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An Inductive Power Transfer System for Electric Vehicle Battery Charging
Inductive Power Transfer
The inductive power transfer (IPT) is a technology enabling power transfer between two mutually-coupled inductors across a large air gap. Among the applications of IPT is the contactless charging of electric vehicles. A typical IPT battery charging system is composed of three building blocks: a primary power supply, a secondary pick-up controller, and the coupled magnetic structure of both the primary and the secondary controllers.In this case study, we proceed with the design of the controllers, namely circular charging pads. We use EMS package to simulate the inductance of the charging pads.
SolidWorks Charging Pads Model
Objective
The IPT system controllers are represented by two distinct circular pads (see Figure 1.c). Each pad is composed of a Litz wire coil and 12 ferrite strips (see Figure 1.a and 1.b). At constant AC frequency, the airgap dimension between the circular pads is function of the flux resulting from the AC current crossing the primary circular pad. The objective of this study is to compute the primary pad inductance in function of the airgap between the two pads in order to determine how to maximize magnetic flux at constant frequency.
(a) (b)
(c)
Figure 1. IPT charging pads: (a) real-world system [1, p. 76], (b) 3D model of the primary circular charging pad (c) 3D model of the full IPT circular charging system
Circular Pads Model
Each of the circular pads (see model in Figure 1.b) is made of three building blocks: a Litz wire coil mounted on top of 12 equally-spaced radial ferrite strips, both attached to a backing plate.Each of these components is made of a different material. Table 1 summarizes the materials used for the design of each of these building blocks.
Table 1. The list of materials used for the design of the circular charging pads.
| Component | Material |
| Backing Plate | Aluminum |
| Litz wire coil | Copper |
| Ferrite strips | Ferrite isotropic material with a relative permeability of 2000 |
In addition, each of the Litz wire coils of the circular charging pads is made of a wound current coil of 12 turns. The current going through the transmitter coil is 30 Amps (RMS) while the receiver coil is open-circuited (0 Amps RMS).
IPT Charging System Analysis
Study parametrization
Using EMWorks EMS package, we created a parametric AC magnetic study to analyze the electromagnetic behavior of the circular pads.As given in Table 2; we considered the simulation frequency of 20 kHz. In addition, we varied the vertical airgap offset from 90 mm to 250 mm. The primary pad inductance is measured with the secondary pad open-circuited.
Table 2. Study configuration.
| Study Type | Parametric AC Magnetic |
| Frequency (kHz) | 20 kHz |
| Airgap offset (mm) | 90- 250 mm |
Analysis Results
After meshing (see Figure 2) and running the parametric AC magnetic study, we obtained the results illustrated in Figure 3.
Figure 2. IPT circular charging system meshed.
Considering the study configuration detailed in Table 2, the primary circular charging pad inductance varies from 148 mH at a vertical airgap offset of 90 mm to 132 mH at an offset of 250 mm. The evolution of the primary pad inductance is reported in Figure 3 along with the simulation results from [1, p. 77].
Figure 3. The evolution of the primary pad inductance in function of the airgap offset compared to reference [1, p. 77].
References
[1] Chang-Yu (David) Huang, “Design of IPT EV Battery Charging Systems for Variable Coupling Applications,” PhD Thesis in Electrical and Computer Engineering, the University of Auckland, 2011.