Wireless power supply system and power transmission device

A wireless power supply system comprising a power transmission device provided on a ground side and having a power transmission coil, and a power receiving device provided in a vehicle and having a power receiving coil, the power transmission coil transmitting electric power to the power receiving coil via a wireless connection, so as to supply the electric power to an electrical load installed in the power receiving device, the power receiving device including:

• a first efficiency calculator configured to calculate a first efficiency based on a transmission power command value and the electric power supplied to the electrical load;
• and
• a switching controller configured to stop supplying the electric power received by the power receiving coil to the electrical load when the first efficiency falls to a predetermined first threshold efficiency or less,
• the power transmission device including:
• a second efficiency calculator configured to calculate a second efficiency based on a phase difference between a voltage and a current supplied to the power transmission coil;
• and
• power controller configured to control electric power supplied to the power transmission coil according to the transmission power command value, and regulate the electric power supplied to the power transmission coil when the second efficiency falls to a predetermined second threshold efficiency or less when electric power supplied to the electrical load is stopped by the switching controller.