Abstract:
A permanent split capacitor motor operable in a full mode and in a modulated mode for improving efficiency. The motor includes a stator and a rotor in rotational relationship with the stator. The motor also includes a single set of windings wound on the stator. The windings are in a magnetically coupled relationship with each other such that one of the windings is a main motor winding while the other is an auxiliary motor winding. The windings define a plurality of A-ratios as a function of turns in the main motor winding compared to turns in the auxiliary motor winding. A switching circuit selectively energizes the first and second windings in a full mode configuration and in a modulated mode configuration based on motor load conditions. In the full mode configuration, the A-ratio of the windings is greater than in the modulated mode configuration.

Description:
BACKGROUND OF THE INVENTION 
     The invention generally relates to permanent split capacitor single phase induction motors and, particularly, to a permanent split capacitor motor having a full capacity mode and a modulated capacity mode for improving operating efficiency. 
     A permanent split capacitor (PSC) motor of the type described herein has a stator assembly forming a core of magnetic material. Typically, the core consists of a stack of laminations punched from sheet-like ferro-magnetic material. Each lamination has a plurality of teeth spaced around a central opening and extending radially inwardly. When the laminations are stacked, the central openings are coaxial and constitute a bore extending longitudinally through the core. The bore receives a rotor assembly (e.g., a squirrel cage rotor) made from a stack of rotor laminations. A slip between the rotation of the rotor and the rotation of a magnetic field created by the stator induces a current in the rotor. In turn, the induced current creates a magnetic field of the rotor in contrast to the magnetic field of the stator. These contrasting rotating magnetic fields cause rotating torque of the rotor. Such a motor is particularly useful for driving a compressor of a refrigeration or air conditioning system. In this instance, the rotor has a bore for receiving a hermetic compressor crankshaft that rotatably supports the rotor body within the stator bore. 
     The rising cost of energy, the heightened awareness of environmental issues and the attendant governmental regulations for appliances and the like have all tended to accentuate the ongoing need for efficient and economical motors. As described above, single phase induction motors, including PSC motors, are frequently used as part of refrigeration and air conditioning systems for driving hermetically sealed compressors. In such systems, proper sizing of the equipment seeks to improve efficiency for operation over a wide range of load conditions. However, it is difficult to provide ample capacity and efficient operation for peak load conditions while still operating efficiently at lighter load conditions. 
     In general, the efficiency of a compressor motor involves the ratio of running load torque to breakdown torque. A ratio of about 3.0 (breakdown torque/running load torque) is desired for a relatively high efficiency for running load while still meeting the low voltage run down loaded requirements. The Air conditioning and Refrigeration Institute (ARI) sets forth standard test procedures for evaluating compressor efficiency. The test procedures examine the compressor&#39;s performance at standard conditions of 45° F. evaporating and 130° F. condensing temperature. Present government guidelines for energy efficiency reference ARI standards. In addition, compressor performance may be measured at operating conditions more closely approximating the actual operating conditions of a high efficiency system. For example, Copeland Corporation evaluates the performance of its compressors according to a standard referred to as “CHEER.” The CHEER standard rates compressor performance at 45° F. evaporating, 100° F. condensing temperature; 85° F. liquid; 65° F. return gas. Since the CHEER rating conditions more closely approximate the conditions under which the compressor will operate most frequently, higher compressor efficiency at CHEER generally equates to lower operating cost. 
     One method for modulating the compressor of the refrigeration system involves operating the compressor at two distinct speeds. However, multiple speed motors often cost more than single speed motors and/or fail to provide sufficient operating torque at low speeds. As an example, distinct winding multiple speed motors require separate main and auxiliary windings for each motor speed, which can increase the cost of the motor and present problems with respect to slot fill. 
     Since the motor is enclosed and hermetically sealed within the compressor unit in such a system, the number of leads from the motor is another cost factor. Electrical connections are made through the shell of the compressor and special connectors are needed to preserve the hermetic seal. The use and insertion of the connectors in the shell add significantly to the cost of the compressor. Consequently, motors designed for use in hermetic compressors should incorporate a minimum number of leads so as to minimize construction problems and the cost inherent in making multiple electrical connector openings through the compressor shell. 
     For these reasons, a motor is desired for reducing breakdown torque and improving efficiency over a wide operating range from peak load conditions to lightly loaded conditions. Further, such a motor is desired that does not require a large number of leads. 
     Commonly assigned U.S. Pat. No. 4,322,665, U.S. Pat. No. 4,103,212 and U.S. Pat. No. 4,103,213, the entire disclosures of which are incorporated herein by reference, disclose single phase motors that may be used for driving compressors. 
     SUMMARY OF THE INVENTION 
     The invention meets the above needs and overcomes the deficiencies of the prior art by providing an improved PSC motor system. Among the several objects and features of the present invention may be noted the provision of such a motor system that permits high efficiency operation over a wide range of load conditions; the provision of such a motor system that permits operation in a full capacity mode and in a modulated capacity mode; the provision of such a motor system that permits reducing breakdown torque in a modulated capacity mode; the provision of such a motor system that permits electrical connections using a minimum number of leads; the provision of such a motor system that permits unidirectional rotation for driving a compressor; and the provision of such method that can be carried out efficiently and economically and such system that is economically feasible and commercially practical. 
