Abstract:
A brushless motor circuit is for driving a brushless direct current (BLDC) motor. The motor includes a rotor and exciting coils for respective phases in a three-phase winding in a star configuration. A neutral point in the star configuration is configured to switch to one of a ground voltage, a supply voltage, and an open circuit voltage to provide more combinations. The combinations provide extra steps in one revolution for a better resolution with enhanced efficiency.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to a brushless motor drive circuit formed as a semiconductor integrated circuit, and more specifically to a brushless motor drive circuit providing higher mechanical rotational steps for a better resolution.  
       BACKGROUND OF THE INVENTION  
       [0002]     Direct current (DC) motors are very popular in variable speed drives due to simple speed control and simple control circuits. However, the DC motor initially used hard brushes, due to which the DC motors suffered from a low reliability and required frequent maintenance or replacement. These drawbacks of the DC motors were eliminated by using brushless DC motors (BLDC), which are highly reliable and can be used in applications requiring high speed.  
         [0003]     A BLDC motor includes two coaxial magnetic armatures separated by an air gap. An external armature is called a stator and an internal armature is called a rotor. In the BLDC motor, the rotor is a permanent magnet and is supplied by a constant DC current. The stator is poly-phased, three-phases in the present invention, and is coveted by poly-phased currents. Three phase brushless DC motors are used in automotive equipment, refrigerators, air conditioners, compressors and fans due to their high efficiency, silent operation, compact form, reliability and longevity.  
         [0004]      FIG. 1  illustrates a circuit diagram of star connected windings for a conventional BLDC motor. The star connected windings of the BLDC motor are connected to commutation switches. The commutation switches can be field effect transistors (FET). The star connected windings, such as coil A, coil B and coil C are connected in a star configuration with a neutral node  4 . A node  1  of coil A is connected to switches S 1  and S 2 . A node  2  of coil B is connected to switches S 5  and S 6  and a node  3  of coil C is connected to switches S 3  and S 4 . The node  4  is unutilized and is kept at an open circuit voltage. The switches S 1 , S 3  and S 5  are connected to a supply voltage V and the switches S 2 , S 4  and S 6  are connected to a ground voltage. These switches can be controlled by specifically designed devices for motor control applications, like ST7FMC devices, as illustrated in  FIG. 2 .  
         [0005]     Using a single-pole three-phase BLDC motor as illustrated above, one mechanical rotation can be achieved in six steps. Each step corresponds to  60  degrees of rotation, i.e., 360/6 . The six steps are generated by switching different combinations of switches as illustrated in  FIG. 3A  and  FIG. 3B . Step  1  shows a node  1  connected to the positive supply voltage V and the node  3  connected to the ground voltage, by turning the switches S 1  and S 4  to an on state. A resultant magnetic field will align the rotor in a direction as illustrated in step  1 . In Step  2 , the switches S 1  and S 6  are in the on state, so the node  1  is connected to the positive supply voltage V and the node  2  is connected to the ground voltage. The resultant magnetic field will turn the rotor in a counter clockwise direction by an additional 60 degrees as illustrated in step  2 . In Step  3 , the switches S 3  and S 6  are in the on state, so the node  2  is connected to the ground voltage and the node  3  is connected to the positive supply voltage V. As a result the rotor will be rotated by 60 degrees in the counter clockwise direction. The next corresponding three steps (Step 1 , Step 2 , Step 3 ) are illustrated in  FIG. 3B . By reversing switching patterns of these commutation switches a rotation in a clockwise direction can be achieved.  
         [0006]     Therefore, there is a need of a brushless motor drive circuit to provide additional steps in one rotation for a better resolution in each step,  
       SUMMARY OF THE INVENTION  
       [0007]     It an object of the present invention to provide a cost effective brushless DC (BLDC) motor circuit for achieving additional steps in one mechanical revolution such as without utilizing any additional hardware. The proposed circuit is a cost effective technique as it can provide additional steps without utilizing two pole configurations, which need two different sets of coils for implementation.  
         [0008]     It is another object of the present invention to provide a brushless DC (BLDC) motor circuit providing additional rotational for a better resolution and efficiency.  
