Abstract:
An apparatus and a method for controlling operation of a reciprocating compressor is capable of reducing a stroke estimation error by eliminating an error that occurs due to resistance and inductance of a compressor motor by estimating a stroke with a counter electromotive force induced by a searching coil. Furthermore, by leaving errors of inductance and resistance, among all motor parameters, out of consideration in stroke estimation, a stroke estimation error can be reduced.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an apparatus and a method for controlling operation of a reciprocating compressor. In particular, the present invention relates to an apparatus and a method for controlling operation of a reciprocating compressor that is capable of reducing a stroke estimation error by estimating a stroke with a counter electromotive force induced by a searching coil and removing an error due to resistance and inductance in a compressor motor (hereinafter, referred to as a motor). 
   2. Description of the Prior Art 
     FIG. 1  is a block diagram illustrating an operation control apparatus of a reciprocating compressor in accordance with the conventional art. As depicted in  FIG. 1 , the operation control apparatus of the reciprocating compressor includes a current detector  150  for detecting current applied to a motor; a voltage detector  140  for detecting a voltage applied to the motor; a stroke estimator  130  for estimating a stroke on the basis of the detected current, voltage and a motor constant; a comparator  100  for comparing the estimated stroke with a preset stroke reference value and outputting a difference value according to the comparison result; and a controller  110  for controlling a stroke of the compressor by varying a voltage applied to the motor according to the difference value. 
   Hereinafter, the operation of the control apparatus of the reciprocating compressor will be described with reference to accompanying  FIG. 2 . 
   First, the current detector  150  detects current applied to the motor, and the voltage detector  140  detects a voltage applied to the motor. Herein, the stroke estimator  130  calculates a stroke estimation value of the compressor with Equation 1 by substituting the detected current value, the detected voltage value and a motor constant and applies the calculated stroke estimation value to the comparator  100 . 
   
     
       
         
           
             
               
                 X 
                 = 
                 
                   
                     1 
                     α 
                   
                   ⁢ 
                   
                     ∫ 
                     
                       
                         ( 
                         
                           
                             V 
                             M 
                           
                           - 
                           Ri 
                           - 
                           
                             L 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               i 
                               _ 
                             
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         ⅆ 
                         t 
                       
                     
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 1 
               
             
           
         
       
     
   
