Abstract:
The compressor in a refrigeration system is controlled solely by a variable speed drive which controls the motor of the compressor by virtue of the varying of the frequency of the electricity provided to the motor. To minimize the initial cost and to minimize operating costs, the variable speed drive is cooled by refrigerant from the refrigeration system which permits the use of a smaller drive, and the variable speed drive is operated at, or approaching, a unity power factor.

Description:
BACKGROUND OF THE INVENTION 
     In screw compressors the bores for the rotors overlap. The overlapping bores create cusps in the nature of the waist of a figure eight. One of the cusps is the normal location for one form of a mechanical unloader which forms a portion of the bore and coacts with the rotors as it moves axially in the cusp to unload and to control the V 1  or discharge volume to suction volume ratio, of the compressor. The unloader is normally driven by a solenoid. To provide a greater degree of control, it is common to provide a variable speed drive which controls the motor by changing the frequency of the electric power being supplied to the motor by the variable speed drive. 
     SUMMARY OF THE INVENTION 
     The cost of a variable speed drive is on the order of that of a compressor. So, adding a variable speed drive to a conventional compressor greatly increases the cost and adds a degree of redundancy since the unloader valve, or other mechanical unloading structure, has some functional overlap with the variable speed drive in that both can control compressor capacity. While the variable speed drive is external to the compressor, an unloader valve is internal to the compressor. Being internal to the compressor, the unloader valve requires additional manufacturing steps to accommodate it in the compressor. Specifically, the unloader valve is located in a cusp and effectively forms a portion of the bores. This requires precision machining to achieve the requisite sealing with the rotors and introduces a leakage path along the interface of the unloader valve with the rotor bores. Other types of mechanical unloaders such as poppets also require additional manufacturing steps in order to be accommodated in a compressor. 
     The present invention eliminates the mechanical unloader structure and thereby simplifies the manufacture of the compressor while reducing costs. All of the control of the compressor is through the variable speed drive so that further efficiency increases and cost reductions can be achieved by properly selecting the variable speed drive, motor, compressor and chiller for a particular application. The required drive amperage of the variable speed drive, and also its cost, is directly related to the chiller performance and to the motor power factor. Improvements in the chiller performance and motor power factor lowers the average cost of a variable speed drive for an application. 
     In the case of the compressor, considerations for unloading include the amperage or load requirements over the range of operation, efficiency over the range of operation and the minimum speed requirements for bearing life which is dependent upon lubrication circulation with the refrigerant. The motor must be matched with both the variable speed drive and the compressor in order to optimize the speed of the compressor. For example, the ideal compressor speed for a given load is not usually the same as the synchronous speed. Also, the variable speed drive may be required to compensate for the various input frequencies and voltages used around the world and one motor voltage can be used for all applications over a range of supply voltages. For example, one variable speed drive and motor combination might be efficiently used for power supplied at 50 Hz or 60 Hz and over a voltage range of 346 volts to 480 volts since the variable speed drive output would remain the same. The system current usage can be minimized through a unity, or approaching unity, input power factor of the variable speed drive. The variable speed drive output can be increased by using system refrigerant for cooling such as is taught in commonly assigned U.S. Pat. No. 6,116,040. This permits the use of a smaller and therefore less expensive variable speed drive to produce a desired output. 
     The foregoing factors are optimized to achieve a given performance at a minimized installed cost with the following being affected: the compressor size, speed and configuration; the variable speed drive size, input, output and cooling configuration; the motor size and speed; and the input wire sizes. 
     It is an object of this invention to control compressor output in a refrigeration system solely by use of a variable speed drive. 
     It is another object of this invention to reduce the initial cost of a refrigeration system employing a variable speed drive. 
     It is an additional object of this invention to add a variable speed drive to a refrigeration system at a cost penalty no greater than 5% of the cost of a compressor with mechanical unloading. 
     It is a further object of this invention to integrate a variable speed drive into a refrigeration system. These objects, and others as will become apparent, hereinafter, are accomplished by the present invention. 
     Basically, the compressor in a refrigeration system is controlled solely by a variable speed drive which controls the motor of the compressor by virtue of the varying of the frequency of the electric current provided to the motor. To minimize the initial cost and to minimize operating costs, the variable speed drive is cooled by refrigerant from the refrigeration system which permits the use of a smaller drive, and the variable speed drive is operated at, or approaching, a unity power factor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the present invention, reference should now be made to the following detailed description thereof taken in conjunction with the accompanying= drawing wherein: 
     The FIGURE is a schematic representation of a refrigeration system employing the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For a refrigeration system a particular system capacity is the starting point in designing the system. A compressor capable of producing the required capacity will be chosen based upon factors such as cost, efficiency and operating speed. The selection of the compressor will be in conjunction with the selection of the motor. Motors are available having power factors in the range of 0.80 to 0.93 and the motor will be selected based upon cost, power factor, efficiency at design compressor speed. Variable speed drives are available which have power factors running from 0.99 to unity. However, the differences between the standard sizes of the variable speed drives are relatively large such that a considerably oversized variable speed drive might be the smallest standard drive available sufficient to meet design requirements. By cooling the variable speed drive with refrigerant from the condenser, it may be operated at higher than its air cooled drive design capacity because of the greater cooling available. For example, a 100 ampere drive that supplies 80 amperes when air cooled could supply from 80 to 100 amperes for driving the compressor when refrigerant cooled. 
