MOTOR ASSEMBLY
BACKGROUND OF THE INVENTION
1. Field of the Invention 5
This invention relates to a motor assembly, more particularly to a motor assembly which includes a dc motor and a driving unit for driving the dc motor at different speeds.
2. Description of the Related Art A conventional motor assembly usually includes an
induction motor and a variable frequency device. The aforementioned conventional motor assembly has the following disadvantages:
1. The induction motor assembly is relatively large, and 15 the noise that is generated during operation is relatively loud. Furthermore, the cost of the induction motor assembly is relatively high.
2. The variable frequency device varies the current for the induction motor so as to enable the induction motor to 20 rotate at different speeds. However, the torque ratio thus obtained is relatively low. Therefore, the problem
of an insufficient torsional force is present when the motor speed is relatively low.
3. The induction motor usually comprises a rotor which 25 includes a rotatable shaft and a plurality of fan-shaped permanent magnets attached to the rotatable shaft by means of adhesives. Therefore, undesired disengagement of the permanent magnets from the rotatable shaft may occur during the rotation of the rotatable shaft. 30 Furthermore, the unique shape of the permanent magnets results in a higher processing cost and complicates the assembling process.
Therefore, the main objective of the present invention is to provide a motor assembly which includes a brushless dc motor and a driving unit for driving the dc motor at different speeds and which is able to overcome the disadvantages 40 associated with the prior art.
According to the present invention, a motor assembly includes a brushless dc motor and a driving unit for driving the dc motor at different speeds. The motor includes a stator and a rotor. The stator includes a housing and a three-phase 45 coil unit installed in the housing and provided with three windings. The rotor extends through the housing of the stator. The driving unit includes a power supplying circuit for generating a dc output. A first power transistor unit includes three first power transistors. Each of the first power 50 transistors is activable to connect electrically a respective one of the windings of the coil unit and the power supplying circuit. A driving circuit is connected electrically to the power transistors of the first power transistor unit for activating the first power transistor unit when the power sup- 55 plying circuit is activated. A sensor circuit is used for sensing rotating speed of the rotor and for generating three speed pulses which represent the rotating speed of the rotor and which are out of phase. A frequency-to-voltage converter is connected electrically to the sensor circuit for 60 generating an analog voltage signal corresponding to the rotating speed of the rotor. A speed setting unit is activable to generate a reference voltage signal equal to a desired motor speed. A comparing unit is connected electrically to the speed setting unit and the frequency-to-voltage converter 65 for comparing the reference voltage signal with the analog voltage signal and for generating an analog error voltage
signal having a magnitude corresponding to a difference between the analog voltage signal and the reference voltage signal. A pulse width modulating circuit is connected electrically to the comparing unit for generating a variable-width modulated pulse signal with the analog error voltage signal as a modulating signal. A second power transistor unit includes three second power transistors. Each of the second power transistors corresponds to one of the first power transistors and is activable to connect electrically the power supplying circuit and a respective one of the windings of the coil unit. The driving circuit receives the three speed pulses from the sensor circuit and processes the speed pulses so as to activate intermittently the first power transistors such that two of the first power transistors are activated and one of the first power transistors is deactivated at any time. The driving circuit is further connected electrically to the pulse width modulating circuit and the second power transistors for activating one of the second power transistors corresponding to the deactivated one of the first power transistors upon receiving the modulated pulse signal from the pulse width modulating circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiment, with reference to the accompanying drawings, of which:
FIG. 1 is a schematic block diagram showing a motor assembly according to the present invention;
FIG. 2 is a partly exploded view showing a brushless dc motor of the motor assembly according to the present invention; and
FIGS. 3 to 6 are schematic circuit diagrams of a driving unit of the motor assembly according to the present invention.
DETAILED DESCRIPTION OF THE
PREFERRED EMBODIMENT
Referring to FIGS. 1 and 2, a motor assembly according to the present invention includes a brushless dc motor 500 and a driving unit (D) for driving the dc motor 500 at different speeds.
The dc motor 500 includes a stator 510 and a rotor 520. The stator 510 includes a housing 511 and a three-phase coil unit 512 installed in the housing 511 and provided with three windings. The rotor 520 includes a rotatable shaft 521 extending rotatably through the housing 511 of the stator 510, a magnet-mounting member 522 sleeved securely on the rotatable shaft 521 and formed with a plurality of axially extending magnet-receiving cavities 5220 around the rotatable shaft 521, and a plurality of permanent magnets 523 received respectively in the magnet-receiving cavities 5220.
The driving unit (D) includes a power supplying circuit 100, a first protective circuit 300, first and second power transistor units 400 and 450, a sensor circuit 600, a driving circuit (U), a frequency-to-voltage converter 110, a speed setting unit 120, a comparing unit 130, a pulse width modulating circuit 140 and a second protective circuit 150.
Referring now to FIGS. 1 and 3, the power supplying circuit 100 includes two bridge rectifiers 101,103. Each of the rectifiers 101,103 is adapted to be connected to an ac outlet and generates a dc output. The dc output of the rectifier 101 is applied to a transformer 202 via a capacitor 102 and a switching transistor circuitry 201. The transformer
202 supplies dc voltages to a plurality of rectifiers 203 so as to generate a plurality of regulated dc voltage outputs for driving the electronic circuits of the driving unit (D).
Referring to FIGS. 1, 3 and 4, the first protective circuit 300 includes a capacitor 301 which is connected electrically 5 to and which is charged by one of the regulated dc voltage outputs of the rectifiers 203. A D-type flip-flop 362 is connected electrically to the capacitor 301. A relay unit 303 is connected electrically to the D-type flip-flop 302 via two transistors (Ql and Q2). Three capacitors 304,305,306 are 10 connected electrically to the relay unit 303 and to the bridge rectifier 103 so as to be charged by the dc output of the bridge rectifier 103 when the relay unit 303 is activated.
