Common-mode voltage due to asymmetry in inductive power transfer systems

The secondary side of an inductive power transfer (IPT) system is typically galvanically isolated. In IPT systems for vehicular applications, the vehicle is isolated from earth ground by the tires. The primary side is necessarily and in accordance to regulations grounded. The electric field of an IPT system is intense and IPT systems usually employ an electrostatic shield between the secondary coupler and the vehicle. However, capacitive coupling exists between all of the components of the system, including the primary and secondary windings, electrostatic shield(s), the vehicle, and ground. Thus, to some approximation, an electric potential (essentially a common-mode potential) exists between each component and ground. The extraneous electric field of an IPT system is, however, slightly non-conservative. Thus, the calculation of electric potential is not unique. We present numerical calculations showing that the potential induced on the secondary electrostatic shield of a representative IPT system can be significant even when some care is taken to enforce symmetry with respect to ground in the primary. Also, we show that the potential computed for a number of different integration paths is nearly identical, thus indicating that a nearly uniquely-defined common-mode voltage exists between the electrostatic shield and ground.