Characterization of SiFe sheet insulation

Magnetic circuits of electric machines are laminated using coated sheets in order to limit eddy currents. The standard “Franklin” device is widely used to measure the coating surface resistance. Its main drawback is a bad local repeatability. Resistance measurements performed on the same sample reveal a wide range of values. Hence the mean value may not be considered as a sufficient criterion for the insulation quality; a statistical approach is needed. The characterization of several types of coating by different means (Franklin device, scanning electron microscope and thickness tester) has been performed. The measurements have shown that the scattered resistance values can be explained by inhomogeneities in the insulating layers. Interlaminar losses have been modeled taking into account this scattering. Thermal calculation has shown that defects in the coating surface cannot be responsible for significant damage (dangerous overheating) in the magnetic circuit