         [0009]     To achieve the objectives, the present invention provides a brushless motor circuit driving a brushless direct current (BLDC) motor. The motor may have a rotor and exciting coils, with the coils wound in a three-phase winding connected in a star configuration to provide additional rotational steps for a high resolution. The motor may comprise a detector circuit detecting an induced voltage generated across the exciting coil; a rotor position signal generating circuit producing a specified position signal for exciting coil of each phase; and a control circuit performing excitation control of the exciting coils by controlling switching elements for conducting excitation currents via the exciting coils and based on the rotor position signal. A neutral node in the star configuration may be switched to one of a ground voltage, a supply voltage, and an open circuit voltage to provide the additional steps.  
         [0010]     Further, the present invention provides a method of providing additional rotational steps in a brushless DC motor for a better resolution. The motor may have a rotor and exciting coils wound in a three-phase winding connected in a star configuration. The method may comprise detecting an induced voltage generated across the exciting coil through a detector circuit; producing a specified position signal for the exciting coil of each phase through a rotor position signal generating circuit; and performing excitation control of the exciting coils by controlling switching elements for conducting excitation currents via the exciting coils through a control circuit. A neutral point of the star configuration, may be switched to one of a ground voltage, a supply voltage, and an open circuit voltage to provide the additional rotational steps. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The present invention is described with the help of accompanying drawings.  
         [0012]      FIG. 1  illustrates a circuit diagram of star connected windings for a conventional BLDC motor.  
         [0013]      FIG. 2  illustrates a circuit diagram for microcontroller switches connected to the conventional BLDC motor.  
         [0014]      FIGS. 3A and 3B  illustrates  6  orientation steps for a conventional BLDC motor as illustrated in  FIG. 1 .  
         [0015]     FIGURE.  4  illustrates a block diagram of a brushless DC (BLDC) circuit according to the present invention.  
         [0016]      FIG. 5  illustrates a circuit diagram of star connected windings for a proposed brushless DC circuit (BLDC) according to present invention.  
         [0017]      FIGS. 6A, 6B ,  6 C,  6 D,  6 E and  6 F illustrates  12  orientation steps for a proposed BLDC circuit according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]      FIG. 4  illustrates a block diagram of a brushless DC (BLDC) circuit  400  according to the present invention. The BLDC circuit  400  drives a brushless DC (BLDC) motor having  12  steps in one mechanical rotation. The motor includes a rotor and multiple exciting coils connected in a star topology in a three-phase winding, such that a neutral node of the star topology is utilized and switched to one of a supply voltage, a ground voltage and an open circuit voltage. The neutral node is connected to more switches for providing more combinations for additional steps. The circuit  400  includes a detector circuit  402 , a rotor position signal generating circuit  404 , and a control circuit  406 . The detector circuit  402  detects an induced voltage generated across the exciting coils. The circuit  404  generates a specified position signal for the exciting coils. The control circuit  406  performs an excitation control of the exciting coils by controlling switching elements for conducting excitation currents via the exciting coils.  
         [0019]      FIG. 5  illustrates a circuit diagram of star connected windings for a proposed brushless DC circuit (BLDC) according to present invention. The star connected windings are connected to commutation switches. The commutation switches can be field effect transistors (FETs). The arrangement is such that, a neutral node  4  of the star configuration is used to provide one of a supply voltage, ground voltage, and an open circuit voltage. The neutral node  4  is connected to two extra switches S 7  and S 8  to provide more combinations for achieving additional steps per rotation ( 12  steps total). The additional step generation is explained in the following paragraphs.  
         [0020]     The three winding coils A, B and C are connected in the star configuration having an activated neutral node  4 . Node  1  of coil A is connected to switches SI and S 2 , and node  2  of coil B is connected to switches S 5 , and S 6  and node  3  of coil C is connected to switches S 3  and S 4 . The neutral node  4  is connected to additional switches S 7  and S 8 . The switches S 1 , S 3 , S 5  and S 7  are connected to a positive supply voltage V and the switches S 2 , S 4 , S 6  and S 8  are connected to the ground voltage side.  
