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In this paper, a comprehensive definition of eccentricity fault in cage induction motor is introduced, and instantaneous power is considered as eccentricity-fault index. In addition to the line current, the stator line voltage influences the instantaneous power of the motor; therefore, harmonics of this power can be employed as a fault-diagnosis index when the motor is supplied by a closed-loop control method. It is theoretically shown that the frequencies of harmonics of the instantaneous power due to mixed eccentricity fault are the same for the mains-fed and open/closed-loop drive-connected motors, although the number of factors affecting the amplitudes of the harmonics is relatively large when closed-loop drive connected. A flexible test rig is described, which was established to carry out various measurements on the eccentric motor with different eccentricity degrees, under different supply and control methods, varying load, and variable speeds. The experimental results indicate that the amplitudes of the dominant instantaneous-power harmonics depend on the degrees of mixed-eccentricity static and dynamic components, although these amplitudes are influenced by the supply methods, load level, and speed of the motor.