Abstract:
A Switched Reluctance (SR) generator having improved efficiency at low and high speed is disclosed. The SR generator includes a step-up converter for controlling an excitation voltage of an excitation capacitor capable of providing a phase winding with an excitation current. Therefore, the SR generator can control the excitation current, and can provide a phase winding with a sufficient excitation current, thereby improving efficiency. The step-up converter is arranged between a battery capable of storing generated power and the excitation capacitor, boosts a voltage of the battery, and provides the excitation capacitor with the boosted voltage. Therefore, the SR generator does not include an additional power-supply unit for a boosting function, resulting in simplification of an overall structure and reduction of a production cost.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a Switched Reluctance (SR) generator, and a more particularly to an SR generator for improving generation performance at low and high speed.  
         [0003]     2. Description of the Related Art  
         [0004]     Generally, an SR generator has been widely used as an energy conversion device, such that it transmits electric energy from a voltage source to a winding having one or more phases via an electric switching operation during a predetermined rotor angle period, and takes more electric energy than that received in the winding from the winding during the remaining rotor angle periods. The above-mentioned additional energy is indicative of mechanical energy applied to a generator rotor when torque is rotationally applied to the SR generator. The SR generator has an output signal configured in the form of a current source, so that it can generate electricity.  
         [0005]     Although conventional wind power stations have widely used an induction generator or a synchronous generator, the induction generator must maintain a predetermined speed when generating electricity whereas it has a simple structure and a low production cost, such that it requires an additional gearbox and has a limited range of wind velocity at which electricity is generated. The synchronous generator has disadvantages in that it has a complicated structure and is expensive. The synchronous generator outputs a low output voltage at a low wind velocity, such that it is unable to charge a battery with electricity.  
         [0006]     Therefore, although induction or synchronous generators have been widely used to generate electricity in an area having a low wind velocity, many problems occur when generating electricity. The applicant of the present invention has recognized that the SR generator can be improved to be suitable for an application field in a process during which a generator capable of efficiently performing wind power generation has been developed.  
       SUMMARY OF THE INVENTION  
       [0007]     Therefore, the present invention has been made in view of the above problems, and it is an object of the invention to provide an SR generator having improved efficiency at low and high speed.  
         [0008]     It is another object of the present invention to provide an SR generator having a simplified structure and superior endurance.  
         [0009]     In accordance with one aspect of the present invention, these objects are accomplished by providing an SR generator comprising: a step-up converter for controlling an excitation voltage of an excitation capacitor capable of providing a phase winding with an excitation current.  
         [0010]     Therefore, the SR generator can control the excitation current, and can provide a phase winding with a sufficient excitation current, thereby improving efficiency.  
         [0011]     In accordance with another aspect of the present invention, the step-up converter is arranged between a battery capable of storing generated power and the excitation capacitor, boosts a voltage of the battery, and provides the excitation capacitor with the boosted voltage.  
         [0012]     Therefore, the SR generator does not include an additional power-supply unit for a boosting function, resulting in simplification of thes overall structure and reduction of a production cost.  
         [0013]     In accordance with yet another aspect of the present invention, the step-up converter is operated when a rotor rotation speed is less than a predetermined speed. The step-up converter increases the excitation current in inverse proportion to the rotor rotation speed.  
         [0014]     Therefore, the SR generator improves unique efficiency at a low speed.  
         [0015]     In accordance with yet another aspect of the present invention, the step-up converter is operated when the rotor rotation speed is higher than a predetermined speed. Preferably, the step-up converter increases the excitation current in proportion to the rotor rotation speed.  