         [0021]     The above circuit arrangement provides  12  steps in one complete mechanical rotation. Each step corresponds to 30 degrees of rotation, i.e., 360/12 degrees. The  12  steps are generated by switching different combinations of switches as illustrated in  FIGS. 6A, 6B ,  6 C,  6 D,  6 E and  6 F.  
         [0022]      FIG. 6A  illustrates steps  1  and  2  for one mechanical rotation. In step  1 , the switches S 1  and S 4  are made on so that the node  1  is connected to the positive supply voltage V and the node  3  is connected to the ground voltage. A resultant magnetic field will rotate the rotor in a counter clockwise direction by 30 degrees. In Step  2 , the switches S 1  and S 8  are in an on state, such that the node  1  is connected to the positive supply voltage V and the node  2  is connected to the ground voltage. The resultant magnetic field will further turn the rotor in a counter clockwise direction by 30 degrees.  
         [0023]      FIG. 6B  further illustrates step  3  and step  4 . In step  3 , the switches S 1  and S 6  are turned on so that the node  1  is connected to the positive supply voltage V and the node  2  is connected to the ground voltage. A resultant magnetic field will rotate the rotor in counter clockwise direction by 30 degrees. In step  4 , the switches S 6  and S 7  are in the on state, such that the node  4  is connected to the positive supply voltage V and the node  2  is connected to the ground voltage. The resultant magnetic field will further turn the rotor in a counter clockwise direction by 30 degrees.  
         [0024]      FIG. 6C  further illustrates step  5  and step  6 . In step  5 , the switches S 3  and S 6  are turned on so that the node  3  is connected to the positive supply voltage V and the node  2  is connected to the ground voltage. A resultant magnetic field will rotate the rotor in a counter clockwise direction by 30 degrees. In step  6 , the switches S 3  and S 8  are in the on state, such that the node  3  is connected to the positive supply voltage V and the node  4  is connected to the ground voltage. The resultant magnetic field will further turn the rotor in a counter clockwise direction by 30 degrees.  
         [0025]      FIG. 6D  further illustrates step  7  and step  8 . In step  7 , the switches S 3  and S 2  are turned on so that the node  3  is connected to the positive supply voltage V and the node  1  is connected to the ground voltage. A resultant magnetic field will rotate the rotor in a counter clockwise direction by 30 degrees. In step  8 , the switches S 2  and S 7  are in on state, such that the node  4  is connected to the positive supply voltage V and the node  1  is connected to the ground voltage. The resultant magnetic field will further turn the rotor in a counter clockwise direction by 30 degrees.  
         [0026]      FIG. 6E  further illustrates step  9  and step  10 . In step  9 , the switches S 2  and S 5  are turned on so that the node  2  is connected to the positive supply voltage V and the node  1  is connected to the ground voltage. A resultant magnetic field will rotate the rotor in a counter clockwise direction by 30 degrees. In step  10 , the switches S 8  and S 5  are in the on state, such that the node  2  is connected to the positive supply voltage V and the node  4  is connected to the ground voltage. The resultant magnetic field will further turn the rotor in a counter clockwise direction by 30 degrees.  
         [0027]      FIG. 6F  further illustrates step  11  and step  12 . In step  11 , the switches S 4  and S 5  turned made on so that the node  2  is connected to the positive supply voltage V and the node  3  is connected to the ground voltage. A resultant magnetic field will rotate the rotor in a counter clockwise direction by 30 degrees. In step  12 , the switches S 4  and S 7  are in the on state, such that the node  4  is connected to the positive supply voltage V and the node  3  is connected to the ground voltage. The resultant magnetic field will further turn the rotor in a counter clockwise direction by 30 degrees.  
         [0028]     By reversing the switching pattern of the commutation switches a clockwise rotation of the motor can be achieved.  
         [0029]     The proposed BLDC circuit that drives BLDC motor offers many advantages. Firstly the BLDC motor provides a cost effective technique for achieving additional steps in a mechanical revolution for a better resolution for each step.