The invention provides a low-power ventilating fan which comprises a minisize
permanent magnet synchronous motor and an
impeller which can be directly driven by the motor and can rotate according to a predetermined rotating direction, wherein the
impeller is of an axial-flow type and manufactured in an
injection moulding mode, the
diameter of the
impeller is not more than 150mm; the
pole number of a motor
stator is 4 or 6 or 8, the winding of the
stator is connected to an
alternating current power supply according to a specified winding direction; the
pole number of a
permanent magnet rotor is the same with that of the
stator; air gap of each pole can be narrowed along the rotating direction, so that each pole axis of the rotor can deflect an
acute angle along the rotating direction in the
free state by taking the axis of one pole of the closest stator as a reference; a
position sensor can detect the rotor at a set position on the circumference of the rotor, a
control circuit can determine the direction of each polar permanent
magnet flux and detect the power supply polarity by taking the position as a reference and the output of the sensor so as to determine the axis direction ofeach polar main flux of the stator; when the motor starts, a switch can connect the
half wave of the power supply if the main flux axis direction and the permanent
magnet flux axis direction are intersected and form the
acute angle; or when the motor runs, the switch can connect the
half wave of the power supply right now if the main flux axis direction is opposite to the permanent
magnet flux axis direction or close to the opposite direction. The ventilating fan has better performance in the starting and running processes, while the cost is relatively low.