An object of the utility model is to provide an alternating -current converter, including source sideline circle and secondary winding, two extreme points of source sideline circle are used for receiving the voltage signal who treats the conversion, and two end -to -end joints of secondary winding have the voltage changer to gather the return circuit for detect the voltage signal between two extreme points of secondary winding, still include the monitoring return circuit, and be equipped with at least one output nodes on the secondary winding, monitoring loop connection output nodes and/or secondary winding's extreme point for each detects the voltage signal between the output nodes, and/or the voltage signal between output nodes and the secondary winding extreme point. The utility model provides an alternating -current converter divides at certain section coil portion at its the secondary winding department to be equipped with corresponding monitoring return circuit for detect the voltage signal or the current signal of this section coil. When coil portion divides the appearance broken string, according to monitoring voltage signal or the current signal that the return circuit detected, to the location of broken string position.