The variation of the voltage has been known to be harmful in the sound operation of motors. It also results in the variation in slip. If the voltage and slip are known, every characteristics of motors can be determined. This paper, therefore, analyzed numerically the characteristics of motors by the formula expressed in the function of both slip and voltage, and then satisfactorily compared with the experimental values. A new per-unit method derived in this paper is shown to be easier and simpler than the conventnional one in determining the relative running characteristics. It has also suggested the minimum allowable voltage when voltage below the rated value was applied to the motor. It is expected to apply to the determination of the characteristics under the voltage variation.