         [0016]     Therefore, the SR generator improves unique efficiency at a high speed.  
         [0017]     In accordance with yet another aspect of the present invention, the SR generator increases inductance of the phase winding at a low speed, and decreases the inductance of the phase winding at a high speed.  
         [0018]     Therefore, a change rate between the inductance and a phase angle is increased at a low speed, and output energy is also increased at a low speed, thereby improving generation efficiency.  
         [0019]     In accordance with yet another aspect of the present invention, the SR generator has a single-phase structure.  
         [0020]     Therefore, the number of necessary switches of the SR generator is reduced, resulting in a simple structure and reduction of a production cost. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The above objects, and other features and advantages of the present invention will become more apparent after reading the following detailed description when taken in conjunction with the drawings, in which:  
         [0022]      FIG. 1  is a circuit diagram illustrating an SR generator in accordance with a preferred embodiment of the present invention;  
         [0023]      FIG. 2  is a graph illustrating the relationship between a compensated excitation voltage and a speed in accordance with the present invention;  
         [0024]      FIG. 3  is a graph illustrating the relationship between generation energy obtained by a compensated excitation voltage and a rotor rotation speed in accordance with the present invention;  
         [0025]      FIG. 4  is a circuit diagram illustrating an SR generator in accordance with another preferred embodiment of the present invention;  
         [0026]      FIG. 5  shows the appearance of a single-phase stator for use in the SR generator shown in  FIG. 4 , and particularly shows exemplary wiring connection structure of the SR generator including a phase winding composed of three unit windings in accordance with the present invention;  
         [0027]      FIG. 6  is a graph illustrating the relationship among a phase inductance, a generator current, and a motor current in association with a phase angle of a rotor in the SR generator in accordance with the present invention;  
         [0028]      FIG. 7   a  shows the appearance of wiring connection formed at a low speed in a stator including the wiring connection structure shown in  FIG. 5  in accordance with the present invention;  
         [0029]      FIG. 7   b  shows the appearance of wiring connection formed at a high speed in a stator including the wiring connection structure shown in  FIG. 5  in accordance with the present invention;  
         [0030]      FIG. 8   a  is a graph illustrating the relationship between an inductance and a generation current at a low speed in a stator including a wiring connection structure shown in  FIG. 5  in accordance with the present invention; and  
         [0031]      FIG. 8   b  is a graph illustrating the relationship between an inductance and a generation current at a high speed in a stator including the wiring connection structure shown in  FIG. 5  in accordance with the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     Now, preferred embodiments of the present invention will be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.  
         [0033]      FIG. 1  is a circuit diagram illustrating an SR generator in accordance with a preferred embodiment of the present invention. Referring to  FIG. 1 , the SR generator includes a rotor  10 , a stator  30  on which a phase winding  300  is wound, a drive switch  100  for controlling a current signal flowing in the phase winding  300 , and a power controller  500  for controlling a switching operation of the drive switch  100 .  
         [0034]     The SR generator must initially receive an excitation current. For this purpose, an independent generation system generates a necessary power-supply signal by driving a small-sized synchronous motor connected in parallel to the SR generator. In  FIG. 1 , a current source  910  is a motor for generating an initial excitation current, for example, a BLDC motor. The excitation current generated from the motor is charged in the excitation capacitor  930 . Once the excitation capacitor  930  is charged, a voltage charged in the excitation capacitor  930  is supplied to the phase winding  300  via the drive switch  100  within a predetermined range of a rotor rotation angle, and redundant electric energy induced in the phase winding  300  is charged in the battery  955  within a predetermined range of another rotor rotation angle. The power controller  500  switches on or off the drive switch  100  according to a rotor rotation angle detected by the speed detector  530 , and controls the above-mentioned operation.  
