Impact of circuit parameters in contactless power transfer system

2014 IEEE International Conference on Power Electronics, Drives and Energy Systems (PEDES)

Impact of Circuit Parameters in Contactless Power Transfer System Ezhil Reena Joy

Brijesh Kumar Kushwaha

Gautam Rituraj

Praveen Kumar

Dept. of Electronics and Electrical Engineering IIT Guwahati Assam, India 781039

Dept. of Electronics and Electrical Engineering IIT Guwahati Assam, India 781039

Dept. of Electronics and Electrical Engineering IIT Guwahati Assam, India 781039

Dept. of Electronics and Electrical Engineering IIT Guwahati Assam, India 781039

Abstract—This paper presents a practical study on variable frequency, variable load and variable distance of a planar contactless power transfer (CPT) system. The study investigates the behavior of CPT system under three different cases and its characteristics plots are generated for wide range of frequency, load and distance such that the real time situations of CPT systems can be analyzed. With these results obtained from the experiments, the theory of CPT systems can be well understood and it provides a foundation for future implementation of CPT systems for various applications. Index Terms—Compensation, contactless power transfer system, experimental analysis, experimental set-up.

I. I NTRODUCTION Contactless Power Transfer (CPT) systems is a method to transfer power magnetically rather than by direct physical contact [1], [2]. This technology offers several advantages such as safety, durability, robustness and power compatibility [1]–[3]. Recent advancements in CPT systems has led to various applications such as biomedical engineering, portable electronic equipments, moving robots, machine tools, electric vehicle battery charging systems, personal rapid systems, aerospace etc. [1]–[3]. CPT systems basically has a high frequency power converter to feed the coil at the input side and a rectifier in the secondary side connected to the load. The specifications of converter, controllers and the parameters of CPT system differs for each applications. The power transfer capability of CPT systems are normally limited due to loose magnetic coupling. In addition, there could be an application, where the system is connected to the variable load and the secondary side of the coil must be moved with the primary coil [4]–[6]. In such cases, the leakage inductance of the coils are larger than the magnetizing inductance, leading to an inefficient power transmission. Therefore, the impact of variation in parameters of contactless system must be studied which is the first step in the design of such systems. There are many works found in the literature, which has studied the mechanical design issues, converter topologies and control structures of contactless system [5]–[10]. Several other works have focused on compensation topologies suitable for contactless system [9], [10]. Considerable number of other useful studies have discussed the behavior of CPT system for variable frequency over large distances and load. In [11]– [14], a CPT model is analyzed to investigate the sensitivity

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of the system to variation in operational frequency and load. However, it has been noticed only a few works have studied the impact of system parameters in CPT systems. Among those, the most critical parameters are the frequency, load, low magnetizing inductance and high leakage inductance. Of particular interest, this work has studied the impact of CPT system with variation in frequency, distance and load; so that the practical behavior of CPT system can be visualized. The prefixed parameters in this study are coil shape, dimensions of the coil and circuit topology. The variable parameters are air gap distance, operating frequency and load. Skin effect is not considered because both coils are wound using multistrand copper wire. The study has been conducted by varying frequency, load and distance to analyze the real time situations of CPT systems in many possible ways. The remainder of the paper is organized as follows: Section II describes and analyzes the operation of contactless coils with its design equations. Section III presents the description of experimental set-up. Finally, experimental results are presented in Section IV and its conclusions are given in Section V. II. A NALYSIS O F O PERATION Fig.1a shows the basic block diagram of CPT system with primary and secondary compensation capacitor connected on both side of contactless coils. The coupling between the primary and secondary coil and its operation can be analyzed using mutual inductance coupling model. Mutual inductance model uses the concept of induced and reflected voltage to describe the coupling effect between the primary and secondary network. Fig.1b shows the equivalent circuit of a practical CPT system. Let V1 , I1 , R1 , L1 denotes the parameters of the primary side of the coil and V2 , I2 , R2 , L2 denotes the parameters of the secondary side of the coil. RL is the load resistance at the secondary side of the coil. Then the steady state primary and secondary side voltage equations are given by Eq. (1) and (2).   1 V1 = I1 + I1R1 + jωL1 I1 − jωMI2 (1) jωC1 jωMI1 =

I2 + I2R2 + jωL2 I2 + I2 RL jωC2

(2)

Power supply

Primary compensation

Ip

Secondary compensation

Is

and the secondary resonance frequency is given by Eq. (10)

Load

ωo = √

Contactless coils

(a) C1 I2 Vc1

C1 =

Vc2 R1

R2

V1

VL L1

RL

C2 =

L2

M

(b) Fig. 1.

