High permeability and low remanence tape wound transformer cores

Single layer tape wound toroidal cores have a small effective air gap. By simultaneously winding several layers of tape, which are properly coated with an inert medium, the effective air gap of the cores can be increased. This winding method permits us to alter the shape of the hysteresis loop, to make it more round. Cores with a rounded hysteresis loop are required for differential transformers used in ground fault circuit interrupters which have to respond to AC and unipolar current pulses. We present here calculations and data for the effective air gap of multilayer wound cores so that the permeability and the remanence of such cores can be predicted, or the number of layers required to achieve a certain hysteresis loop can be calculated.