforms corresponding to each device in Figs. 15 and 16. Fig. 17 shows effective value is increased to 1.84A and the effective value is 19.53 V. Fig. 18 shows that Vds measured by 13.51V. Since higher current is required in heavy load, cut-off voltage, Vds, measured during heavy load is lower. Fig. 19 shows effective value measured is 2.776 A and the peak value current is 4.47 A. Fig. 20 shows effective value measured is 24.52V and the effective value is 7.755 A, phase angle is 83.2°, and primary side transmission power is 22.51 WΖFig. 21 shows effective value is 24.2V and effective value is 86V. Fig. 22 shows effective value is 7.14A and effective value is 0.5715 A. TABLE 1 voltage, current and power test values by different loads. Fig. 23 shows the curve of input power and efficiency under different loads. Based on Fig. 23, it is found that transmission efficiency increases as input power increases in linear relationship. However, the saturation may occur if the power is larger than 42 watts in the experiment.
Fig.20 VC1 and IL2 waveform
Fig.21 VC1 and VL3 waveform
Fig.22 IL2 and IL3 waveform
Fig. 24 shows the experiment of the contactless power transmission system. The model train relies on iron core to pickup power from outer tracks and starts motor. TABLE I VOLTAGE, CURRENT AND POWER TEST VALUES UNDER DIFFERENT LOADS
Vac Iac Pac Vout Iout Pout efficiency (V) (A) (W) (V) (mA) (W) (%) 20.01 1.45 34.78 13.90 0.00 0.00 0.00
Fig.17 Vac and Iacwave form diagram of system
No-load Intermediate 19.94 1.65 38.28 10.60 190.20 2.02 load(51ȍ) Heavy load 19.85 1.80 42.62 10.09 420.20 4.24 (20ȍ) Input Power vs. Efficiency curve
Efficiency
Fig.18 Vac and Vds waveform
Input Power
Fig.19 Vds and IL waveform
Fig.23 Power-efficiency curve under different loads
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5.27 9.94
[2]
Fig.24 The experiment of the contactless power transmission system
VI. CONCLUSION This paper focuses on the contactless power transmission system and also discusses the transformer iron core selection, winding method, resonant frequency selection, converter selection, and compensation capacitor connection method related parameters. Simulation and experimental result depict that the structure of the contactless power transmission system is confirmed to be safe with high transmission efficiency. Finally, a contactless power transmission system model with output voltage of 31V, output current of 11A is completed. REFFERENCE [1]
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