A large amount of energy will be saved if adjustable-speed drives are adopted by high and medium power pumps and fans. The power range of most of the drives is within 0.2～2 MW. The rated voltage of most of the motors in the power range is 10 kV because the power supply is 10 kV. The adjustable-speed scheme, a 10 kV motor is fed by a 10 kV inverter, is not reasonable because this kind of inverter consists of a lot of power electronic devices in series and its cost is high and reliability is poorer than lower voltage one. Owing to the existence of input transformer of the inverter, it is not necessary to ask for that the rated voltage of the motor and inverter is the same as power supply, i. e. a lower rated voltage can be used for the motor and inverter. The reasonable rated voltages of motors and inverters with different power grades are investigated and the by-pass methods when the rated voltages of the motors are different from power supply are discussed in this paper.