Iron loss in two kinds of electrical steel, which have the same iron loss in case of conventional linear amplifier excitation but different thickness, is found to have a difference of 10%-20% in case of PWM inverter excitation, which is often used in a motor drive control system. The PWM inverter has many time harmonics in the output voltage, whereas the linear amplifier does not. The difference in iron loss is not always ignored while considering the electrical steel grade in commercial catalogs and the JIS (Japanese Industrial Standards). Since the iron loss also increases by about 10%-60% in PWM inverter excitation, a material evaluation system and a method for reducing iron loss are considered to be important.