Table I shows that zinc phosphate is not suitable to provide electrical insulation between aluminum cage and rotor steel. Although zinc phosphate shows good electrical insulation property (at least 10 times greater than thick Fe3O4 film, in Franklin tests) it shows noticeable low mechanical strength to friction. Therefore, it is supposed that this film cannot withstand the molten aluminum flow during the cast process and it is, plucked out or damaged. Manganese phosphate film has similar insulation property of zinc phosphate but with superior mechanical strength and