Sensorless inductive power transfer system for electric vehicles: strategy and control for automatic dynamic operation

This article focuses on Inductive Power Transfer to implement dynamic charging for electric vehicles. The system proposed is composed of adjacent 50-cm-side square coils set on the road, with only the coil under the vehicle activated for power transfer. It relies on a symmetrical circuit coupled to a symmetrical control using ZPA regulation to operate frequency-varying inverters, using soft switching around 90 kHz. This system is able to deal with large coupling factor variations (between 0.1 and 0.26, and even more), which allows to use same-size coils on the ground and on the vehicle, to reduce stray field and improve safety. Moreover, the regulation works with no communication between the vehicle and the road. The idea of short-circuiting a ground coil and its resonant capacitor has been used to determine the position of the vehicle in order to switch from one activated inverter to the next for dynamic operation. Different scenarios were investigated and experimented regarding the different possible states of ground inductances: on, off or short-circuited with the capacitor. This study leaded to the choice of a scenario implemented on a 2.4 kW prototype involving a Renault Twizy as the vehicle carrying the secondary inductance. The control method employed allows dynamic operation with no additional sensor, resulting in a very good time response of 4 ms, coherent with a motorway application.