Wireless power transfer system

A power transmitter (101) for a wireless power transfer system comprises a transmitter coil (103) and a driver (201) generates a drive signal) for the transmitter coil (103) 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 (201) generates a drive signal for the transmitter coil (103) to generate the power transfer signal. A communicator (205) receives messages from the power receiver (105) and an adapter (213) adapts a timing property of the reduced power time interval in response to at least a first message received from the power receiver (105). A synchronizer (206) 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.