WIRELESS POWER TRANSFER SYSTEM

A power transmitter () for a wireless power transfer system comprises a transmitter coil () and a driver () generates a drive signal) for the transmitter coil () employing a repeating time frame with a power transfer time interval and a reduced power time interval during which a power level of the power transfer signal is reduced. A driver () generates a drive signal for the transmitter coil () to generate the power transfer signal. A communicator () receives messages from the power receiver () and an adapter () adapts a timing property of the reduced power time interval in response to at least a first message received from the power receiver (). A synchronizer () for synchronizing a foreign object detection and the generation of a test signal to occur during the reduced power time interval. The operation may typically be a foreign object detection or communication, and the timing property may e.g. be a duration of the reduced power time interval.