Inductive Charging
An inductive charger transfers power magnetically. This type of charger has been explored for Levels 1 and 2 devices. A recommended practice for EV inductive charging was published by the SAE in 1995. The clear advantage of contactless charging is its convenience for the user. Instead of deep-cycling the battery, the vehicle battery can be topped off frequently while parked at home or at work, when shopping and even at traffic lights. Cables and cords are eliminated. Advantages include convenience and galvanic isolation. It is also possible to build charging strips into highways which enables charging while driving. Therefore, inductive charging could strongly reduce the need for a fast-charging infrastructure. Disadvantages include relatively low efficiency and power density, manufacturing complexity, size, and cost. Given that energy savings is an important motivator for EVs, the extra power loss is an important consideration. Basic principles of inductive power transfer (IPT) are similar to transformers, although most versions have poor magnetic coupling and high leakage flux. The secondary side may be stationary, or moving (roadbed charging). Typical stationary and roadbed IPT charging systems are represented in Figs. 16 and 17.
Stationary Inductive Charging: Stationary inductive charging employs primary and secondary transducers. In the version originally developed for the EV1 (see Fig. 16 in previous issue of EI) the primary transducer is a paddle and the secondary transducer is a vehicle charge port. When the paddle is inserted into the charge port, a magnetic circuit forms and power is transferred through a high-frequency link converter. Power transfer levels of typical systems vary from 0.5 W to 50 kW with air gaps of 1–150 mm. One of the first commercially available inductive couplers was developed by Delco Electronics and applied to the General Motor EV1 system. The main advantage of that approach was the fact that a higher number of turns could be used to maximize the magnetizing inductance of the transformer and hence minimize requirements on the medium power converter to supply magnetizing current. Stationary inductive charging methods have better coupling, tuning, lateral alignment, and higher efficiency than contactless moving-roadbed EV charging methods. Single-stage high-power-factor converter can be used for inductive Level 1 charging. An alternative is to use a two-stage power converter that can be any one of a number of different types of resonant and PWM converters. Due to high peak currents, two-stage approaches dominate for inductive Level 2 charging. Other topologies with a high-frequency resonant current link have been used for both the power transmitter and receiver to compensate coils and support efficient power transmission. To meet distortion standards, an active front end is likely for Levels 2 and 3 inductive charging.
Contactless Roadbed EV Charging: Inductive charging systems have been considered for roadway contactless power transfer. The vehicle can be moving or stationary. Contact- less moving-roadbed EV charging can be used for battery weight and size reduction. Constraints on vehicle energy storage can be relaxed with roadbed charging systems since a portion of the operational power is delivered from the roadbed and, this type of system transfers power from a stationary primary source (track or loop) embedded below the pavement surface to one or more secondary loops (pickup) installed in a moving vehicle as shown in Fig. 17. The powering EVs while in motion to address the inherent compromise that on-board energy storage imposes on EV range and availability. High power can be transferred with perfect alignment and tuning. There have been several proposed methods for increasing the tolerance of IPT to lateral movement or other position errors, as well as to the inherent large air -gap. Configurations that include a long wire loop, sectional loops, and spaced loops have been presented in the literature. The spaced-loop geometry improves the coupling coefficient and overall system efficiency, while minimizing the magnetization current, supply voltage ratings, and stray fields. Challenges of roadbed charging include high power ratings, poor coupling high supply-voltage requirements, loop losses, high magnetization current due to loose coupling , lateral misalignment, the large air gap, and stray field coupling.
By using slim primary ferrite core bars, efficiency can be improved, but cost must be taken into consideration when magnetic components are built into the primary track. A sectional track IPT system for moving vehicles is proposed and studied for increasing power efficiency. Much higher efficiencies have been reported for inductive chargers in stationary applications and it is described a design process to select the parameters of a coreless inductively coupled power transfer (ICPT) device with a large air gap that delivers high power efficiently. A polarized coupler called a double-D-quadrature (DDQ) is introduced and optimized for EVs. The DDQ produces a flux-path height twice that of a circular pad along a single-sided flux path. It has the potential to support cost-effective ICPT designs.
A novel approach implemented for contactless power interface, which is based on IPT technology and suitable for bidirectional power transfer between a common DC bus and multiple electric or hybrid vehicles. The proposed bi- directional contactless power-transfer concept is viable and can be used in applications such as V2G systems to charge and discharge electric or hybrid vehicles to the power grid.
- Resonant and Compensation Circuit Topologies: Resonant circuits are normally employed in inductive charging networks to maximize power transfer capability while minimizing power-supply voltage and current ratings. To deliver the required power with small devices, it is necessary to operate at high frequency. To supply the necessary real power efficiently, series or parallel reactive compensation is required for both the primary and secondary sides of an inductive charger as shown in Fig. 18. Conventional compensation circuit topologies are not suitable for application to EVs because of the high power level, long air gap, and need for low sensitivity to misalignment.
- For inductive charging, among the most critical parameters are the frequency range, the low magnetizing inductance, the high leakage inductance, and any capacitance needed to set up resonance and support reactive power requirement. The series–series (SS) high-frequency resonant topology has been established as a good solution because its resonant circuit can be designed independently of the coupling.
An electric circuit model of the SS system is shown I Fig. 19. Both primary and secondary windings are series-compensated to keep the efficiency high. Parallel–parallel (PP) topologies for both the transmitter and receiver have higher impedance and can be driven more easily than SS topologies. A novel receiver circuit topology for a cordless EV charger is developed for Parallel–parallel (PP) topologies. Compared to a PP circuit, the parallel–parallel–series (PPS) circuit improves the power factor. A PPS circuit allows a larger gap between the transmitter and receiver coils and the objective of a design of the primary resonant circuit that mitigates effects of phase or frequency shifts.
