Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a screw compressor for refrigerating apparatus that is driven by a motor being controlled by an inverter. 
     2. Description of the Related Art 
     Conventionally, a screw compressor for refrigerating apparatus adapted to control the rotational speed of each of motors for driving the compressor body and the liquid injection pump according to the increase and the decrease of load is known in Japanese Patent Laid-Open Publication No. Sho 57-18484. This screw compressor has a suction pressure detector for detecting the suction pressure in its suction section, which pressure has a monotone increasing relationship with the cooling heat load, wherein a pressure signal outputted from the suction pressure detector is inputted to a variable voltage frequency converter via a pressure regulator, so that rotational speeds of a compressor body driving motor and a liquid injection pump driving motor are controlled simultaneously. 
     In addition, a screw compressor for refrigerating apparatus which is aimed at the improvement of efficiency in a low partial load region by adding a capacity control system using an inverter to a stepless capacity control mechanism by a slide valve is disclosed in Japanese Patent Laid-Open Publication No. Sho 59-211790. 
     The above-mentioned screw compressor disclosed in Japanese Patent Laid-Open Publication No. Sho 57-18484 is controlled to increase the rotational speed of motor when the pressure detected by the suction pressure detector is higher than the predetermined value of pressure. Therefore, there is a problem in that the motor is apt to be overloaded, so that the endurance of the motor is deteriorated. 
     Furthermore, the screw compressor disclosed in Japanese Patent Laid-Open Publication No. Sho 59-211790 has a complicated construction because the slide valve is provided. Furthermore, the screw compressor is constructed so that the inverter is not used, i.e., a commercial power source is directly used without intervening the inverter, when the capacity of this screw compressor is in the range of 100 to 75% by the slide valve, whereby there is a problem in that the inverter is not sufficiently used. 
     SUMMARY OF THE INVENTION 
     In view of above problems, it is an object of the present invention to provide a screw compressor for refrigerating apparatus capable of controlling the rotational speed of a motor to be suitable for cooling heat load by an online power system using an inverter, without causing overload of the motor. 
     To solve the above problem, the present invention provides a screw compressor for refrigerating apparatus comprising: a screw rotor; a motor for driving said screw rotor, the rotational speed of said motor being controlled through an inverter; a heat load detecting means for detecting a cooling heat load; a load condition detecting means for detecting the heat load condition of said motor; and a motor rotational speed controlling device for controlling the rotational speed of said motor based on a heat load signal from said heat load detecting means and a load condition signal from said load condition detecting means. 
     Here, said motor rotational speed controlling device can be configured to control the rotational speed of said motor to be reduced if the compressor capacity is determined as excessive based on said heat load signal or if the load condition of said motor is determined as excessive based on said load condition signal, and to control the rotational speed of said motor to be increased if the compressor capacity is determined as lacking based on said heat load signal and the load condition of said motor is determined as not excessive based on said load condition signal. 
     As said head load detecting means, either a detector for detecting the suction pressure of said screw compressor or a detector for detecting the temperature of cooled liquid, emanated from an evaporator in said refrigerating apparatus can be used. 
     As said load condition detecting means, a detector for detecting the coil temperature of said motor, a detector for detecting the electric current that is supplied to said motor, a detector for detecting the temperature of refrigerant gas that is discharged from said screw compressor, a detector for detecting the rotational speed of said motor and the like can be used. 
     It is possible to provide a plurality of said load condition detecting means to perform the control of rotational speed of said motor based on a plurality of said load condition signals. 
     According to the present invention, it becomes possible to control the corresponding rotational speed of the motor to be suitable for the cooling heat load without causing the overload of motor. As a result, the improvement of motor endurance and the reduction of consuming power can be realized. Furthermore, above advantages are accomplished by always using the inverter and not using a slide valve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS. 
