Type of CPT structures
There are different types of CPT structures. The first to be discussed is the two-plate structure, shown in the figure below. Like the name states, only two capacitive plates are used to transfer the power. Two-plate CPT can be divided into three types: (a) two-plate for EV’s, (b) two-plate for single wire application, and © two-plates for quasi-wireless applications. Research has shown that the two-plate structure experiences lower voltage stress than the other structures. Additionally, the two-plate architecture enhances tolerance for both linear and angular misalignment.
The two-plate structure for EV’s uses a parasitic capacitance between the EV chassis and the ground to create a returning path for the current to follow, resulting in reduced system losses. A prototype system achieved a DC-DC efficiency of 70% with a power transfer of 350W over an airgap of 110mm. The single -wire application consists of a power transmitter, power receiver, and a load. There is no physical path for the current to return, so the system depends on the voltage difference between the terminals. Because of this, it allows for greater tolerance in misalignment ranges. With quasi-wireless two-plate structures (Figure 26c) only two plates are used for the forward current path, while the returning current’s path is established by the earth ground. There are realised quasi-wireless two plate structure setups with a power transmission over an airgap of two meters.
The four-plate structures are categorised into two types based on the alignment of its metal plates: horizontal and vertical configurations. It shows a total of six capacitances due to the coupling between the different plates. The self-coupled capacitances (C12, C34), basic-coupled capacitances (C13, C24) and cross-coupled capacitances (C14, C23). The current is transferred from transmitter to the receiver through plates P1 and P3, the return path is created by plates P4 and P2.
Distance d in the horizontal four-plate structure should be increased to minimize the effects of the parasitic capacitances. In this structure, the self-coupled capacitances small which in turn reduces the self-capacitance. However, this structure is highly sensitive to rotational misalignments. For instance, when there is a phase shift of 90 degrees between the transmitting and receiving plates, the system power reaches zero. This limits the use of this structure. The vertical four-plate architecture, as shown in the figure above, is characterized by a smaller distance between plates P3 and P4. Plate P1 is positioned close to plate P2. This eliminates external and internal capacitances and increases self-capacitances. Plates P1 and P3 are made much larger than plates P2 and P4 to ensure coupling between each set of plates. The close proximity of plates P1and P2, as well as P3 and P4, significantly increases the capacitances C12 and C34. Compared to a horizontal four-plate architecture, the vertical four-plate architecture offers better consolidation and is particularly suitable for rotational misalignment when the plates are straight at the centre, especially in a circular configuration. However, this architecture has drawbacks such as low cross-coupling capacitance, leading to increased cross-coupled capacitances and lower efficiency compared to the horizontal four-plate architecture. Also, the voltage stress is high, requiring the metal plates to be covered with a good insulating material.
The six-plate structure, a combination of the horizontal and vertical four-plate structure as shown in the figure below, consists of a four-plate parallel structure with two additional shielding plates designed to enhance protection against electric fields. These extra plates are not involved in power transfer and are only coupled through parasitic capacitance. Research has demonstrated that this approach leads to improved safety and efficiency.
It also shows a total of 15 capacitances. To minimize the effects the parasitic capacitances, the vertical distances between P1-P5 and horizontal distances between P1-P2 should be carefully considered.
When the transfer distance needs to be increased, the concept of the electric field repeater can be used. In the upper part of the figure below, a two-plate electric field repeater is shown. P1 is used as a power transmitter while P2 and P3 are the receivers. The receivers can also function as a repeater to continuously increase the transfer distance. Because of its connection with the earth ground, this setup can be viewed as a quasi-wireless repeater system.
The setup with the four-plate structure is shown in the bottom part of the fgure. In this system, the connection between the transmitter and receiver is removed. It can therefore be seen as completely wireless since there are no direct connections between the transmitters and the receivers. How further the distance, more and more repeater stages are required which results in higher losses and reduced system efficiency. This is why it’s important to ensure proper power transfer between stages to keep a balance between the power transmitted and the power consumed at each stage.