(a) Block diagram of contactless system (b) Equivalent circuit

The mutual inductance M of the coil is given by Eq. (3) √ M = k L1 L2 (3)

jωMI1   I2 = R2 + RL + j ωL2 − ωC1 2

(4)

On substituting secondary side current given in Eq. (4) to Eq. (1), we get Eq. (5)    1 ω2M2  I1 V1 = I1 jωC1 + I1 (R1 + jωL1 ) + 1 R2 +RL + j ωL2 − ωC

2

(5) It can be observed from Eq. (5), from the point of view of the primary side, the secondary side is seen as a transferred impedance or reflected impedance (Zr ). Hence, ω2 M 2 ω2 M 2 =  1 Zs R2 + RL + j ωL2 − ωC 2

(6)

Where, Zs is the secondary side impedance and is given by Eq. (7) 

1 Zs = R2 + RL + j ωL2 − (7) ωC2 The total impedance (Zt ) of the system seen by the power supply is obtained from Eq. (8)   ω2 M 2 1  (8)  Zt = R1 + j ωL1 − + 1 ωC1 R2 + RL + j ωL2 − ωC 2 Therefore, the current drawn from the source is given by Eq. (9) V1 I1 = (9) Zt In order to obtain high power transfer capability, the operating frequency should be equal to the secondary resonant frequency

1

(11)

ω2o L1 1 ω2o L2

(12)

The primary compensation is chosen such that the impedance seen from the source side is purely resistive in nature so as to ensure that high frequency inverter which acts as a power source has minimum possible VA rating i.e., the input voltage and current are in phase. The secondary compensation is chosen to increase the power transfer capability. The choice of resonance capacitor ensures that the impedance of the secondary circuit (Zs ), the reflected impedance of the secondary circuit to the primary circuit (Zr ) and the net impedance seen by the power supply (Zt ) are purely resistive at resonance frequency at ω = ωo .

Where k is the coupling coefficient of the coil. Simplifying Eq. (2), secondary current I2 can be obtained.

Zr =

(10)

Further, the resonance capacitor (C1 and C2 ) of primary and secondary side is given by Eq. (11) and (12)

C2

I1

1 L2C2

Zr =

Zs = R2 + RL

(13)

ω2o M 2 ω2 k2 L1 L2 = o Zs R2 + RL

(14)

Zt = R1 +

ω2o k2 L1 L2 R2 + RL

(15)

The main objective is to attain high efficiency at resonance condition through CPT system. Thus, on substituting Eq. (11) into Eq. (2), the relation between I1 and I2 is obtained. I2 jωo M = I1 R2 + RL I1 = GI2 ; where G =

(16) jωo M R2 + RL

(17)

Where G is the gain relating the input current and output current, where I1 leads I2 at some angle. Using this equations input power, output power and the total efficiency of the system are calculated. Pi = I12 Zt

(18)

Po = I22 RL = (GI1 )2 RL

(19)

η=

G2 (R

2 + RL )

Zt

(20)

III. E XPERIMENTAL S ET- UP D ESCRIPTION In this section, the experimental set-up developed for contactless system is presented. The block diagram of experimental set-up and its physical view is depicted in Fig. 2. The primary winding of contactless coil is fed through a high frequency full bridge inverter, while secondary winding is connected to the load through diode bridge rectifier. The variations of the contactless coils are observed with the help of wooden

