This chapter explains iron losses generated in a motor made of nonoriented electrical steel and driven by an inverter consisted of the fast switching power semiconductor devices, GaN-FETs. The rectangular voltage pulses due to PWM applied to the motor generate not only a fundamental current but also the harmonic currents. One of them at the resonant frequency of the L, R, and C components in the motor grows considerably. The phenomenon is called ringing. The ringing current as well as the other currents causes their respective iron losses. The paper experimentally confirms that the ringing occurs in the motor and quantitatively evaluates the iron loss related to the ringing. The proportional dependence of the loss on the carrier frequency of PWM is found out.