     In the FIGURE, the numeral  10  generally designates a refrigeration system. Refrigeration system  10  has a screw compressor  12  which has no mechanical unloading structure. Refrigeration system  10  includes a closed fluid circuit serially including compressor  12 , discharge line  14 , condenser  16 , line  18  containing expansion device  20  and flash tank economizer  22 , line  24  containing expansion device  26 , chiller  28  and suction line  30 . Line  32  branches from flash tank economizer  22  and provides fluid communication with a trapped volume in compressor  12  at an intermediate pressure. 
     Compressor  12  is driven by motor  11  under the control of variable speed drive  40  which is connected to the electrical power grid (not illustrated). Variable speed drive  40  controls the alternating frequency of the current supplied to motor  11  thereby controlling the speed of motor  11  and the output of compressor  12 . In chiller  28 , water is chilled by refrigerant circulating in the closed fluid circuit of refrigeration system  10 . The chilled water provides the cooling to the zones. The temperature of the water leaving chiller  28  via line  29  is sensed by thermal sensor  50  and supplied to microprocessor  100 . Microprocessor  100  controls variable speed drive  40  and thereby motor  11  and compressor  12  to maintain a desired water temperature for the water leaving chiller  28 . Microprocessor  100  can control variable speed drive  40  solely responsive to the temperature sensed by thermal sensor  50  or it may also receive zone inputs from the zones being cooled and regulate the rate of water circulation through the chiller  26 , and thereby the amount of available cooling. If desired, microprocessor  100  may also control expansion devices  20  and  26 . 
     While refrigeration system  10 , as described above, has many features common with conventional refrigeration systems, there are a number of significant differences. Screw compressor  12  is simpler than conventional refrigeration compressors in that it has no mechanical unloading structure. Accordingly, the rotors only seal with each other and the bores. There is no slide valve which replaces portions of the bores in the region of a cusp with the attendant extra manufacturing costs and potential for leakage between the slide valve and adjacent structure or any other mechanical unloading structure. The output of compressor  12  is controlled through motor  11  whose speed is controlled by variable speed drive  40 . The motor  11  is matched to the variable speed drive  40  and compressor  12 . There is an ideal compressor speed for the design compressor output. So the motor is chosen to have efficient operation at the ideal compressor speed and to have an optimized power factor. On the input side of the variable speed drive, a near unity power factor reduces energy usage and the cost of the energy because of the reduced energy demand at, or approaching, unity power factor. This is because the power factor of the variable speed drive, not the power factor of the motor, is seen by the utility, since the variable speed drive isolates the motor from the utility. 
     In the operation of refrigeration system  10 , gaseous refrigerant is induced into compressor  12  via suction line  30  and compressed with the resultant hot, high pressure refrigerant gas being supplied via discharge line  14  to condenser  16 . In condenser  16 , the gaseous refrigerant condenses as it gives up heat due to heat transfer via air, water or brine-cooled heat exchangers (not illustrated). The condensed refrigerant passes from condenser  16  into line  18  and serially passes through expansion device  20  into flash tank economizer  22 . A portion of the refrigerant flowing into economizer  22  is diverted into line  32  at an intermediate pressure and passes via line  32  to a trapped volume in compressor  12 . The remaining liquid refrigerant in economizer  22  passes through expansion device  26  thereby undergoing a pressure drop and partially flashing as it passes via line  24  into chiller  28 . In chiller  28 , the remaining liquid refrigerant evaporates due to heat transfer to the water passing through chiller  28  via line  29 . The economizer flow into compressor  12  via line  32  increases the capacity of compressor in that it increases the mass of refrigerant gas being compressed. 
     Microprocessor  100  receives a signal form thermal sensor  50  indicative of the temperature of the water leaving chiller  28  via line  29  to provide cooling to one or more zones (not illustrated). Responsive to the water temperature sensed by sensor  50 , the microprocessor  100  sends a signal to variable speed drive  40  to cause it to change the speed of motor  11  to increase or decrease the cooling capacity of compressor  12 , as required. Variable speed drive  40  increases or decreases the speed, and therefore the capacity, of compressor  12  by changing the frequency of the current supplied to power motor  11 . By having a motor  11  operating at an optimum power factor the electrical usage and demand are minimized and the size of the variable speed drive  40  required is reduced. Additionally, a portion of the liquid refrigerant in condenser  16  is diverted via line  17  to the variable speed drive  40  where the electronic components are cooled and the refrigerant evaporated. The evaporated refrigerant passes from variable speed drive  40  via line  41  to chiller  28 . The rate of flow of refrigerant to variable speed drive  40  from condenser  16  is controlled by valve  42  responsive to the temperature of the refrigerant leaving variable speed drive sensed by sensor  43 . Because the variable speed drive  40  is cooled by the liquid refrigerant, a still smaller variable speed drive  40  can be used. 
     Although a preferred embodiment of the present invention has been illustrated and described, other changes will occur to those skilled in the art. For example, the economizer may be omitted and/or zone temperature, water flow rates, the expansion devices can be connected to the microprocessor. It is therefore intended that the present invention is to be limited only by the scope of the appended claims.