         [0035]     The phase winding  300  receives electric energy from the excitation capacitor  930  acting as a voltage source within a predetermined rotor angle range because of an electronic switching operation of the drive switch  100 , and provides a battery  955  with more energy than received energy within other rotor angle ranges. The power controller  500  for controlling an electronic switching operation of the drive switch  100  generates a pulse-width modulation signal capable of controlling the above switching operation according to a rotor position detected by a speed detector  530 . The power controller  500  can be implemented with various forms, and a variety of power controllers have been well known to those skilled in the art. As well known in the art, the speed detector  530  can be implemented with various forms, for example, a speed detector using a hall sensor, and another speed detector using a current detection scheme. For example, the speed detector  530  detects a displacement, and differentiates the detected displacement. Otherwise, the speed detector  530  counts the number of pulse trains generated from an encoder during a predetermined time, such that it can detect a speed of the SR generator.  
         [0036]     An output signal of the battery  955  is supplied to a load  600 . Selectively, an output signal of the battery  955  may be supplied to the load  600  via a step-up DC-DC converter. Diodes  951  and  953  prevent power charged in the battery from being counter-induced to the phase winding.  
         [0037]     In accordance with a preferred embodiment of the present invention, the phase winding  300  for use in the stator has a single-phase structure. The single-phase SR generator has a small number of switches for energy conversion, such that it can be implemented with a low price. However, it should be noted that the present invention is not limited to the above-mentioned example, and can be modified to take different forms.  
         [0038]     In the preferred embodiment shown in  FIG. 1 , the SR generator includes a step-up converter for controlling an excitation voltage of the excitation capacitor  930  capable of providing the phase winding with an excitation current. In accordance with a second preferred embodiment of the present invention, the step-up converter is positioned between the battery  955  for storing generated power therein and the excitation capacitor  930 . The step-up converter boosts power of the battery  955 , and provides the excitation capacitor  930  with the boosted power. In accordance with the first preferred embodiment of the present invention, the step-up converter is implemented with a boost converter. In other words, the present invention exemplarily shows the simplest boost converter circuit for convenience of description and better understanding of the present invention. The boost converter circuit includes a boost inductor  810  for receiving a current signal from the battery  955 ; a switch  850  for switching on or off the provision of an inductor current; and a diode  830  for providing the excitation capacitor with a current signal by switching on or off the switch  850 , and preventing the occurrence of a counter-current flow. Operations of the above-mentioned boost converter circuit have been well known to those skilled in the art, so that their detailed description will herein be omitted for convenience of description.  
         [0039]     In the preferred embodiment shown in  FIG. 1 , the SR generator includes a speed detector  530  for detecting a rotation speed of the rotor; and an excitation controller  800  for operating the boost converter when the rotor rotation speed detected by the speed detector  530  is less than a predetermined speed. In this case, it is preferable that the excitation controller  800  controls the boost converter to allow an excitation current to be increased in inverse proportion to the rotor rotation speed.  
         [0040]     In accordance with the second preferred embodiment of the present invention, the excitation controller  800  operates the boost converter when the rotor rotation speed detected by the speed detector  530  is equal to or higher than a predetermined speed. In this case, the excitation controller  800  controls the boost converter to allow the excitation current to be increased in proportion to the rotor rotation speed.  
         [0041]     The principle of the above-mentioned operations of the boost converter will hereinafter be described in detail using the following equations.  
         [0042]     A voltage equation for use in the SR generator can be represented by the following equation 1:  
             v   =       Ri   +       ⅆ   ϕ       ⅆ   t         =       Ri   +     L   ⁢       ⅆ   i       ⅆ   t         +     i   ⁢       ⅆ   θ       ⅆ   t       ⁢       ⅆ   L       ⅆ   θ           =     Ri   +     L   ⁢       ⅆ   i       ⅆ   t         +   e                 [     Equation   ⁢           ⁢   1     ]             
 