     FIG. 1 schematically shows a refrigerating apparatus incorporating a screw compressor according to the present invention; 
     FIG. 2 shows a relationship between the suction pressure and the cooling heat load of the screw compressor in the refrigerating apparatus shown in FIG. 1; 
     FIG. 3 is a flow chart illustrating the control of the screw compressor in the refrigerating apparatus shown in FIG. 1; 
     FIG. 4 schematically shows a refrigerating apparatus incorporating another screw compressor according to the present invention; 
     FIG. 5 schematically shows a refrigerating apparatus incorporating still another screw compressor according to the present invention; 
     FIG. 6 is a flow chart illustrating the control of the screw compressor in the refrigerating apparatus shown in FIG. 5; 
     FIG. 7 schematically shows a refrigerating apparatus incorporating yet another screw compressor according to the present invention; 
     FIG. 8 is a flow chart illustrating the control of the screw compressor in the refrigerating apparatus shown in FIG. 7; 
     FIG. 9 schematically shows a refrigerating apparatus incorporating yet another screw compressor according to the present invention; 
     FIG. 10 is a flow chart illustrating the control of the screw compressor in the refrigerating apparatus shown in FIG. 9; 
     FIG. 11 schematically shows a refrigerating apparatus incorporating yet another screw compressor according to the present invention; 
     FIG. 12 is a flow chart illustrating the control of the screw compressor in the refrigerating apparatus shown in FIG. 11; 
     FIG. 13 schematically shows a refrigerating apparatus incorporating yet another screw compressor according to the present invention; 
     FIG. 14 is a flow chart illustrating the control of the screw compressor in the refrigerating apparatus shown in FIG. 13; 
     FIG. 15 schematically shows a refrigerating apparatus incorporating yet another screw compressor according to the present invention; and 
     FIG. 16 is a flow chart illustrating the control of the screw compressor in the refrigerating apparatus shown in FIG.  15 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now, embodiments of the present invention will be explained with reference to the drawings. 
     FIG. 1 illustrates a refrigerating apparatus  10 A incorporating a screw compressor  1 A according to the present invention, in which a circulation flow path  14  is formed for flowing the refrigerant through the screw compressor  1 A, a condenser  11 , an expansion valve  12  and an evaporator  13 . 
     The screw compressor  1 A comprises a pair of engaging male and female screw rotors  21 , a motor  22  for rotating the screw rotors  21 , a discharging portion  23  and a motor coil temperature detector D 1 , wherein the motor  22  is adapted to be operated by a power supplied from a power source supplying line  24  via an inverter  25 . The condenser  11  is provided with a cooling water inlet Ci and a cooling water outlet Co and the evaporator  13  is provided with a cooled liquid inlet Bi and a cooled liquid outlet Bo. Furthermore, in the secondary side of the evaporator  13  in connection with the circulation flow path  14 , an superheat degree detector D 2  for detecting the superheat degree of refrigerant gas emanated from the evaporator  13  is provided, so that the opening extent of the expansion valve  12  is controlled according to the detected superheat degree. And, in the secondary side of the evaporator  13 , a suction pressure detector D 3  for detecting the suction pressure of the screw compressor  1 A is provided, so that a pressure signal indicating the detected pressure is inputted from this suction pressure detector D 3  into a controller  26  and simultaneously, a temperature signal indicating the detected temperature is inputted from a motor coil temperature detector D 1  into the controller  26 . Herein, the suction pressure is a factor indicating the cooling heat load of the refrigerating apparatus  10 A and the motor coil temperature is a factor indicating the load condition of the motor  22 . And based on both of these signals, frequency conversion of power is performed in the inverter  25  and the motor  22  is controlled as explained later. 
     However, as shown in FIG. 2, the suction pressure of the screw compressor  1 A and the cooling heat load of the refrigerating apparatus  10 A have a relationship of wherein and the cooling heat load increases as the suction pressure increases. And, if the suction pressure is too low, the cooling heat load is so small that the capacity of the screw compressor  1 A becomes excessive, and thus it is necessary to reduce the consumption power by reducing the rotational speed of the motor  22  to reduce the capacity of the screw compressor  1 A. In contrast, if the suction pressure is too high, it is needed to increase the capacity of the screw compressor  1 A by increasing the rotational speed of the motor  22 , because the capacity of the screw compressor  1 A becomes lacking due to the increased cooling heat load. Therefore, it is possible to predetermine X region in which the suction pressure is too low, Z region in which the suction pressure is too high, and Y region in which the suction pressure is neither high nor low and thus the rotational speed of the motor  22  does not need to be changed, and boundary values of each region is previously set in the controller  26 . 