The first power transistor unit 400 includes three first power transistors 401,402,403. Each of the first power 15 transistors 401,402,403 has a gate terminal, a drain terminal connected electrically to the capacitors 304,305,306, and a source terminal connected electrically to a respective one of the windings of the coil unit 512.
The second power transistor unit 450 includes three 20 second power transistors 451,452,453. Each of the second power transistors 451,452,453 corresponds to one of the first power transistors 401,402,403 and has a gate terminal, a drain terminal connected electrically to a respective one of the windings of the coil unit 512, and a source terminal 25 connected electrically to the capacitors 304,305,306.
Referring to FIGS. 1, 2 and 5, the sensor circuit 600 includes three angularly displaced Hall sensors 602 which are mounted on a stationary magnetic disk 601 that is 3Q provided adjacent to one end of the rotatable shaft 521. In the present embodiment, the angular displacement between two adjacent Hall sensors 602 is approximately 30 degrees.
The driving circuit (U) includes a first circuit portion 800 and a second circuit portion 900. The first circuit portion 800 35 includes three exclusive-OR gates (801a,8016,801c) and six inverters (802a to 802/). Each of the exclusive-OR gates (801a,8016,801c) has a first input terminal connected electrically to a respective one of the Hall sensors 602 and a second input terminal connected electrically to an output 40 terminal of an exclusive-OR gate 704 which has a first input terminal connected electrically to an output terminal of an exclusive-OR gate 702 and a grounded second input terminal. The exclusive-OR gate 702 has a normally high first input terminal and a grounded second input terminal. It 45 should be appreciated that the first input terminal of the exclusive-OR gate 702 may be grounded if necessary. The inverter (802a) has an input terminal connected electrically to an output terminal of the exclusive-OR gate (801a) and an output terminal. The inverter (8026) has an input terminal 50 connected electrically to the output terminal of the inverter (802a) and an output terminal. The inverter (802c) has an input terminal connected electrically to an output terminal of the exclusive-OR gate (8016) and an output terminal. The inverter (802d) has an input terminal connected electrically 55 to the output terminal of the inverter (802c) and an output terminal. The inverter (802e) has an input terminal connected electrically to an output terminal of the exclusive-OR gate (801c) and an output terminal. The inverter (802/) has an input terminal connected electrically to the output termi- 60 nal of the inverter (802e) and an output terminal.
The second circuit portion 900 of the driving unit (U) includes six AND gates 901 to 906 and six inverters 907 to 912. The AND gate 901 has a first input terminal connected electrically to the output terminal of the inverter (8026), a 65 second input terminal connected electrically to the output terminal of the inverter (802c), and an output terminal. The
AND gate 902 has a first input terminal connected electrically to the output terminal of the inverter (802rf), a second input terminal connected electrically to the output terminal of the inverter (802e), and an output terminal. The AND gate 903 has a first input terminal connected electrically to the output terminal of the inverter (802a), a second input terminal connected electrically to the output terminal of the inverter (802/), and an output terminal. The AND gate 904 has a first input terminal connected electrically to the output terminal of the inverter (802d), a second input terminal connected electrically to the output tenninal of the inverter (802a), a third input terminal and an output terminal. The AND gate 905 has a first input terminal connected electrically to the output terminal of the inverter (802c), a second input terminal connected electrically to the output terminal of the inverter (802/), a third input terminal and an output terminal. The AND gate 906 has a first input terminal connected electrically to the output terminal of the inverter (8026), a second input terminal connected electrically to the output terminal of the inverter (802e), a third input terminal and an output terminal. Each of the inverters 907 to 912 has an input terminal connected electrically to the output terminal of a respective one of the AND gates 901 to 906, and an output terminal connected electrically to the gate terminal of a respective one of the power transistors 401,402,403 and 451,452,453 via an optical coupler 407.
Referring now to FIGS. 1, 5 and 6, the frequency-tovoltage converter 110 has an input terminal connected electrically to an output terminal of an exclusive-OR gate 111 which has a grounded first input terminal and a second input terminal connected electrically to the output terminal of each of the inverters (802a to 802/) via a diode and a capacitor.
The speed setting unit 120 is activable to generate a reference voltage signal equal to a desired motor speed. In the present embodiment, the reference voltage signal is obtained by the adjustment of a variable-resistor.
The comparing unit 130 includes a comparator 131 which has input terminals connected electrically to the speed setting unit 120 and the frequency-to-voltage converter 110. An adder 134 has an input terminal connected electrically to an output terminal of the comparator 131 via a buffer 132 and to the output terminal of the speed setting unit 120.
The pulse width modulating circuit 140 includes a triangular-wave generating integrated circuit 141 which generates a train of triangular-wave voltage signals. A comparator 142 has input terminals connected electrically to an Output terminal of the adder 134 and an output terminal of the integrated circuit 141. The comparator 142 has an output terminal connected electrically to the third input terminal of each of the AND gates 904,905,906 via a node (N).
The second protective circuit 150 includes a current detecting unit and a temperature detecting unit. The current detecting unit includes a current detector 151 (FIG. 4) which is connected electrically to the capacitors 304,305,306 for detecting an amount of load current from the power supplying circuit 100 and which is connected electrically to a comparator 154 via a low pass filter 152 and a nonphaseinverting amplifier 153. The comparator 154 compares the output from the amplifier 153 with a predetermined value and has an output terminal connected electrically to a reset terminal of a D-type flip-flop 155. The flip-flop 155 has an output terminal connected electrically to the output terminal of the comparator 142 via a diode 156. The temperature detecting unit is used for detecting temperature of the dc motor 500 and includes a thermal switch 504 which is
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