     Briefly described, a permanent split capacitor motor embodying aspects of the invention is operable in a full mode and in a modulated mode. The motor includes a stator and a rotor in rotational relationship with the stator. The motor also includes first and second windings wound on the stator. The windings are in a magnetically coupled relationship with each other such that one of the windings is a main motor winding while the other is an auxiliary motor winding. The windings define an A-ratio as a function of turns in the main motor winding compared to turns in the auxiliary motor winding. The motor further includes a switching circuit for selectively energizing the first and second windings in a full mode configuration and in a modulated mode configuration based on motor load conditions. In a preferred embodiment, the A-ratio of the windings energized in the full mode configuration is greater than the A-ratio of the windings energized in the modulated mode configuration. In this manner, the motor is selectively operable in the full and modulated modes based on motor load conditions for improving efficiency. 
     In another embodiment, a permanent split capacitor having a stator and a rotor in rotational relationship with the stator is operable in a full mode and in a modulated mode. The motor includes first and second windings wound on the stator in a magnetically coupled relationship with each other. The first winding generates a first rotating main magnetic field and the second winding generates a first rotating auxiliary magnetic field when the windings are energized in a full mode configuration for rotating the rotor. On the other hand, the second winding generates a second rotating main magnetic field and the first winding generates a second rotating auxiliary magnetic field when the windings are energized in a modulated mode configuration different from the full mode configuration for rotating the rotor. The motor also includes a switching circuit for selectively energizing the windings in the full mode configuration and in the modulated mode configuration based on motor load conditions. In this manner, the motor is selectively operable in the full and modulated modes based on motor load conditions for improving efficiency. 
     Another embodiment of the invention is directed to an improved compressor with a drive shaft driven by a permanent split capacitor motor. The motor has a stator wound with first and second windings and a rotor in rotational relationship with the stator. In addition, the rotor is in driving relation with the shaft. The improvement includes the first winding generating a first rotating main magnetic field and the second winding generating a first rotating auxiliary magnetic field when the windings are energized in a full mode configuration for rotating the rotor. The motor operates in a full mode for driving the compressor when the windings are energized in the full mode configuration. The improvement also includes the second winding generating a second rotating main magnetic field and the first winding generating a second rotating auxiliary magnetic field when the windings are energized in a modulated mode configuration different from the full mode configuration for rotating the rotor. In the instance, the motor operates in a modulated mode for driving the compressor when the windings are energized in the modulated mode configuration. The improvement further includes a switching circuit for selectively energizing the windings in the full mode configuration and in the modulated mode configuration based on motor load conditions. In this manner, the motor is selectively operable in the full and modulated modes based on motor load conditions for improving efficiency. 
     In yet another embodiment, a permanent split capacitor motor embodying aspects of the invention includes a stator and a rotor in rotational relationship with the stator. A single set of windings are wound on the stator. The set of windings includes a main motor winding and an auxiliary motor winding different from the main winding. The main and auxiliary motor windings are in a magnetically coupled relationship with each other and define a plurality of A-ratios as a finction of turns in the main motor winding compared to turns in the auxiliary motor winding. 
     Another embodiment of the invention is directed to a method of improving efficiency in a permanent split capacitor motor. The motor has a stator and a rotor, the rotor being in rotational relationship with the stator. The method includes the steps of winding a first winding on the stator and winding a second winding different from the first winding on the stator. The method also includes selecting one winding as a main motor winding and selecting the other winding as an auxiliary motor winding. The windings define an A-ratio as a function of turns in the main motor winding compared to turns in the auxiliary motor winding. The method further includes the step of selectively energizing the first and second windings in a full mode configuration and in a modulated mode configuration based on motor load conditions. In a preferred embodiment, the A-ratio of the windings energized in the full mode configuration is greater than the A-ratio of the windings energized in the modulated mode configuration. In this manner, the motor is selectively operable in the full and modulated modes based on motor load conditions for improving efficiency. 
     Alternatively, the invention may comprise various other methods and systems. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram a permanent split capacitor motor according to a preferred embodiment of the present invention. 
     FIGS. 2A and 2B are schematic models of the motor of FIG. 1 operating in a full capacity mode and a modulated capacity mode, respectively. 
     FIG. 3 is a diagrammatic view of the winding distribution of one preferred embodiment of the motor of FIG.  1 . 
     FIG. 4 is a graph illustrating exemplary efficiency and torque data from motor dynamometer test results for the motor of FIG. 1 operating at full capacity and modulated capacity. 