         [0043]     where e can be denoted by the following equation 2:  
             e   =       i   ⁢       ⅆ   θ       ⅆ   t       ⁢       ⅆ   L       ⅆ   θ         =       ⅈω   m     ⁢       ⅆ   L       ⅆ   θ                   [     Equation   ⁢           ⁢   2     ]             
 
         [0044]     Therefore, the flow of energy can be denoted by the following equation 3:  
             vi   =         Ri   2     +     Li   ⁢       ⅆ   i       ⅆ   t         +       i   2     ⁢       ⅆ   θ       ⅆ   t       ⁢       ⅆ   L       ⅆ   θ           =       Ri   2     +       ⅆ     ⅆ   t       ⁢     (       1   2     ⁢     Li   2       )       +       1   2     ⁢     i   2     ⁢     ω   m     ⁢       ⅆ   L       ⅆ   θ                     [     Equation   ⁢           ⁢   3     ]             
 
         [0045]     where a first term is indicative of a copper loss, a second term is indicative of the sum of a core loss and magnetic energy, and a third term is indicative of generation energy acting as a mechanical output value. During a first period denoted by  
             ⅆ   L       ⅆ   θ       &gt;   0     ,       
 
 the mechanical output value is a positive (+) value, such that the SR generator is used as a motor. During a second period denoted by  
             ⅆ   L       ⅆ   θ       &lt;   0     ,       
 
 the mechanical output value is a negative (−) value, such that the SR generator is used as a generator. 
 
         [0046]     The third term indicative of power energy acquired from the generator can be represented by the following equation 4:  
             Energy   =       1   2     ⁢     i   2     ⁢     ω   m     ⁢       ⅆ   L       ⅆ   θ                 [     Equation   ⁢           ⁢   4     ]             
 
         [0047]     where i is proportional to an excitation current supplied to the phase winding. The higher the excitation current, the higher the generation energy. In other words, the value of angular velocity is decreased at a low speed, so that the generation energy is also decreased. In accordance with a characteristic aspect of the present invention, if an excitation voltage is increased to increase the excitation current, the current i is also increased, thereby maintaining generation energy. In this case, the excitation current is increased in inverse proportion to the speed, so that a decrement of the angular velocity ω can be compensated for.  
         [0048]     Also, although the angular velocity ω is high at a high speed, a time necessary for the excitation is decreased, an excitation current is also reduced, thereby lowering generation energy. In this case, the excitation voltage is increased so that the value of the excitation current is also increased, thereby maintaining generation energy. In this case, the value of the excitation current is increased in proportion to a speed, and a substantial excitation current is maintained, so that generation energy can be compensated for.  FIG. 2  is a graph illustrating the relationship between a compensated excitation voltage and a speed in accordance with the present invention. As shown in  FIG. 2 , the angular velocities ω 1  and ω 2  at which a compensation operation begins can be determined to be appropriate values.  FIG. 3  is a graph illustrating the relationship between generation energy obtained by a compensated excitation voltage and a rotor rotation speed in accordance with the present invention. As shown in  FIG. 3 , the conventional SR motor has a small amount of generation energy in a low-speed area and a high-speed area, but it can be noted that relatively smoothing characteristics are obtained.  
         [0049]      FIG. 4  is a circuit diagram illustrating an SR generator in accordance with another preferred embodiment of the present invention. As shown in  FIG. 4 , it should be noted that the same reference numerals are assigned to components similar to those in the first preferred embodiment of the SR generator. Referring to  FIG. 4 , the phase winding  300  for use in the SR generator includes: a plurality of unit windings  310 ,  330 , and  350 ; one or more winding switches  340  and  360  for switching on or off a connection among the unit windings  310 ,  330 , and  350 ; and a wiring connection controller  700  for controlling a switching operation of the winding switches  340  and  360  so that inductance is increased when a speed detected by the speed detector  530  is low, and is decreased when the speed detected by the speed detector  530  is high, differently from the above SR generator shown in  FIG. 5 .  
         [0050]     Operations of the wiring connection controller  700  will hereinafter be described with reference to the accompanying drawings. The wiring connection controller  700  controls a switching operation of the winding switches  340  and  360  contained in the phase winding  300  according to the speed detected by the speed detector  530 , and changes a wiring connection scheme of the unit windings  310 ,  330 , and  350  to another scheme.  
         [0051]     In accordance with a feature of the present invention, the wiring connection controller  700  controls the unit windings  310 ,  330 , and  350  to be connected in series at a low speed, and controls the unit windings  310 ,  330 , and  350  to be connected in parallel to each other at a high speed. As shown in  FIG. 4 , the phase winding  300  of a single phase includes: three unit windings  310 ,  330 , and  350 ; and two winding switches  340  and  360  for controlling a wiring connection among the three unit windings  310 ,  330 , and  350 .  FIG. 5  shows a wiring connection structure of a stator of the above-mentioned SR generator. In accordance with the preferred embodiment of the present invention, radially-arranged stator magnetic poles are divided into predetermined magnetic pole groups, each of which includes predetermined magnetic poles adjacent to each other. Individual unit windings are allocated to individual groups, respectively. The unit windings are wound to allow neighboring magnetic poles to be alternated with each other in each group. In more detail, individual magnetic poles have polarities opposite to neighboring magnetic poles. The magnetic poles of three unit coils are divided into three groups, and one unit winding is wound on each group. In  FIG. 5 , reference characters O and X are indicative of winding directions of each winding. The stator increases the number of single-phase poles, and improves a wiring connection scheme in such a way that high energy efficiency can be established at a low speed.  
         [0052]      FIG. 6  is a graph illustrating the relationship among a phase inductance, a generator current, and a motor current in association with a phase angle of a rotor in the SR generator in accordance with the present invention. As shown in  FIG. 6 , inductance has the highest Lmax at a first condition of θ=θmax , and has the smallest Lmin at a second condition of θ=θmin.  
         [0053]      FIG. 7   a  shows the appearance of wiring connection formed at a low speed in the stator including the wiring connection structure shown in  FIG. 5 .  FIG. 7   b  shows the appearance of wiring connection formed at a high speed in the stator including the wiring connection structure shown in  FIG. 5 .  FIG. 8   a  is a graph illustrating the relationship between an inductance and a generation current at a low speed in the stator including the wiring connection structure shown in  FIG. 5 .  FIG. 8   b  is a graph illustrating the relationship between the inductance and the generation current at a high speed in the stator including the wiring connection structure shown in  FIG. 5 .  
         [0054]     Energy generated from the SR generator can be represented by the above-mentioned equation 4:  
             Energy   =       1   2     ⁢     i   2     ⁢     ω   m     ⁢       ⅆ   L       ⅆ   θ                 [     Equation   ⁢           ⁢   4     ]             
 
