Power transmission device and wireless power transmission system

A wireless power transmission system comprising:

• a power transmission device including a first and second power-transmission-side electrodes, and a DC to AC inversion circuit that converts a direct-current voltage into an alternating-current voltage and applies the voltage between the first and second power-transmission-side electrodes;
• and
• a power reception device including a first power-reception-side electrode that opposes the first power-transmission-side electrode with a gap therebetween, a second power-reception-side electrode that opposes the second power-transmission-side electrode with a gap therebetween, and a voltage supply circuit that supplies a voltage induced between the first power-reception-side electrode and the second power-reception-side electrode to a load;
• wherein the power transmission device includes:
• a first voltage monitoring unit that monitors a voltage of the first power-transmission-side electrode;
• a first change detection unit that detects a change in the voltage monitored by the first voltage monitoring unit at regular time intervals;
• a second voltage monitoring unit that monitors a voltage of the second power-transmission-side electrode;
• a second change detection unit that detects a change in the voltage monitored by the second voltage monitoring unit at regular time intervals;
• and
• a power transmission stopping unit that stops transmission of power to the power reception device as a function of the changes detected by the first and second change detection units.

A power transmission device, comprising:

• first and second electrodes for transmitting power to a power receiving device placed in an operational relationship with the first and second electrodes;
• an AC power generator for generating an AC voltage and applying it to the first and second electrodes;
• a controller for controlling the AC voltage applied to the first and second electrodes, the controller:
• monitoring the voltage on the first electrode and determining how that voltage changes at the beginning and end of each of a plurality of predetermined time intervals;
• monitoring the voltage on the second electrode and determining how that voltage changes at the beginning and end of each of the plurality of predetermined time intervals;
• and
• determining when to cut off the application of the AC voltage to the first and second electrodes as a function of the manner in which the voltage on the first and second electrodes changes at the beginning and end of at least one of the predetermined time intervals.