   Herein, R is the resistance of the motor, L is the inductance of the motor, α is a motor parameter, V M  is the voltage of the motor and i is the current of the motor. 
   Then, the comparator  100  compares the stroke estimation value with the stroke reference value (S 220 ) and applies a difference value according to the comparison result to the controller  110 . The controller  110  controls a stroke by varying the voltage applied to the motor on the basis of the difference value. 
   In more detail, the control unit  110  increases a motor supply voltage (S 240 ) when a stroke reference value is greater than a stroke estimation value, and the control unit  110  decreases a motor supply voltage (S 230 ) when a stroke reference value is less than a stroke estimation value. 
   However, in the conventional operation control method of the reciprocating compressor, because stroke control is performed by estimating a stroke utilizing all motor parameters (motor constant, resistance, inductance, etc.), an error in an estimated stroke is increased due to errors and non-linearity of the parameters. 
   SUMMARY OF THE INVENTION 
   In order to solve the above-mentioned problem, it is an object of the present invention to provide an apparatus and a method for controlling operation of a reciprocating compressor that is capable of reducing a stroke estimation error by leaving errors of inductance and resistance, among all motor parameters, out of consideration by estimating a stroke with a counter electromotive force induced by a searching coil. 
   In order to achieve the above-mentioned object, an operation control apparatus of a reciprocating compressor accordance with the present invention includes a compressor in which includes a searching coil, a first stroke estimator that estimates a first stroke value by using a voltage, a current applied to a motor of the compressor and a motor constant and a phase difference detector that detects a phase difference value between a phase of the first stroke value and a phase of the current applied to the motor. A searching coil voltage detector detects a voltage applied to both ends of the searching coil based upon the phase difference detected by the phase difference detector and a counter electromotive force extractor extracts a counter electromotive force induced by the searching coil in accordance with the phase difference detected by the phase difference detector. A second stroke estimator estimates a second stroke value based upon the extracted counter electromotive force and a control unit compares the second stroke estimation value with a stroke reference value and varies one of a voltage applied to the motor and an operational frequency of the compressor in accordance with a result of the comparison. 
   In addition, the present invention relates to a method of controlling operation of a reciprocating compressor in includes estimating a first stroke estimation value by using current and a voltage applied to a motor of a compressor and a motor constant, calculating a difference between a phase of the first stroke estimation value and a phase of the current applied to the motor and judging whether the phase difference is 90°. Detecting a counter electromotive force by using a voltage applied to the both ends of a searching coil when the phase difference is 90° and estimating a second stroke estimation value based upon the counter electromotive force; and comparing the second stroke estimation value with a stroke reference value and varying a voltage applied to the motor based upon the result of the comparison. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     In the drawings: 
       FIG. 1  is a block diagram illustrating an operation control apparatus of a reciprocating compressor in accordance with the conventional art; 
       FIG. 2  is a flow chart illustrating an operation control method of a reciprocating compressor in accordance with the conventional art; 
       FIG. 3  is a block diagram illustrating an operation control apparatus of a reciprocating compressor in accordance with an embodiment of the present invention; 
       FIG. 4  is a flow chart illustrating an operation control method of a reciprocating compressor in accordance with an embodiment of the present invention; and 
       FIG. 5  is a mimetic diagram illustrating a method for calculating a counter electromotive force induced by a searching coil in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In an apparatus and a method for controlling operation of a reciprocating compressor in accordance with the present invention, after calculating a stroke estimation value by the same method as in the conventional art, in order to reduce an error due to inductance and resistance elements used in the estimation value calculation, a phase of the calculated stroke is compared with a phase of a current applied to the motor compressor. When the comparison result yields a phase difference of 90°, a new stroke is estimated, and when the comparison result yields a phase difference that is not 90°, an operational frequency applied to the motor is varied, and accordingly accuracy of the stroke control can be improved. 
   In more detail, after detecting a first stroke estimation value based upon a voltage and current applied to the motor and upon a motor constant, a difference between a phase of the first stroke estimation value and a phase of the current applied to the motor is calculated. Herein, when a phase difference is 90°, a size and a phase of a voltage applied to the both ends of a searching coil are detected. After detecting a phase of the current applied to the motor, a phase of magnetic flux induced by the voltage applied to the motor (hereinafter referred to as magnetic flux of the motor) is calculated by using the phase current, and the difference between the magnetic flux phase and the voltage phase applied to the both ends of the searching coil is calculated. 
   Afterward, by using the difference between the magnetic flux phase and the voltage phase, a size of a counter electromotive force induced by the searching coil (hereinafter referred to as the counter electromotive force) is detected, a second stroke estimation value is calculated utilizing the size of the counter electromotive force. The second stroke estimation value is compared with the stroke reference value, and a voltage applied to the motor is varied according to the comparison result. Accordingly, the stroke of the compressor is controlled. 
   On the other hand, when difference between the phase of the first stroke estimation value and the phase of the motor current is not 90°, the operational frequency of the motor is varied. In particular, when the phase difference is greater than 90°, an operational frequency is increased, and when the phase difference is less than 90°, an operational frequency is decreased. 
   Hereinafter, the apparatus and the method for controlling the operation of the reciprocating compressor in accordance with an embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIG. 3  is a block diagram illustrating an operation control apparatus of a reciprocating compressor in accordance with the present invention. As depicted in  FIG. 3 , the operation control apparatus includes a voltage detector  390  for detecting a voltage applied to the motor of a compressor  300 ; a current detector  380  for detecting current applied to the motor; a first stroke estimator  370  for estimating a first stroke by using the voltage, the current and a constant of the motor; a phase difference detector  360  for detecting a difference value between a phase of the stroke estimation value from the first stroke estimator  370  with a phase of the motor current; a searching coil voltage detector  350  for detecting a voltage applied to a searching coil according to the detected phase difference; a counter electromotive force extractor  340  for extracting a counter electromotive force by receiving the detected voltage; a second stroke estimator  330  for estimating a second stroke by using the counter electromotive force; a comparator  310  for comparing the second stroke estimation value with the stroke reference value and outputting a comparison value according to the comparison result; and a control unit  320  for controlling a stroke by varying the voltage applied to the motor according to the comparison result from the comparator  310 . 
   Herein, the voltage E1 detected by the searching coil voltage detector  350  is the sum total of the motor magnetic flux and the counter electromotive force, and it can be calculated by utilizing Equation 2. In addition, the motor magnetic flux E2, defined by Equation 3 can be obtained by utilizing the basic information of the motor itself, and a phase of E2 has the same shape as a phase of the current applied to the motor. By using Equations 2 and 3, the following Equation 4 can be obtained for the counter electromotive force E3. The counter electromotive force extractor  340  calculates a counter electromotive force by using Equation 4. 
   
     
       
         
           
             
               
                 E1 
                 = 
                 
                   
                     N 
                     ⁢ 
                     
                       
                         ⅆ 
                         
                           Φ 
                           A 
                         
                       
                       
                         ⅆ 
                         t 
                       
                     
                   
                   + 
                   
                     α 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       x 
                       _ 
                     
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 2 
               
             
           
           
             
               
                 
                   E2 
                   = 
                   
                     N 
                     ⁢ 
                     
                       
                         ⅆ 
                         
                           Φ 
                           A 
                         
                       
                       
                         ⅆ 
                         t 
                       
                     
                   
                 
                 ⁢ 
                 
                     
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 3 
               
             
           
           
             
               
                 
                   E3 
                   = 
                   
                     α 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       x 
                       _ 
                     
                   
                 
                 ⁢ 
                 
                     
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 4 
               
             
           
         
       
     
   
   Herein, N is the number coils that are wound around the motor, Φ A  is magnetic flux of the motor, α is a motor constant, and 
             x   _     ⁡     (     =       ⅆ   x       ⅆ   t         )           
is a piston speed.
 
