Brushless DC motor fan driven by an AC power source

A brushless DC motor fan includes a rotor, a stator, a drive circuit, and a conversion circuit. An input of the conversion circuit is connected to an AC power source. The conversion circuit drops the voltage of the AC power source before connecting to a sensor element and a drive circuit. The drive circuit outputs alternately conducted current with positive/negative polarity to make the coil of the stator be alternately conducted, thereby generating an alternating magnetic field for driving the rotor.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a brushless direct-current (DC) motor fan
 that can be driven by an alternating-current (AC) power source.
 2. Description of the Related Art
 With technology advance, current brushless direct current motor fans can be
 mass-production in miniature size, light, weight, and compact with stable
 quality. Although widely used, they are still not suitable for household
 use, as they can not be driven by alternating current power sources which
 is available at ordinary house.
 SUMMARY OF THE INVENTION
 It is a primary object of the present invention to provide a brushless DC
 motor fan that is not limited to be driven by direct current such that
 miniature brushless DC motor can be widely and conveniently used.
 A brushless DC motor fan in accordance with the present invention includes
 a conversion circuit for converting alternating current for connection
 with a sensor element and a drive circuit. Thus, the drive circuit allows
 the current of the motor coil to be alternately conducted with
 positive/negative polarity, thereby generating an alternating magnetic
 field which interact with a permanent magnet of the rotor and cause the
 motor to rotate.
 Other objects, specific advantages, and novel features of the invention
 will become more apparent from the following detailed description and
 preferable embodiments when taken in conjunction with the accompanying
 drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIG. 1, a first embodiment of a brushless DC motor in
 accordance with the present invention generally includes a rotor 1, a
 stator 2, an axle tube 3, a drive circuit board 4, and a fixed plate 5.
 The rotor 1 includes an annular permanent magnet 11 with more than one set
 of north pole and south pole. The permanent magnet 11 is around the stator
 2 for induction purpose. The rotor 1 includes a shaft 12 that is pivotally
 mounted in a bearing 31 provided in the axle tube 3. The rotor 1 may
 include blades to form an impeller. Alternatively, the shaft 12 may
 include an impeller attached to an end thereof. The shaft 12 includes an
 annular groove 13 in the other end thereof for engaging with a retainer
 element 6 (e.g., a C-clip), thereby retaining the shaft 12 in place.
 The stator 2 includes pole plates 21 with coil 22 wound around pole posts
 (not labeled). The stator 2 includes a central hole 23 which the axle tube
 3 extends. Thus, the stator 2 is engaged with the rotor 3. The coil 22 of
 the stator 2 is electrically connected to and thus controlled by a drive
 circuit on the drive circuit board 4.
 The axle tube 3 is extended through the hole 23 of the stator 2, a hole 41
 in the drive circuit board 4, and engaged in a hub 51 on the fixed plate
 5. The bearing 31 is mounted in the axle tube for pivotally supporting the
 shaft 12 of the rotor 1. The axle tube 3 and the hub 51 on the fixed plate
 5 may be integrally formed if desired.
 In addition to the hole 41 for mounting the axle tube 3, the drive circuit
 board 4 includes necessary electric elements 42 to constitute a drive
 circuit. The drive circuit board 4 further includes a sensor element 43 to
 detect polarity of the permanent magnet 11 for producing a pulse signal,
 thereby allowing the drive circuit to output alternating current with
 positive/negative polarity to the coil 22 of the stator 2 for generating
 alternating magnet IC fields to drive the rotor 1.
 In addition to the hub 51 for mounting the axle tube 3, the fixed plate 5
 includes necessary electric elements 52 to constitute a conversion
 circuit. The fixed plate 5 has a power line 53 that acts as an input for
 the conversion circuit. The power line 53 may be electrically connected to
 a household alternating current power source. An output of the conversion
 circuit is electrically connected to the drive circuit of the drive
 circuit board 4. Preferably, the fixed plate 5 is integral with the
 housing of the fan or motor.
 Referring to FIG. 2, a section view of assembly of the first embodiment in
 accordance with the present invention, the axle tube 3 is extended through
 the stator 2 and the drive circuit board 4 and engaged on the fixed plate
 5. The bearing 31 is mount inside axle tube 3. The bore of bearing 31
 receives the shaft 12 which is retained in place by the retainer element 6
 engaged in the annular groove 13 of the shaft 12. The rotor 1 and the
 stator 2 are thus engaged together. The permanent magnet 11 of the rotor 1
 is around the stator 2 such that the rotor 1 is rotatable relative to the
 stator as a result of magnetic induction. The shaft 12 may be engaged with
 an impeller 14 such that the impeller 14 may rotate together with the
 rotor 1. Alternatively, the rotor I may include blades to form an
 impeller, as mentioned above.