staffs. This work has examined the secondary coil at different vertical distances. The arrangement in wooden staff is made to vary the vertical distance between the primary and secondary coil. The primary and secondary coils of contactless system are wounded using Litz wire having N1 and N2 turns of AWG#25 copper wire. The coils are wound in spiral fashion to represent a closed square and rectangular coil to provide a complete current path. The maximum and minimum observed vertical distance between the primary and secondary coil is 2cm and 10cm. The converters are implemented using IRF540, Metal Oxide Semiconductor Field Effect Transistor (MOSFET) for primary side switches (S1 - S4 ) and QRD0610T30 fast recovery diode module for secondary side diodes (D1 - D4 ). The full bridge resonant converter in the primary side converts rectified dc voltage to high frequency ac voltage. The output voltage of the secondary side coil is rectified using diode rectifier. To compensate leakage inductance of the contactless coils, series capacitors are used on both sides. The frequency of the MOSFET converter is varied from 200Hz to 175kHz by a frequency controller. For this purpose, a frequency controller circuit has been implemented using pulse width modulator based frequency control IC SG3525 with high and low side gate driver circuits using IR2110 IC. This work has chosen primary side square coil and secondary side rectangular coil to analyze CPT systems in practical situations. Cp

TABLE I S PECIFICATIONS OF CPT SYSTEM

Specification N1 N2 L1 L2 C1 C2

8

D1

D3

+ RL

Vs

-

L2

L1 S4

Inverter

R = 1.2 Ω

f = 15.432 kHz

D2

Contactless coil (a)

D4

Value 10 10 26.6µH 24.6µH 3.8µF 4.3µF

Vin

L

6

S3 R2

Term Number of primary turn Number of secondary turn Primary side inductance Secondary side inductance Compensation capacitor primary Compensation capacitor secondary

Fig. 3 and Fig. 4 shows the voltage and current waveform at the input and output sides of the contactless coil. As it can be seen, the voltage and current at the input and output side are near about same phase at resonance condition.

Cs

M R1

S2

The performance of the contactless system has been evaluated on a laboratory prototype. The frequency range of experimental circuit has been observed from 200Hz to 175kHz, over a wide load range between 1.2Ω to 36Ω. The investigation has been carried out for a maximum vertical distance of 10cm. The CPT system designed in this work has a resonance frequency of 15.432kHz. Other parameters of the system are shown in Table I.

Voltage (V), Current (A)

S1

IV. P RACTICAL M EASUREMENTS AND I NVESTIGATIONS

Iin

4 2 0 −2 −4 −6

Rectifier

−8

0

0.5

1

1.5

2

2.5

Time (s)

−4

x 10

Compesation Capacitor

Fig. 3. Voltage and current waveforms of input side at resonance frequency. Compesation Capacitor

Load

8 RL = 1.2 Ω

f = 15.432 kHz Converter

Contactless Coils

Driver Circuit

Frequency Controller

(b) Fig. 2.

(a) Circuit diagram of contactless system (b) Laboratory set-up.

Voltage (V), Current (A)

Rectifier

6

Io

4 2 0 −2 −4 −6 −8

In order to examine the practical behavior of CPT system, this study has examined the behavior of contactless coils in variable air gap distance, operating frequency and load. Various experiments are conducted step by step in the laboratory by varying these parameters and its results are summarized in three different cases in Section IV.

Vo

0

0.5

1

1.5 Time (s)

2

2.5 −4

x 10

Fig. 4. Voltage and current waveforms of output side at resonance frequency.

Case (i): Variation w.r.t. frequency: In the first case of analysis, the behavior of CPT system has been studied for

80 70

L

50 40 30 20 10

0

20

Fig. 7.

40

60

80 100 Frequency (kHz)

RL=1.2 Ω

4

Coupling coefficient

Power (W)

5

160

175

R = 1.2 Ω L

f = 15.432kHz

R = 2.3 Ω L

0.6

RL = 22.8 Ω

0.5

R = 35.4 Ω L

0.4 0.3

3

0.2

2

0.1 0

1 0

140

0.8 0.7

D = 2 cm D = 3 cm D = 4 cm D = 5 cm

120

Efficiency versus operating frequencies at varying distance.

7 6

2 cm 3 cm 4 cm

R = 1.2 Ω

60 Efficiency (%)

variation in frequency. Fig. 5 and 6 shows the output power characteristics for the frequency range of 200Hz to 175kHz over different distances and loads. In Fig. 5 the magnitude of output power is highest at resonance frequency fr for variable distances. This variation has been shown at a constant load resistance of 1.2Ω. In Fig. 6 output power is highest at resonance frequency for variable loads. This variation has been at fixed distance of 2cm. Further, it also has been observed that there is not much variation in the output power for initial load change, while for higher load resistances the power transferred has been decreased. Fig. 7 shows the efficiency curve for increasing distance over wide load range. The measured efficiency in this case (Including the losses) is approximately 40%-70%.

