1.  Field of the Invention
The present invention relates to a reciprocating compressor for a refrigerator using a linear motor, and particularly, to an apparatus and a method for controlling a driving of the reciprocating compressor for a refrigerator using a linear motor in which capacitance is varied according to a variation of a driving load, thereby improving a driving efficiency of a compressor. 
2.  Description of the Background Art
As well-known, a compressor compresses a refrigerant circulated in a cooling apparatus such as an air conditioner and a refrigerator into a high temperature and high pressure.  The compressor includes a reciprocating compressor, a rotary compressor, a brushless direct current (BLDC) compressor, and an ability variable-type reciprocating compressor called as an inverter compressor and having a varied rotary speed. 
The reciprocating compressor using a linear motor varies a piston stroke thereof according to a voltage applied to a motor, thereby controlling a cooling capacity by a user""s intention. 
The reciprocating compressor will be explained with attached drawings. 
FIG.  1 shows an apparatus for controlling a driving of a general reciprocating compressor. 
Referring to FIG.  1, an apparatus for controlling a driving of a reciprocating compressor comprises a reciprocating compressor 100 for controlling a cooling capacity by varying a stroke of an inner piston by a voltage input to an inner motor M according to a stroke reference value set by a user: a voltage detecting unit 102 for detecting a motor voltage applied to the reciprocating compressor 100 by varying the piston stroke of the reciprocating compressor 100; a current detecting unit 104 for detecting current applied to the reciprocating compressor 100 by varying the piston stroke of the reciprocating compressor 100; a microcomputer 106 for calculating a stroke value by using detected voltage and current from the voltage detecting unit 102 and the current detecting unit 104, comparing the calculated stroke value with the stroke reference value, then outputting a switching control signal according to the comparison between the calculated stroke value and the stroke reference value; and an electric circuit unit 108 for controlling a size of the motor voltage applied to the reciprocating compressor 100 according to interrupting an AC power source to a triac Tr1 controlled by a switching control signal of the microcomputer 106. 
Operations for controlling a driving of the general reciprocating compressor will be explained. 
The reciprocating compressor 100 controls a cooling capacity by a varied piston stroke, wherein the piston reciprocates up and down by the motor voltage input from the motor according to the stroke reference value set by a user. 
The stroke means a distance that a piston in the reciprocating compressor 100 moves with reciprocation. 
A period of turn-on of the triac Tr1 in the electric circuit unit 108 becomes long by a control signal of the microcomputer 106, so that a stroke is increased.  At this time, the voltage detecting unit 102 and the current detecting unit 104 respectively detect voltage and current applied to the reciprocating compressor 100, and output the detected voltage and current to the microcomputer 106. 
The microcomputer 106 calculates a stroke by using voltage and current detected from the voltage detecting unit 102 and the current detecting unit 104, compares the calculated stroke with a stroke reference value set by a user, and outputs a switching control signal to the triac Tr1 according to the comparison between the calculated stroke and the stroke reference value. 
That is, when the calculated stroke is smaller than the stroke reference value, the microcomputer 106 outputs the switching control signal which lengthens the period of turn-on of the triac Tr1, thereby increasing a voltage applied to the reciprocating compressor 100. 
In the meantime, when the calculated stroke is greater than the stroke reference value, the microcomputer 106 outputs the switching control signal which shortens the period of turn-on of the triac Tr1, thereby decreasing a voltage applied to the reciprocating compressor 100. 
A relation between a voltage (V) applied to a motor (M) of the reciprocating compressor 100 and a stroke will be shown as follows.                                                          V              =                                                L                  ⁢                                                            ⅆ                      i                                                              ⅆ                      t                                                                      +                                  R                  ⁢                                      xe2x80x83                                    ⁢                  i                                +                                  α                  ⁢                                      xe2x80x83                                    ⁢                  ω                  ⁢                                      xe2x80x83                                    ⁢                  S                                                                                                                        ω                =                                  2                  ⁢                                      xe2x80x83                                    ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                  f                                            ⁢                              xe2x80x83                                                                        [                  Equation          ⁢                      xe2x80x83                    ⁢          1                ]            
Wherein, xcex1 indicates a motor constant for converting electric force into mechanical force, S indicates stroke, R indicates inner resistance of a motor, and L indicates inductance of a motor (M). 
