Brushless dc motor assembly - Patent Review 6307337

What is claimed is:

1. A brushless dc motor comprising:

a stator assembly having a stator winding thereon and a rotor opening formed therein;

a rotor assembly having a rotor shaft, a rotor core fixed to said rotor shaft, and first and second bearings secured to said rotor shaft on opposite sides of said rotor core, said rotor core being disposed at least partially within said rotor opening with said first and second bearings positioned on opposite sides of said stator assembly; and

a control board having control electronics connected thereto for receiving an electrical input and establishing current in said stator winding for generating a rotating magnetic field in said stator assembly;

wherein said control electronics include first and second hall effect devices for sensing a rotational position of said rotor and providing first and second feedback signals, respectively, to said control board representative of said rotational position,

and wherein said control electronics are connected to said stator winding to form an H-bridge circuit for establishing said current in said stator winding in dependence of said electrical input and a user-controlled enable input to said control board,

said H-bridge circuit comprising first and second pairs of driving elements having conducting and non-conducting states, said first pair of driving elements being in a conducting state to conduct a first current through said winding when said first feedback signal and said enable signal are high, and said second pair of driving elements being in a conducting state to conduct a second current through said winding when said second feedback signal and said enable signal are high, said first current being in a direction through said winding which is opposite from the direction of said second current.

2. A brushless dc motor assembly according to claim 1, wherein said control electronics further include:

electronics for establishing a dc bus voltage from said electric power source; and

an under voltage circuit for providing an under voltage feedback signal representative of the magnitude of said dc bus voltage,

wherein said first pair of driving elements are in a conducting state to conduct said first current through said winding when said under voltage feedback signal, said first feedback signal and said enable signal are high, and said second pair of driving elements being in a conducting state to conduct said second current through said winding when said under voltage feedback signal, said second feedback signal, and said enable signal are high.

3. A brushless dc motor assembly according to claim 2 wherein said electronics further include:

a current limit circuit for establishing an undercurrent signal representative of the magnitude of said bi-directional current through said stator winding,

wherein said first pair of driving elements are in a conducting state to conduct said first current through said winding when said current limit signal, said first feedback signal and said enable signal are high, and said second pair of driving elements being in a conducting state to conduct said second current through said winding when said current limit signal, said second feedback signal and said enable signal are high.

4. A brushless dc motor assembly according to claim 1 wherein said electronics further include:

a current limit circuit for establishing an undercurrent signal representative of the magnitude of said bi-directional current through said stator winding,

wherein said first pair of driving elements are in a conducting state to conduct said first current through said winding when said current limit signal, said first feedback signal and said enable signal are high, and said second pair of driving elements being in a conducting state to conduct said second current through said winding when said current limit signal, said second feedback signal and said enable signal are high.

5. A brushless dc motor assembly according to claim 1, wherein said control electronics further include,

electronics for establishing a dc bus voltage on a dc bus from said electric power source,

and wherein said first pair of drive elements comprise first and second solid state devices and said second pair of drive elements comprise third and fourth solid state devices, each of said solid state devices having first and second terminals for conducting current when said device is in said conducting state and a third terminal for controlling the conductive state of said solid state device,

and wherein said first terminal of said first solid state device is connected to said first terminal of said fourth solid state device and to a first end of said motor winding, said first terminal of said third solid state device is connected to said first terminal of said second solid state device and to a second end of said motor winding, said second terminal of said first solid state device and said second terminal of said third solid state device are connected to said dc bus,

and wherein said enable input and said first feedback signal are connected as inputs to a first AND gate, an output of said first AND gate being connected for driving said third terminal of said fourth solid state device,

and wherein said enable input and said second feedback signal are connected as inputs to a second AND gate, an output of said second AND gate being connected for driving said third terminal of said second solid state device,

and wherein said first feedback signal is connected for driving said third terminal of said third solid state device and said second feedback signal is connected for driving said third terminal of said first solid state device.

6. A brushless dc motor assembly according to claim 5, wherein said first, second, third, and fourth solid state devices are MOSFETs.

7. A brushless dc motor assembly according to claim 6, wherein said second and said fourth solid state devices are N-channel MOSFETs and said first and third solid state devices are P-channel MOSFETs.

8. A brushless dc motor assembly according to claim 1, wherein said control electronics further include:

an under voltage circuit for providing an under voltage feedback signal representative of the magnitude of said dc bus voltage as an input to said first and second AND gates;

wherein said under voltage feedback signal is an output of a comparator circuit, said comparator circuit comprising first and second voltage dividers connected between a voltage representative of said dc bus voltage and ground, said first voltage divider including a zener diode connected between a node of said first voltage divider and ground; and

an OPAMP, a negative terminal of said OPAMP being connected to said node of said first voltage divider establishing a zener voltage at said negative terminal, and said positive terminal being connected to a node of said second voltage divider,

said under voltage feedback signal thereby disabling said first and second pairs of driving elements when said voltage at said node of said second voltage divider drops below said zener voltage.

