Generalized Active EMF Cancel Methods for Wireless Electric Vehicles

In the inductive power transfer systems (IPTSs) of wireless electric vehicles (WEV), the electromagnetic field (EMF) should be lowered for the safety of pedestrians. In general, the EMF should be canceled for every space, time, and load condition of interest. Three generalized design methods for cancelling the EMF of WEV are proposed in this paper. By adding active EMF cancel coils to each primary main coil and secondary main coil, respectively, the EMF generated from each main coil can be independently cancelled by their corresponding cancel coils. Moreover, the EMF can be successfully mitigated if a dominant EMF source only is cancelled with 3-dB margin, which can be applied to any resonant type wireless power transfer systems. Furthermore, no significant power drop may occur if the cancel coils are placed aside from magnetic coupling path. Design examples are shown for U-type and W-type IPTS as well as a wireless stationary EV charger. Experimental verifications are shown for a recently developed I-type IPTS, which has a narrow rail width structure with alternating magnetic polarity along with a roadway. The proposed design methods have been demonstrated, without the loss of generality, to only the secondary coil where relatively large EMF is generated due to high ampere turns. An optimum spacing for cancel coils from main coils and an optimum number of turns are determined. Through experiments, additional EMF mitigation techniques such as the magnetic mirror method, separating pick-up rectifiers, and passive Al plate are provided. Thus, the EMF at 1 m distance from the center of a pick-up becomes under 44 mG even for the maximum power of 12 kW.