 FIGS. 3 and 4 illustrate block diagram and detailed schematic diagram of a
 circuit in accordance with the present invention, respectively. The
 circuit includes a conversion circuit 54, a sensor element 43, and a drive
 circuit 44. The conversion circuit 54 includes a rectifier element 541, a
 voltage-dropping element 542, and a filter element 543.
 Alternating current is rectified by the rectifier element 541 and then
 dropped in voltage by the voltage-dropping element 542. The voltage
 waveforms are filtered and regulated by the filter element 543 to form a
 direct current power source with lower voltage. The direct current power
 source is electrically connected to the sensor clement 43 and the drive
 circuit 44. The drive circuit 44 cooperates with the sensor element 43
 that detects polar change of the permanent magnet 11 of the rotor 1 to
 thereby provide alternately conducted current with positive/negative
 polarity to the coil 22. Alternating magnetic field is thus formed to turn
 the rotor 1.
 Furthermore, the conversion circuit 54 and the drive circuit 44 may be made
 in an integrated circuit chip separately or together to thereby further
 minimize the overall size of the brushless DC motor and to thereby allow
 easier and simpler assembly and manufacture.
 FIG. 5 illustrates a second embodiment of the brushless DC motor of the
 invention. The stator (now designated by 7) is different from that in the
 first embodiment, and the drive circuit board 4 has been omitted. As
 illustrated in FIG. 5, the stator 7 includes pole plates 71 and coil 72,
 wherein the coil 72 is a single winding axially wound between upper and
 lower pole plates 71. The coil 22 of the stator 2 in the first embodiment
 is radially wound around pole posts of the pole plates 21. Thus, the
 stator 7 in the second embodiment can be manufactured in an easier manner.
 In addition, the drive circuit board 4 has been omitted, and the sensor
 element 43 and the drive circuit 44 are mounted on the fixed plate 5. The
 fixed plate 5 may be integral with the housing of the fan or motor.
 FIG. 6 illustrates a third embodiment of the brushless DC motor of the
 invention. When a brushless DC motor requires a higher torque output, the
 motor may include more than one stator 7 mounted on the axle tube 3 in
 series and more than one sensor element for detection Thus a brushless DC
 motor with a higher torque output is former as illustrated in FIG. 6.
 FIG. 7 illustrates a second embodiment of the conversion circuit 54 in
 accordance with the present invention, wherein a diode is connected to the
 AC power source in series to form the rectifier element 541 for proceeding
 with half-wave rectification on the alternating current A voltage-dropping
 element 542 consisting of a set of resistors that are connected in
 parallel is then connected in series for dropping voltage, and a filter
 element 543 consisting of at least one filtering capacitor is connected in
 parallel, thereby forming a DC voltage for driving the brushless DC motor
 fan.
 FIG. 8a is a voltage waveform diagram of point A in FIG. 7, FIG. 8b is a
 voltage waveform diagram of point B in FIG. 7, and FIG. 8c is a voltage
 waveform diagram of point C in FIG. 7. Voltage waveform of the AC power
 source at point A is represented in FIG. 8a. After rectification by the
 rectifier element 541 (the diode), the voltage waveform at point B is
 represented in FIG. 8b. After filtering and regulation by the filter
 element 543, the voltage waveform at Point C is represented in FIG. 8c. As
 can be seen from FIGS. 8a through 8c, the AC power source, after passing
 through the conversion circuit 54, is converted into a reliable DC power
 source for driving the brushless DC motor fan.
 FIG. 9 illustrates a third embodiment of the conversion circuit 54 in
 accordance with the present invention. The voltage-dropping element 542 of
 the conversion circuit 54 in this embodiment is in the form of a
 transformer. The AC power source after voltage drop is rectified by the
 rectifier element 541 and then finally filtered and regulated by the
 filter capacitor 543 for regulating the voltage waveform. A reliable DC
 power source is thus obtained for driving the brushless DC motor fan.
 FIG. 10 illustrates a fourth embodiment of the conversion circuit 54 in
 accordance with the present invention. In this embodiment, the filter
 element 543 of the conversion circuit 54 consists of a set of capacitors
 connected in parallel for filtering the alternating current. The filter
 element 543 is connected in series with a voltage dropping element 542
 consisting of a set of resistors connected in parallel for voltage
 dropping. At last, a Zener diode 544 is used to regulate the voltage to a
 stable DC power source for driving the brushless DC motor fan.
 According to the above description, the brushless DC motor in accordance
 with the present invention can be directly driven by alternating current
 to allow wider application and to solve the inconvenience of power source
 to conventional brushless DC motors.
 Although the invention has been explained in relation to its preferred
 embodiment as mentioned above, it is to be understood that many other
 possible modifications and variations can be made without departing from
 the spirit and scope of the invention. It is, therefore, contemplated that
 the appended claims will cover such modifications and variations that fall
 within the true scope of the invention.