A variable-frequency motor device that runs on alternating current includes a first converting circuit, a second converting circuit, and a DC variable-frequency motor. An operation module is coupled to the variable-frequency motor device, and provides the alternating current to the first or second converting circuit. The first converting circuit receives the alternating current, and generates a first rotation-speed signal. The second converting circuit receives the alternating current, and generates a second rotation-speed signal. The DC variable-frequency motor is driven at a rotation speed according to the first or second rotation-speed signal.