As shown in the equation 1, inductance voltage   (      L    ⁢                  ⅆ        i                    ⅆ        t              )
is almost similar to counter-electromotive force (xcex1xcfx89S), and voltage (Ri) of inner resistance (R) of the reciprocating compressor 100 is a small value possible to ignore when compared with the   (      L    ⁢                  ⅆ        i                    ⅆ        t              )
and the counter-electromotive force (xcex1xcfx89S). 
Therefore, voltage (V) applied to the motor (M) is determined by a sum of the inductance voltage   (      L    ⁢                  ⅆ        i                    ⅆ        t              )
and the counter-electromotive force (xcex1xcfx89S). 
Accordingly, to get a greater stroke in the reciprocating compressor, voltage applied to the motor has to be great. 
To improve efficiency of the reciprocating compressor, inductance value of a coil wound on the motor has to be small. 
That is, as shown in FIG.  2, capacitor (C) is connected to the motor (M) in series and attenuates an inductance (L) of a coil wound on the motor, thereby improving efficiency of the reciprocating compressor. 
FIG.  2 is a block diagram of a reciprocating compressor in accordance with the conventional art. 
Referring to FIG.  2, an operation for attenuating inductance of the coil will be explained.  Voltage applied to the motor and both ends of the capacitor is shown as a following equation.                      V        =                              L            ⁢                                          ⅆ                i                                            ⅆ                t                                              +                                    1              C                        ⁢                          ∫                              i                ⁢                                  ⅆ                  t                                                              ⁢                      xe2x80x83                    +                      R            ⁢                          xe2x80x83                        ⁢            i                    +                      α            ⁢                          xe2x80x83                        ⁢            ω            ⁢                          xe2x80x83                        ⁢            S                                              [                  Equation          ⁢                      xe2x80x83                    ⁢          2                ]            
At this time, capacitance (C) is shown as a following equation.                      C        =                              1                                          (                                  2                  ⁢                                      xe2x80x83                                    ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                  f                                )                            2                                ⁢          L                                    [                  Equation          ⁢                      xe2x80x83                    ⁢          3                ]            
Wherein, the capacitance (C) and the inductance (L) are predetermined as resonant values. 
Accordingly, the capacitance (C) and the inductance (L) are attenuated by being resonated each other, so that voltage applied to the motor (M) and both ends of the capacitor is shown as a following equation. 
V=Ri+xcex1xcfx89sxe2x80x83xe2x80x83[Equation 4]
As shown in the equation 4, the applied voltage (V) has a similar size as the counter-electromotive force (xcex1xcfx89S) because the inductance voltage   (      L    ⁢                  ⅆ        i                    ⅆ        t              )
and capacitor voltage   (            1      C        ⁢          ∫              i        ⁢                  ⅆ          t                      )
are attenuated after being resonated each other.  Therefore, the reciprocating compressor can obtain a necessary stroke with just a low voltage (V). 
Also, because the capacitor voltage   (            1      C        ⁢          ∫              i        ⁢                  ⅆ          t                      )
is applied to the motor (M) together with the applied voltage (V) applied to the motor and both ends of the capacitor, a great stroke can be obtained with a low voltage, thereby improving a corresponding capacity to overload. 
In case of that the conventional-art reciprocating compressor is adopted to a refrigerator and driven, necessary voltage for the motor (M) of the reciprocating compressor 100 to obtain a constant stroke becomes different according to a driving load of a refrigerator. 
That is, the motor M of the reciprocating compressor 100 requires voltage greater than line voltage (in Korea, AC 220) when the driving load of a refrigerator is greater, and requires voltage smaller than line voltage when the driving load of a refrigerator is smaller. 
Accordingly, the microcomputer 106, in case that the driving load of a refrigerator is great, shortens off-time of the triac Tr1, thereby increasing voltage applied to the motor, and in case that the driving load of a refrigerator is small, it lengthens off-time of the triac Tr1, thereby decreasing voltage applied to the motor. 