9. A brushless dc motor assembly according to claim 8, wherein said electronics further include:

a current feedback circuit for providing a current limit feedback signal representative of the magnitude of said bi-directional current through said stator winding as an input to an OPAMP comparator for comparing a voltage level of said current limit feedback signal with a predetermined voltage level and providing output to a timer circuit,

said timer circuit providing a pulsed output to said first and second AND gates in dependence of said output from said comparator, said pulsed output of said timer circuit thereby pulsing said first and second pairs of driving elements from an "on" to an "off" state when said bi-directional current exceeds a predetermined level.

10. A brushless dc motor assembly according to claim 1, wherein said electronics further include:

a current feedback circuit for providing a current limit feedback signal representative of the magnitude of said bi-directional current through said stator winding as an input to an OPAMP comparator for comparing a voltage level of said current limit feedback signal with a predetermined voltage level and providing output to a timer circuit,

said timer circuit providing a pulsed output to said first and second AND gates in dependence of said output from said comparator, said pulsed output of said timer circuit thereby pulsing said first and second pairs of driving elements from an "on" to an "off" state when said bi-directional current exceeds a predetermined level.

11. A brushless dc motor assembly according to claim 10 wherein said timer circuit includes an LM555 IC timer, and wherein said comparator output is connected to a trigger input of said timer.

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FIELD OF THE INVENTION

The present invention relates to a dc motor assembly and, more particularly, to a compact and efficient brushless dc motor assembly.

BACKGROUND OF THE INVENTION

Permanent magnet brushless dc motors are widely used in a variety of applications due to their simplicity of design, high efficiency, and low noise. These motors operate by electronic commutation of stator windings rather than the conventional mechanical commutation accomplished by the pressing engagement of brushes against a rotating commutator. To achieve electronic commutation, brushless dc motor designs usually include an electronic controller for controlling the excitation of the stator winding(s). Advantageously, electronic commutation allows for specific, user-controlled motor operating characteristics.

Given the relative simplicity of design for permanent magnet brushless dc motors, however, prior art designs have failed to satisfactorily achieve a reliable design which may be produced at a minimized cost. For example, prior art brushless dc motor designs generally control motor operating characteristics using relatively complicated microprocessor-based electronics. Incorporation of a microprocessor into the control board is expensive and time-consuming from a production standpoint. In addition, the more components that are incorporated into the control electronics, the more likely it is that operational malfunctions will occur as a result of damaged or improperly installed electronics.

Also, elaborate mounting mechanisms have been developed in the prior art for mounting hall effect sensors adjacent the rotor to sense rotor rotational position. The hall effect sensors are typically connected to the control board via separate wires to provide a feedback signal for controlling motor operation through the electronics. Typically, the electronics are also mounted to the assembly using a separate bracket or other mechanism which further complicates and adds costs to the production process.

Accordingly, there is a long-felt need in the art for a brushless dc motor which is capable of achieving user-controlled operating characteristics, and which is efficient, compact, and cost-effective.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention is to provide a brushless dc motor having control electronics for allowing variable speed, user-controlled motor operation in an efficient, cost-effective, and reliable design.

Yet another object of the invention is to provide a permanent magnet, brushless dc motor having control electronics mounted to a control board, wherein the control board is secured directly to a bearing bracket of the motor for eliminating cumbersome manufacturing steps and reducing the size of the motor assembly.

Yet another object of the invention is to provide a permanent magnet, brushless dc motor having control electronics mounted to a control board which is secured directly to a bearing bracket of the motor, wherein hall devices are mounted directly to the control board in a position adjacent the rotor to eliminate the need for a separate mounting mechanism for the hall devices and reduce the size of the motor assembly.

Another object of the present invention is to provide a reliable and efficient brushless dc motor which may be efficiently manufactured, thereby reducing the consumer cost of such motors.

These and other objects of the present invention will become apparent from a review of the description provided below.

SUMMARY OF THE INVENTION

The present invention is organized about the concept of providing an efficient, reliable brushless dc motor which is of a simple and cost-efficient design. The motor includes control electronics for allowing variable speed, user-controlled motor operation. The control electronics are connected to a control board which is mounted directly to the motor bearing bracket. Hall devices for providing rotor rotational position information to the electronics are connected directly to the control board and extend therefrom to a position adjacent the rotor.

Specifically, the brushless dc motor of the present invention includes: a stator assembly having a stator winding thereon and a rotor opening formed therein; and a control board having control electronics connected thereto for receiving an electrical input and establishing current in the stator winding for generating a rotating magnetic field in the stator assembly. A rotor assembly of the motor has a rotor shaft, a rotor core fixed to the rotor shaft, and first and second bearings secured to the rotor shaft on opposite sides of the rotor core. The rotor core is disposed at least partially within the rotor opening in the stator with the first and second bearings positioned on opposite sides of the stator assembly.

