Based on the fact that asynchronous motors are characterized with periodic load operation, the optimal voltage of wound rotor asynchronous motors has been determined under no-load and light-load operating modes. With the load ratio a constant, the power factor under the optimal voltage is greater than that under the rated voltage. In light of the features of winding rotor asynchronous motors, a research has been conducted on the side series resistance starting and the capacity of voltage regulation and energy-saving series under series resistance speed regulation. When a resistance of 2 is connected in series with the rotor circuit, an accelerated starting of the motor has thus achieved. With the optimal voltage a constant, the electromagnetic torque remains constant before and after the series resistance has been connected with the rotor circuit. A step-down series resistance speed regulation model of wound rotor asynchronous motors has been set up to work out the relationship between the energy consumption of the rotor turn-on and rotor series resistance under a constant optimal voltage. When the load factor of the motor is in the range of 0%~50%, a certain energy-saving space could be obtained by appropriately reducing the power supply voltage, thus verifying the feasibility of the energy saving operation under the light load rotor series resistance voltage regulation of wound rotor asynchronous motors under the optimal voltage.