2

3

4

5

f

6 Distance (cm)

7

8

9

10

r

0

Fig. 5.

20

40

60

80 100 Frequency (kHz)

120

140

160

175

Fig. 8.

Output power versus operating frequency at varying distance.

Coupling coefficient versus distance at varying load.

8 Vi

7

7

R = 1.2 Ω

V

o

L

6 L

6

RL = 3.54 Ω RL = 5.34 Ω

5 Power (W)

D = 2 cm

RL = 22.8 Ω

4

R = 35.9 Ω L

Voltage (V)

R = 1.2 Ω 5 4 3 2

3 1

2

0

2

3

4

Fig. 9.

Comparison of input and output voltage.

1 0

0

Fig. 6.

20

40

60

80 100 Frequency (kHz)

120

140

160

5

6 Distance (cm)

7

8

9

10

Output power versus operating frequency at varying load.

Case (ii): Variation w.r.t. distance: In the second case of analysis, the behavior of CPT system has been studied for variation in distance. Fig. 8 shows the variation in coupling coefficient w.r.t. distance for different loads. It has been observed the coupling of the coil decreases as the distance increases. Fig. 9 and 10 shows the voltage and current variation w.r.t. distance. It has been noticed, the input side current is low at lower distances and when the distance increases the current has been raised. This is because an opposite current is set in the secondary side circuit which brings an opposite flux to cancel the original field according to Faraday’s Law of Induction. This effect is normal and is more at lower distance and has gradually decreased when the coil is moved away from the primary coil.

Fig. 11 and 12 shows the variation of output power and efficiency w.r.t. distance for variable load. As explained above, for larger distances the output power and efficiency has decreased irrespective of the load and frequency variations. The measured efficiency in this case (Including the losses) is approximately 50%-70% till 5cm distance and then it has dropped down for high loads. Case (iii): Variation w.r.t. load: In the third case of analysis, the variation of efficiency w.r.t. load resistance for variable distance and frequency are given in Fig. 13 and 14. It is important to note in Fig. 13, the efficiency with variation in load has almost linear change for variable distance. In Fig. 14 It has been observed, efficiency decreases as the load increases and at resonance frequency, the position of the curve is higher than other operating frequency.

80

5

6.8 kHz

R = 1.2 Ω

Efficiency (%)

Current (A)

L

3

I

i

I

o

2

1

0

15 kHz

30kHz

70

4

60 D = 2 cm 50 40 30

2

3

4

Fig. 10.

5

6 Distance (cm)

7

8

9

10

Comparison of input and output current.

20

5

Fig. 14.

10

15

20 Load (Ω)

25

30

35

Efficiency versus load resistance for variable frequency.

7 1.2 Ω 3.54 Ω 5.34 Ω 22.8 Ω

6 f = 15.432kHz Power (W)

5

coupled. The experimental results reported in this paper has achieved a maximum efficiency of 70% (including the losses). In this case, a higher efficiency can also be obtained. However, in this paper no attempt was made to improve the efficiency of the system. Since the primary objective of the work is to observe the variation of the system over wide range of frequency, load and distance.

4 3 2 1 0

2

3

4

Fig. 11.

5

6 Distance (cm)

7

8

9

10

Output power versus distance for variable loads.

70 1.2 Ω 3.54 Ω 5.34 Ω 22.8 Ω

60 f = 15.432kHz Efficiency (%)

50 40 30 20 10 0

2

3

4

Fig. 12.

5

6 Distance (cm)

7

8

9

10

Efficiency versus distance for variable loads.

80 D = 2 cm D = 3 cm D = 4 cm D = 5 cm

70 f = 15.432 kHz Efficiency (%)

60 50 40 30 20 10 0

5

Fig. 13.

10

15

20 Load (Ω)

25

30

35

Efficiency versus load resistance for variable distance.