     Next, the control of the motor  22  of the above-mentioned refrigerating apparatus  10 A that is in working will be described with reference to FIG.  3 . 
     Firstly, if the screw compressor  1 A is started and thus the refrigerating apparatus  10 A becomes to the working condition, determination as to where a suction pressure belongs among the above-mentioned X, Y and Z regions in the controller is made in the first step; if it belongs to X region, the process passes to a step for reducing the rotational speed of the motor  22 , and if it belongs to Y region, it is not needed to change the rotational speed of the motor  22  and thus the process is returned to the first step while maintaining the rotational speed. Meanwhile, if the suction pressure belongs to Z region, it is needed to increase the capacity of the screw compressor  1 A by increasing the rotational speed of the motor  22 , but if the rotational speed of the motor  1 A is excessively increased, the motor  22  becomes to an overload condition and the overload condition should be avoided. 
     Accordingly, if the suction pressure belongs to Z region, the process passes to a step for determining whether the motor coil temperature is below the predetermined upper limit value (YES) or not (NO) and this determination is made in the controller  26 . If YES, the process passes to a step for increasing the rotational speed of the motor  22  and a signal increasing frequency of power for the inverter  25  is outputted from the controller  26 , so that the rotational speed of the motor  22  will be increased. Whereas, if NO, the process passes to a step for reducing the rotational speed of the motor  22  and a signal for reducing the frequency of power outputted from the controller  26  to the inverter  25 , thus the rotational speed of the motor  22  is decreased even if the suction pressure is in Z region, because the motor  22  is considered as being in the overload condition. After passing through these steps for changing the rotational speed of the motor  22 , all of the cases are returned to the first step and each of the above-mentioned steps is repeated. 
     In this way, the capacity of the screw compressor  1 A is regulated in response to the change of cooling heat load, without causing the overload of the motor  22 . 
     FIG. 4 shows a refrigerating apparatus  10 B using another screw compressor  1 B according to the present invention, in which drawing the constituents of the refrigerating apparatus common to those of the refrigerating apparatus  10 A shown in FIG. 1 are indicated with same reference numerals and descriptions thereof will be omitted. 
     In this refrigerating apparatus  10 B, a cooled liquid temperature detector D 4  is provided in the flow path for cooled liquid emanated from the evaporator  13 , instead of the suction pressure detector D 3  shown in FIG. 1, so that a temperature signal indicating the detected temperature is inputted from the cooled liquid temperature detector D 4  into the controller  26 . 
     As explained in the above, the temperature of the cooled liquid has monotone increasing relationship with the cooling heat load, and this cooling heat load is resulted from the cooled liquid outlet Bo of the evaporator  13  or the lower side thereof. Therefore, the refrigerating apparatus  10 B shown in FIG. 4 is substantially identical to the refrigerating apparatus  10 A shown in FIG. 1, so that it is possible to make determination as to where a suction pressure belongs among the X, Y and Z regions based on the temperature signal from the cooled liquid temperature detector D 4 , and the flow chart shown in FIG. 3 can be correspondingly applied to the refrigerating apparatus  10 B shown in FIG.  4 . 
     FIG. 5 shows a refrigerating apparatus  10 C incorporating still another screw compressor  1 C according to the present invention, in which drawing the constituents common to those of the refrigerating apparatus  10 A shown in FIG. 1 are indicated with same reference numerals and descriptions thereof will be omitted. 
     The refrigerating apparatus  10 C is provided with a current detector D 5  for detecting the magnitude of motor current of the power supplied to the motor  22  instead of the motor coil temperature detector D 1  shown in FIG. 1, so that a current signal indicating the detected current is inputted from the current detector D 5  into the controller  26 . 
     And, in this refrigerating apparatus  10 C, as shown in FIG. 6, a control is performed in which a step for determining whether the motor current is below the predetermined upper limit value (YES) or not (NO) is included instead of the step for determining whether the motor coil temperature is below the predetermined upper limit value or not as shown in FIG.  3 . Namely, if the motor current is below the upper limit value (YES), it passes to the step for increasing the rotational speed of the motor, because the motor  22  is considered as not in the overload condition. If the motor current is not below the upper limit value (NO), it passes to the step for reducing the rotational speed of the motor, because the motor  22  is considered as in the overload condition. 