At this time, waveforms of current by voltage applied to the motor according to the driving load of the refrigerator is shown in FIGS.  3a and 3b.  
FIG.  3A is a current waveform in case that the driving load of the refrigerator is great, and FIG.  3B is a current waveform in case that the driving load of the refrigerator is small. 
As aforementioned, the reciprocating compressor of the conventional art lengthens off-time of triac to decrease voltage applied to the motor, thereby increasing harmonic wave loss and then lowering efficiency of the reciprocating compressor. 
Therefore, an object of the present invention is to provide an apparatus"" and a method for controlling a driving of a reciprocating compressor for a refrigerator using a linear motor so as to control a necessary voltage of a motor for obtaining a predetermined stroke by varying a capacitance according to a variation of a driving load of a refrigerator. 
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided an apparatus for controlling a driving of a reciprocating compressor for a refrigerator using a linear motor, in a reciprocating compressor for a refrigerator which controls a cooling capacity by varying a stroke of a piston reciprocating up and down by a voltage applied to the inner motor according to an on/off state of a triac Tr1, the apparatus comprises a first capacitor for attenuating an inductance of a coil wound on the motor M; a second capacitor connected to the first capacitor in parallel; a relay Ry connected to the second capacitor in series to be turned on/off; and a microcomputer for outputting a control signal to turn on/off the relay according to the driving load of the refrigerator. 
Wherein, the microcomputer outputs a control signal for turning on the relay if the driving load of the refrigerator is small; and outputs a control signal for turning off the rely if the driving load of the refrigerator is great. 
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided an apparatus for controlling a driving of a reciprocating compressor for a refrigerator using a linear motor, in a reciprocating compressor for a refrigerator which controls a cooling capacity by varying a stroke of a piston reciprocating up and down by a voltage applied to the inner motor according to an on/off state of a triac, the apparatus comprises a first capacitor connected to the motor; a second capacitor connected to the first capacitor in parallel; a relay connected to the second capacitor in series to be turned on/off; and a microcomputer for outputting a control signal to turn on/off the relay according to the driving load of the refrigerator, wherein a series combination between the first and second capacitors is set to attenuate an inductance of a coil wound to the motor. 
Wherein, the microcomputer outputs a control signal for turning on the relay if the driving load of the refrigerator is small; and outputs a control signal for turning off the rely if the driving load of the refrigerator is great. 
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a method for controlling a driving of a reciprocating compressor for a refrigerator using a linear motor, in a reciprocating compressor for a refrigerator which controls a cooling capacity by varying a stroke of a piston reciprocating up and down by a voltage applied to the inner motor according to an on/off state of a triac, the apparatus comprises a first capacitor for attenuating an inductance of a coil wound on the motor; a second capacitor connected to the first capacitor in parallel; and a relay connected to the second capacitor in series to be turned on/off, wherein the method comprises the steps of detecting an off time of the triac; determining whether the driving load of the refrigerator is great or small by the detected off time of the triac; and outputting the control signal for turning on the relay in case of when the driving load of the refrigerator is small as a result of the determination, and outputting the control signal for turning off the relay in case of when the driving load of the refrigerator is great. 
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a method for controlling a driving of a reciprocating compressor for a refrigerator using a linear motor, in a reciprocating compressor for a refrigerator which controls a cooling capacity by varying a stroke of a piston reciprocating up and down by a voltage applied to the inner motor according to an on/off state of a triac, the apparatus comprises a first capacitor for attenuating an inductance of a coil wound on the motor; a second capacitor connected to the first capacitor in parallel; and a relay connected to the second capacitor in series to be turned on/off:, wherein the method comprises the steps of detecting an off time of the triac; determining whether the driving load of the refrigerator is great or small by the detected off time of the triac; and outputting the control signal for turning on the relay in case of when the driving load of the refrigerator is small as a result of the determination, and outputting the control signal for turning off the relay in case of when the driving load of the refrigerator is great, wherein a series combination between the first and second capacitors is set to attenuate an inductance of a coil wound to the motor. 
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.