The measurement of interlaminar resistance of varnish-insulated silicon-steel sheet for large electrical machines

In large electrical machines, such as alternators and transformers, that include sheet-steel laminations in their construction, it is essential to insulate the core laminations from one another to reduce eddycurrent stray loss. One method is to varnish the sheets after stamping, before assembly in the core. The need for developing good varnishing technique demands satisfactory measurement of the insulation. One convenient method is to measure the electrical resistance of the insulating film, and this can be done in several ways. The paper attempts to analyse the fundamental problem of the measurement, deduces that it is essentially a statistical problem, and suggests a method by which it may be solved. The paper then proceeds to discuss the possibility of correlation between experimentally measured values of resistance and observed stray losses in completed machines. There is also some consideration of other relevant factors, such as temperature, pressure, and the nature of the plate surface.