     This control is substantially identical to that shown in FIG. 3, except that the overload condition of the motor  22  is determined based on the motor current. 
     FIG. 7 shows a refrigerating apparatus  10 D incorporating still another screw compressor  1 D according to the present invention, in which drawing the constituents of the screw compressor  10 A common to those of the refrigerating apparatus shown in FIG. 1 are indicated with same reference numerals and descriptions thereof will be omitted. 
     The refrigerating apparatus  10 D is provided with a discharge temperature detector D 6  for detecting the temperature of refrigerant gas compressed and discharged from the screw rotors  21  in the discharging portion  23  instead of the motor coil temperature detector D 1  shown in FIG. 1, so that a temperature signal indicating the detected temperature is inputted from the discharge temperature detector D 6  into the controller  26 . 
     And, in this refrigerating apparatus, as shown in FIG. 8, a control is performed in which a step for determining whether the discharge temperature of the compressed refrigerant gas from the screw rotors  21  is below the predetermined upper limit value (YES), or not (NO) is included instead of the step for determining whether the motor coil temperature is below the upper limit value or not as shown in FIG.  3 . Namely, if the discharge temperature is below the upper limit value (YES), it passes to the step for increasing the rotational speed of the motor, because the motor  22  is considered as not in the overload condition, and if the discharge temperature is not below the upper limit value (NO), it passes to the step for reducing the rotational speed of the motor, because the motor  22  is considered as in the overload condition. 
     This control is substantially identical to that shown in FIG. 3, except that the overload condition of the motor  22  is determined based on the discharge temperature. 
     FIG. 9 shows a refrigerating apparatus  10 E incorporating still another screw compressor  1 E according to the present invention, in which drawing the constituents of the screw compressor  10 E common to those of the refrigerating apparatus  10 A shown in FIG. 1 are indicated with same reference numerals and descriptions thereof will be omitted. 
     The refrigerating apparatus  10 E is provided with a motor rotational speed detector D 7  for detecting the rotational speed of the motor  22  instead of the motor coil temperature detector D 1  shown in FIG. 1, so that a rotational speed signal indicating the detected rotational speed is inputted from the motor rotational speed detector D 7  into the controller  26 . 
     And, in this refrigerating apparatus, as shown in FIG. 10, a control is performed in which a step for determining whether the rotational speed of the motor is below the predetermined upper limit value (YES) or not (NO) is included instead of the step for determining whether the motor coil temperature is below the predetermined upper limit value or not as shown in FIG.  3 . Namely, if the rotational speed of the motor is below the upper limit value (YES), it passes to the step for increasing the rotational speed of the motor because the motor  22  is considered as not in the overload condition, and if the rotational speed of the motor is not below the upper limit value (NO), the step passes to the step for reducing the rotational speed of the motor because the motor  22  is considered as in the overload condition. 
     This control is substantially identical to that shown in FIG. 3, except that the overload condition of the motor  22  is determined based on said rotational speed of the motor. 
     Furthermore, because it is also possible to know the rotational speed of the motor from the frequency of electric current outputted from the inverter  25 , a frequency detector D 7  for detecting the frequency may be provided on the inverter  25  or its secondary side instead of the motor rotational speed detector D 7 , so that a frequency signal indicating the magnitude of detected frequency is inputted from the frequency detector D 7  into the controller  26 . In this case, a step for determining whether the frequency is below the predetermined upper limit value or not is included instead of the step for determining the magnitude of rotational speed of the motor in FIG.  10 . 
     Each of the refrigerating apparatus described in the above is provided with only one type of detectors among the motor coil temperature detector D 1 , the cooled liquid temperature detector D 4 , the current detector D 5  and the like in order to determine the load condition of the motor  22 . However, the present invention is not limited to a certain type of detectors, and covers refrigerating apparatus provided with two or more detectors suitably selected from these detectors. Selected detectors may include all or some of the detectors described in the above to determine the load condition of the motor  22  and the combination thereof is optional. 
