An electronically commutated fan system comprising: an alternating-current-to-direct-current conversion unit receiving an alternating-current input power source (Viac) and converting the alternating-current input power source (Viac) into a first direct-current power source; an inverter unit electrically connected between the alternating-current-to-direct-current conversion unit and a fan, the inverter unit converting the first direct-current power source into an alternating-current output power source (Voac) and providing the alternating-current output power source (Voac) to the fan to rotate; a control unit electrically connected to the alternating-current-to-direct-current conversion unit and the inverter unit, the control unit controlling the alternating-current-to-direct-current conversion unit and the inverter unit to adjust a rotational speed of the fan, characterized by a phase predetermination unit electrically connected to the control unit and receiving a phase selection signal (Ss); wherein when the control unit detects a power variation of the alternating-current input power source (Viac), the control unit controls the rotational speed of the fan to be maintained within a first error range; wherein the phase predetermination unit predetermines the alternating-current input power source (Viac) to a three-phase alternating-current power source or a single-phase alternating-current power source according to the phase selection signal (Ss); the control unit receives an input signal (Sin) of the alternating-current input power source (Viac), and the control unit compares the input signal (Sin) and the phase selection signal (Ss) to determine that the alternating-current input power source (Viac) is three-phase-but-out-phase or single-phase; when the phase predetermination unit predetermines the alternating-current input power source (Viac) as the three-phase alternating-current power source and the control unit detects that the alternating-current input power source (Viac) is single-phase, an alarm signal is generated; when the phase predetermination unit predetermines the alternating-current input power source (Viac) as the single-phase alternating-current power source and the control unit detects that the alternating-current input power source (Viac) is single-phase, the alarm signal is not generated. an alternating-current-to-direct-current conversion unit receiving an alternating-current input power source (Viac) and converting the alternating-current input power source (Viac) into a first direct-current power source; an inverter unit electrically connected between the alternating-current-to-direct-current conversion unit and a fan, the inverter unit converting the first direct-current power source into an alternating-current output power source (Voac) and providing the alternating-current output power source (Voac) to the fan to rotate; a control unit electrically connected to the alternating-current-to-direct-current conversion unit and the inverter unit, the control unit controlling the alternating-current-to-direct-current conversion unit and the inverter unit to adjust a rotational speed of the fan, characterized by a phase predetermination unit electrically connected to the control unit and receiving a phase selection signal (Ss); wherein when the control unit detects a power variation of the alternating-current input power source (Viac), the control unit controls the rotational speed of the fan to be maintained within a first error range; wherein the phase predetermination unit predetermines the alternating-current input power source (Viac) to a three-phase alternating-current power source or a single-phase alternating-current power source according to the phase selection signal (Ss); the control unit receives an input signal (Sin) of the alternating-current input power source (Viac), and the control unit compares the input signal (Sin) and the phase selection signal (Ss) to determine that the alternating-current input power source (Viac) is three-phase-but-out-phase or single-phase; when the phase predetermination unit predetermines the alternating-current input power source (Viac) as the three-phase alternating-current power source and the control unit detects that the alternating-current input power source (Viac) is single-phase, an alarm signal is generated; when the phase predetermination unit predetermines the alternating-current input power source (Viac) as the single-phase alternating-current power source and the control unit detects that the alternating-current input power source (Viac) is single-phase, the alarm signal is not generated.