Generally, the contactless system has losses due to leakage inductances as the primary and secondary coils are loosely

V. C ONCLUSIONS A variable frequency, variable load and variable distance contactless power transfer system have been discussed. The mathematical formula explaining the leakage inductance, compensation capacitance and mutual inductance were detailed. To realize the variations of contactless system w.r.t. frequency, load and distance, the laboratory prototype of contactless system has been constructed and tested. The developed CPT system consists of primary side variable frequency full bridge converter and a diode bridge rectifier is connected to the secondary side. The results obtained from the experiment are summarized in three different cases and compared for variable frequency, load and distance. By combining the results obtained in this work, the practical behavior of contactless system can be visualized and thereby this results can be used in the future development of advanced systems in CPT applications. R EFERENCES [1] E. Joy, A. Dalal, and P. Kumar, “Accurate computation of mutual inductance of two air core square coils with lateral and angular misalignments in a flat planar surface,” IEEE Trans. on Magn., vol. 50, no. 1, pp. 1–9, Jan 2014. [2] J. Villa, J. Sallan, J. Sanz Osorio, and A. Llombart, “High-misalignment tolerant compensation topology for icpt systems,” IEEE Trans. on Ind. Electron., vol. 59, no. 2, pp. 945–951, Feb 2012. [3] C.-S. Wang, G. Covic, and O. Stielau, “Power transfer capability and bifurcation phenomena of loosely coupled inductive power transfer systems,” IEEE Trans. on Ind. Electron., vol. 51, no. 1, pp. 148–157, Feb 2004. [4] J. Sallan, J. Villa, A. Llombart, and J. Sanz, “Optimal design of icpt systems applied to electric vehicle battery charge,” IEEE Transn. on Ind. Electron., vol. 56, no. 6, pp. 2140–2149, June 2009. [5] X. Liu and S. Hui, “Optimal design of a hybrid winding structure for planar contactless battery charging platform,” IEEE Trans. on Power Electron., vol. 23, no. 1, pp. 455–463, Jan 2008.

[6] S. Xun Liu ; Shatin ; Hui, “Simulation study and experimental verification of a universal contactless battery charging platform with localized charging features,” IEEE Trans. on Power Electron., vol. 22, no. 6, pp. 2202 – 2210, Nov. 2007. [7] W. Hurley and M. Duffy, “Calculation of self and mutual impedances in planar magnetic structures,” IEEE Trans. on Magn., vol. 31, no. 4, pp. 2416–2422, Jul 1995. [8] O. . P. R. . C. J. Fernandez, C. ; Garcia, “Design issues of a core-less transformer for a contact-less application,” IEEE Seventeenth Ann. Appl. Power Electron. Conf. and Exp., APEC 2002., vol. 23, no. 1, pp. 339 – 345, Jan 2002. [9] C.-S. Wang, O. Stielau, and G. Covic, “Design considerations for a contactless electric vehicle battery charger,” IEEE Trans. on Ind. Electron., vol. 52, no. 5, pp. 1308–1314, Oct 2005. [10] C.-G. Kim, D.-H. Seo, J.-S. You, J.-H. Park, and B. H. Cho, “Design of a contactless battery charger for cellular phone,” IEEE Trans. on Ind. Electron., vol. 48, no. 6, pp. 1238–1247, Dec 2001. [11] C. Mecke, R.; Rathge, “High frequency resonant inverter for contactless energy transmission over large air gap,” IEEE 35th Ann. (Volume:3 ) Power Electron. Specia. Conf., PESC 04., vol. 58, no. 9, pp. 1737 – 1743, June. 2004. [12] W. . J. N. . B. C. . T. A. Lim, “Low-profile contactless battery charger using planar printed circuit board windings as energy transfer device,” IEEE 33rd Ann. (Volume:2 ) Power Electron. Speci. Conf., pesc 02., vol. 2, no. 9, pp. 579 – 584, Sept. 2002. [13] G. . G. A. Boys, J.T. ; Covic, “Stability and control of inductively coupled power transfer systems,” IEE Proceedings on Elect. Power Appln., vol. 147, no. 1, pp. 37–43, Sept. 2000. [14] G. Chwei-Sen Wang ; Stielau, O.H. ; Covic, “Load models and their application in the design of loosely coupled inductive power transfer systems,” Intern. Conf. on Power Sys. Tech. PowerCon 2000., vol. 2, no. 1, pp. 1053 – 1058, Sept. 2000.