     Next, refrigerating apparatus using two types, three types, and four types of detectors, respectively, for determining the load condition of the motor  22  will be described as an example. Of course, the present invention is not limited to illustrated combinations. 
     FIG. 11 shows a refrigerating apparatus  10 F to which a screw compressor  1 F provided with the motor coil temperature detector D 1  and the discharge temperature detector D 6 , in which drawing the constituents common to those of the refrigerating apparatus explained in the above are indicated with same reference numerals and descriptions thereof will be omitted. 
     In the refrigerating apparatus  10 F, as shown in FIG. 12, the determination as to where a suction pressure belongs among X, Y and Z regions is made; if it is determined that it belongs to Z region, two steps for determining signals from two detectors are interposed before arriving at the step for increasing the rotational speed of the motor. Namely, if it is determined that the suction pressure belongs to Z region, the process passes to the step for determining whether the motor coil temperature is below the predetermined upper limit value (YES) or not (NO). If NO, the process passes to the step for reducing the rotational speed of the motor because the motor is considered as in the overload condition. If YES, the process passes to the step for determining whether the discharge temperature is below the upper limit value (YES) or not (NO), because it can not be considered that the motor  22  is in the overload condition merely based on the motor coil temperature. If NO, the process passes to the step for reducing the rotational speed of the motor because the motor  22  is considered as in the overload condition, and if YES, the process passes to the step for increasing the rotational speed of the motor because the motor  22  is considered as not in the overload condition. The control flow thereafter is identical to those described in the above. 
     Like this, in the refrigerating apparatus  10 F, the determination as to whether the motor  22  is in the overload condition or not is doubly made based on two factors. 
     Furthermore, the order of two determination steps as to the motor coil temperature detector D 1  and the discharge temperature detector D 6  is optional rather than limited thereto. 
     FIG. 13 shows a refrigerating apparatus  10 G incorporating a screw compressor  1 G additionally provided with the current detector D 5  in addition to the motor coil temperature detector D 1  and the discharge temperature detector D 6 , in which drawing the constituents common to those of the refrigerating apparatus explained in the above are indicated with same reference numerals and descriptions thereof will be omitted. 
     As shown in FIG. 14, in this refrigerating apparatus  10 G, a step for determining whether the motor current is below the upper limit value (YES) or not (NO) is added to the control flow chart thereof in addition to the flow chart shown in FIG.  13 . 
     And, in this refrigerating  10 G, the determination as to whether the motor  22  is in the overload condition or not is trebly made based on three factors. 
     Furthermore, the order of three determination steps as to the motor coil temperature detector D 1 , the discharge temperature detector D 6  and the current detector D 5  is optional rather than limited thereto. 
     FIG. 15 shows a refrigerating apparatus  10 H incorporating a screw compressor  1 H additionally provided with the motor rotational speed detector D 7  in addition to the motor coil temperature detector D 1 , the discharge temperature detector D 6 , and the current detector D 5 , in which drawing the constituents common to those of the refrigerating apparatus explained in the above are indicated with same reference numerals and descriptions thereof will be omitted. 
     As shown in FIG. 16, in this refrigerating apparatus  10 H, a step for determining whether the rotational speed of the motor is below the upper limit value (YES) or not (NO) is added in the control flow chart in addition to the flow chart shown in FIG.  14 . 
     And, in this refrigerating apparatus  10 H, the determination as to whether the motor  22  is in the overload condition or not is fourfold made based on four factors. 
     Furthermore, the order of four determination steps as to the motor coil temperature detector D 1 , the discharge temperature detector D 6 , the current detector D 5  and the motor rotational speed detector D 7  is optional rather than limited thereto. 
     In each refrigerating apparatus shown in FIG.  5  and the drawings thereafter, the cooled liquid temperature detector D 4  may be provided instead the suction pressure detector D 3 . In this case, the suction temperature is introduced based on the temperature signal from the cooled liquid temperature detector D 4  and then the determination as to where the suction pressure belongs among X, Y and Z regions is made in the control flow.

Technology Category: 4