Wireless power transfer and communication

A power transmitter (101) comprises a driver (201) generating a drive signal for a transmitter coil to generate a power transfer signal during a power transfer time interval and an electromagnetic test signal during a foreign object detection time interval. A set of balanced detection coils (207, 209) comprise two detection coils arranged such that signals induced in the two detection coils by an electromagnetic field generated by the transmitter coil compensate each other. A foreign object detector (205) is coupled to the detection coils and performs foreign object detection during the foreign object detection time interval. The foreign object detector (205) is arranged to detect a foreign object in response to a property of a signal from the detection coils meeting a foreign object detection criterion. A transformer (1101) has a primary winding and a secondary winding coupled in series with the set of balanced detection coils A compensation circuit (1103) is coupled to the primary winding and arranged to generate a compensation drive signal for the primary winding which offsets a combined voltage of the set of detection coils.