         [0055]     As shown in  FIG. 8   a , in the case of an inductance profile, unit windings are connected in series to each other at a low speed so that inductance is increased. As a result, the values of Lmax and Lmin are also increased, and the value of  
         ⅆ   L       ⅆ   θ         
 
 is increased, such that generation efficiency is improved at a low speed. In the case of an excitation current, the value of Lmax is high at a low speed, but a sufficient time for excitation is guaranteed because of the low speed, and an excitation voltage is boosted so that a sufficient excitation current can be obtained in accordance with another aspect of the present invention. Therefore, although generation energy formed at a low speed has a low angular velocity ω, the value of  
         ⅆ   L       ⅆ   θ         
 
 is very high, so that sufficient generation is provided. 
 
         [0056]     In the meantime, since the unit windings are connected in parallel to each other at a high speed, inductance is decreased. As a result, the values of Lmax and Lmin are also decreased, the value of  
         ⅆ   L       ⅆ   θ         
 
 is decreased, and the angular velocity ω is high. In accordance with another aspect of the present invention, an excitation voltage is boosted so that a sufficient excitation current can be obtained and electric generation is improved. 
 
         [0057]     As apparent from the above description, the above-mentioned SR generator boosts an excitation voltage at a low speed and a high speed, and guarantees a sufficient excitation current, such that generation efficiency is improved at a low speed and a high speed.  
         [0058]     The SR generator boosts a voltage of a battery in which a generated voltage is charged, and provides an excitation capacitor with the boosted battery voltage, such that it can improve generation efficiency using a minimum number of components without including an additional power-supply unit.  
         [0059]     Furthermore, the SR generator increases inductance of a stator winding at a low speed, and reduces the inductance of the stator winding at a high speed, such that a change rate between the inductance and a phase angle is increased at a low speed, and output energy is also increased at the low speed, thereby improving generation efficiency.  
         [0060]     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.