A bearing bracket assembly secures the rotor in position relative the stator assembly, provides bearing surfaces for the bearings, and provides means for mounting the control board. The bearing bracket assembly includes a first bearing bracket for receiving the first bearing and a second bearing bracket for receiving the second bearing. The first and second bearing brackets each have at least one bore formed therein which aligns with a corresponding bore formed through the stator assembly. At least one fastener passes through the at least one bore in the first bearing bracket, the corresponding bore in the stator assembly, and the at least one bore in the second bearing bracket. The fastener thereby secures the first and second bearing brackets to the stator assembly with the first and second bearings received at least partially within the first and second bearing brackets, respectively.

The control board has at least one bore therein which aligns with the at least one bore in the second bearing bracket. Advantageously, the fastener extends through the second bearing bracket and through the at least one bore in the control board for securing the control board to the bearing bracket assembly. With this construction, the control board may be efficiently assembled to the motor assembly without the need for a separate mounting bracket or other mechanism.

Also, the control electronics include at least one hall effect device, and preferably two hall effect devices. The hall effect device has leads connected (soldered) directly to the control board and a sensor portion for establishing an output on at least one of the leads which is representative of a rotational position of the rotor core. Advantageously, no separate mounting mechanism for the hall effect device(s) is necessary since the hall device(s) extends directly from the control board toward the rotor core so that the sensor portion is positioned adjacent the rotor core. Where two hall effect devices are used, as in the preferred embodiment, they are secured to the control board so that the sensor portions thereof are positioned adjacent the rotor core at approximately 180 degrees apart relative to the circumference of the rotor opening.

In the preferred embodiment, production efficiency is also achieved by forming the rotor-core as a single-piece, permanent magnet. Preferably, the rotor core is cylindrical with a central bore formed therein. The rotor core is secured to the shaft with the shaft extending through the central bore. Also, in the preferred embodiment, operating efficiency is achieved by forming a pair of radially inward extending teeth on an inner surface of the rotor opening at approximately 180 degrees apart. The teeth form magnetic poles on the stator which provide starting orientation of the permanent magnet rotor assembly relative to the hall effect devices and cause a magnetic attraction during motor operation.

Also, in the preferred embodiment, the motor assembly is formed as a C-frame motor having a C-frame portion, an I-bar portion, and a bobbin around which the stator winding is formed. The I-bar portion extends through a central opening in the bobbin and is secured to the C-frame portion to complete the stator assembly. Although a C-frame motor construction is preferred, the features and advantages of the invention could be used in connection with other stator constructions.

The control electronics provide open-loop, variable speed control in a cost-efficient design. As discussed above, the control electronics preferably include first and second hall effect devices for sensing a rotational position of the rotor. The hall effect devices provide first and second feedback signals, respectively, to the control board representative of the rotational position. The control electronics are connected to the stator winding to form an H-bridge circuit for establishing the current in the stator winding in dependence of the electrical input and a user-controlled enable input to the control board.

The H-bridge includes first and second pairs of driving elements having conducting and non-conducting states. The first pair of driving elements are connected in the control electronics to be in a conducting state to conduct a first current through the winding when the first feedback signal and the enable signal are high. The second pair of driving elements are connected in the control electronics to be in a conducting state to conduct a second current through the winding when the second feedback signal and the enable signal are high. Bi-directional current flow is established in the stator winding since the first current is in a direction through the winding which is opposite from the direction of the second current.

The control electronics also include electronics thereon for establishing a dc bus voltage from the electric power source. The driving elements in the H-bridge circuit use the dc bus voltage as a supply voltage. To ensure proper operation of the driving elements, i.e. preferably power MOSFETs, an under voltage circuit is provided. The under voltage circuit establishes an under voltage feedback signal representative of the magnitude of the dc bus voltage. The under voltage feedback signal is connected in the electronics so that the first pair of driving elements are in a conducting state to conduct the first current through the winding when the under voltage feedback signal, the first feedback signal and the enable signal are high, and the second pair of driving elements are in a conducting state to conduct the second current through the winding when the under voltage feedback signal, the second feedback signal, and the enable signal are high.

Also, to ensure that the motor current does not exceed acceptable values a current limit circuit is provided for establishing a current limit signal representative of the magnitude of the current through the stator winding. The current limit signal is connected in the electronics so that the first pair of driving elements are in a conducting state to conduct the first current through the winding when the current limit signal, the first feedback signal and the enable signal are high, and the second pair of driving elements are in a conducting state to conduct the second current through the winding when the current limit signal, the second feedback signal and the enable signal are high.

Preferably, the first pair of drive elements in the H-bridge circuit comprise first and second solid state devices and the second pair of drive elements comprise third and fourth solid state devices. Each of the solid state devices, e.g power MOSFETs, has first and second terminals for conducting current when the device is in the conducting state and a third terminal for controlling the conductive state of the solid state device.

In the preferred H-bridge circuit, the first terminal of the first solid state device is connected to the first terminal of the fourth solid state device and to a first end of the motor winding. The first terminal of the third solid state device is connected to the first terminal of the second solid state device and to a second end of the motor winding. The second terminal of the first solid state device and the second terminal of the third solid state device are connected to the dc bus.

The enable input and the first feedback signal are connected as inputs to a first AND gate. An output of the first AND gate is connected for driving the third terminal of the fourth solid state device. The enable input and the second feedback signal are co