   Accordingly, by substituting Equation 4 showing the counter electromotive force calculated in the counter electromotive force extractor  340  for following Equation 5, a second stroke estimation value can be obtained. 
   
     
       
         
           
             
               
                 x 
                 = 
                 
                   
                     1 
                     α 
                   
                   ⁢ 
                   
                     ∫ 
                     
                       
                         ( 
                         
                           a 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             x 
                             _ 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         ⅆ 
                         t 
                       
                     
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 5 
               
             
           
         
       
     
   
   Herein, x is a second stroke estimation value. 
   The operation control method of the reciprocating compressor in accordance with the present invention will be described with reference to accompanying  FIGS. 4 and 5 . 
   First, the current detector  380  detects the current applied to the motor, and the voltage detector  390  detects the voltage applied to the motor as shown at step S 410 . Herein, the first stroke estimator  370  calculates a first stroke estimation value with the current, the voltage and a constant of the motor by using Equation 1 as shown at step S 420  and applies it to the phase difference detector  360 . 
   Accordingly, the phase difference detector  360  detects a phase difference between a phase of the first stroke estimation value with a phase of the current applied to the motor and applies the difference to the controller  320  as shown at step S 430 . Then, when the phase difference is greater than 90°, the controller  320  increases an operational frequency applied to the compressor as shown at steps S 450  and S 460 , and when the phase difference is less than 90°, the controller  320  decreases an operational frequency applied to the compressor as shown at steps S 450  and S 470 . Accordingly, a stroke of the compressor  300  is controlled. 
   When a phase detected in the phase difference detector  360  is 90°, the controller  320  applies the voltage which is applied to the both ends of the searching coil detected by the searching coil voltage detector  350  to the counter electromotive force extractor  340  as shown at steps S 440  and S 441 . Herein, the voltage applied to the both ends of the searching coil is the total sum of the magnetic flux of the motor and the counter electromotive force, which can be calculated by Equation 2. 
   Afterward, the counter electromotive force extractor  340  extracts only the counter electromotive force E3 from the voltage applied to the both ends of the searching coil and applies it to the second stroke estimator  330 . Herein, as depicted in  FIG. 5 , the counter electromotive force extractor  340  calculates the counter electromotive force by using Equation 4 through Equations 2 and 3. 
   In other words, by using a size (i.e., magnitude) and a phase of E1 and E2, a size and a phase of E3 can be calculated. In more detail, by using a difference between a phase of the voltage applied to the both ends of the searching coil (phase of E1) and a phase of the motor magnetic flux (phase of E2), a size and a phase of the counter electromotive force (E3) can be detected. Herein, because a difference between a phase of E2 and a phase of E3 is 90°, a size of the counter electromotive force (E3) has a sin θ connection (i.e., relationship) with a size of the voltage (E1) applied to the both ends of the searching coil. Herein, θ is a difference between a phase of the motor magnetic flux and a phase of the voltage applied to the both ends of the searching coil as shown at steps S 442  and S 443 . 
   Then, the second stroke estimator  330  estimates a second stroke with the counter electromotive force (E3) and applies it to the comparator  310 . Herein, the second stroke estimation value can be calculated by utilizing Equation 5 as shown at step S 444 . 
   According to the above description, the comparator  310  compares the second stroke estimation value with the stroke reference value and applies a difference signal according to the comparison result to the controller  320 , and the controller  320  controls a stroke by varying the voltage applied to the motor. In more detail, when the stroke reference value is greater than the second stroke estimation value, the controller  320  increases a voltage input to the motor as shown at steps S 445  and S 446 . On the other hand, when the stroke reference value is less than the second stroke estimation value, the controller  320  decreases a voltage input to the motor as shown at steps S 445  and S 447 . 
   As described above, in the present invention, after detecting a counter electromotive force induced by a searching coil, by estimating a stroke with the counter electromotive force, there is no need to consider error of inductance and resistance among motor parameters, and accordingly it is possible to reduce a stroke estimation error. 
   Although the invention has been described with reference to an exemplary embodiment, it is understood that the words that have been used are words of description and illustration, rather than words of limitation. Changes may be made within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the invention in its aspects. Although the invention has been described with reference to particular means, materials and embodiments, the invention is not intended to be limited to the particulars disclosed. Rather, the invention extends to all functionally equivalent structures, methods, and uses such as are within the scope of the appended claims.