     FIG. 5 is a graph illustrating exemplary speed, input power and torque data from motor dynamometer test results for the motor of FIG. 1 operating at full capacity and modulated capacity. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIG. 1 illustrates a PSC motor, generally indicated at  10 , in schematic diagram form. In one preferred embodiment of the invention, the motor  10  drives a compressor (see FIGS.  2 A and  2 B), which is part of a refrigeration or air conditioning system. Copeland Corporation manufactures a number of scroll compressors suitable for use with the present invention. These Copeland compressors are covered by one or more of the following patents: U.S. Pat. No. 5,741,120, U.S. Pat. No. 5,678,985, U.S. Pat. No. 5,613,841 and U.S. Pat. No. 5,611,674, the entire disclosures of which are incorporated herein by reference. As an example, Copeland sells a 2.75 HP scroll compressor under the trademark Quantum™ (model number ZRS34K3-PFV) that provides high efficiency operation when driven by motor  10  of the present invention. While the invention is described with particular detail in reference to motors used in hermetic compressor applications, those skilled in the art will recognize the wider applicability of the inventive principles disclosed herein. 
     According to the invention, motor  10  advantageously provides full and modulated modes of operation. In the full mode, motor  10  provides high efficiency operation for driving full loads and in the modulated mode, motor  10  provides high efficiency operation for driving relatively lighter loads. As shown in FIG. 1, motor  10  includes a first winding  12  and a second winding  14  wound on its stator. Those skilled in the art recognize that the windings  12 ,  14  are connected to each other and to a continuous rated capacitor C for producing a rotating torque on the rotor when energized. The first winding  12 , also referred to as a full mode winding, functions as the PSC motor&#39;s main winding and the second winding  14  functions as its auxiliary winding when motor  10  operates in the full mode. Conversely, second winding  14 , also referred to as a modulated mode winding, functions as the main winding and first winding  12  functions as the auxiliary winding when motor  10  operates in the modulated mode. 
     FIG. 1 further illustrates a switching circuit, generally indicated at  18 , for switching operation of motor  10  between its full and modulated modes. For example, the switching circuit  18  comprises a triple pole, double throw switch responsive to a motor control circuit  20  for switching modes. In the alternative, switching circuit  18  comprises three, single pole, double throw switches. Switching circuit  18  is conventional and may be any one of a variety of commercially available switches. Therefore, its structure and operation are not described in detail. For simplicity, FIG. 1 illustrates switching circuit  18  as individual switching elements  24 ,  26 ,  28 . 
     When full capacity is desired, the motor control circuit  20  causes the switch  24  to connect node  32  to the circuit&#39;s positive power bus and the switch  26  to connect node  34  to the capacitor C. In addition, the switch  28  connects node  36  to the circuit&#39;s common bus. This configuration orients full mode winding  12  as the main winding of motor  10  relative to the auxiliary winding  14 . Similarly, the motor control circuit  20  causes switching circuit  18  to alternate its positions when modulated capacity is desired. In the modulated mode, switch  24  connects node  40  to the circuit&#39;s positive power bus, switch  26  connects node  42  to capacitor C and switch  28  connects node  44  to the circuit&#39;s common bus. This configuration orients modulated mode winding  14  as the main winding of motor  10  relative to the auxiliary winding  12 . Switching the main and auxiliary windings generally reverses a motor&#39;s direction of rotation. However, in a scroll compressor application, for example, rotation must remain unidirectional. Advantageously, motor  10  provides correct rotation for the compressor in both full and modulated modes. 
     FIG. 2A provides a diagrammatic model of motor  10  in the full mode and FIG. 2B provides a diagrammatic model of motor  10  in the modulated mode. In FIGS. 2A and 2B, the load current I L , the main winding current I M  and the auxiliary winding current I A  are each indicated with respect to first and second windings  12 ,  14  and capacitor C. 
     FIG. 3 diagrammatically illustrates the stator of motor  10  in an exemplary two pole configuration. Those skilled in the art recognize that the stator of motor  10  has a plurality of radial teeth spaced at approximately equal angular intervals and extending into the stator&#39;s central bore. In FIG. 3, the stator teeth are shown as radial lines. Each pair of adjacent teeth defines a slot in between the teeth for holding windings  12 ,  14 . As an example, the illustrated stator has 24 teeth defining 24 slots although other stator assemblies having a different number of teeth and slots may be utilized in accordance with this invention. 
     Referring further to FIG. 3, first winding  12  comprises two coil sets or physical winding poles  12   a ,  12   b . In a preferred embodiment of the invention, a plurality of turns of suitable magnet wire or the like make up the coil sets  12   a ,  12   b . Coil sets  12   a ,  12   b  are inserted in selected stator slots so that they are generally on opposite sides of the stator bore from one another. As an example, an outer coil of coil set  12   a  is inserted in slots  48 ,  50  with inner coils being inserted in selected slots between slot  48  and slot  50 . Likewise, an outer coil of coil set  12   b  is inserted in slots  52 ,  54  with inner coils being inserted in selected slots between slot  52  and slot  54 . FIG. 3 illustrates an exemplary connection in which coil sets  12   a ,  12   b  are connected electrically in parallel with each other by jumpers  58 ,  60 . 
     Second winding  14  is likewise inserted in selected slots of the stator core, having two coil sets  14   a ,  14   b  that are generally on opposite sides of the stator bore from one another. In this instance, coil sets  14   a ,  14   b  are connected electrically in series with each other by a jumper  62 . An outer coil of coil set  14   a  is inserted in slots  66 ,  68  with inner coils being inserted in selected slots between slot  66  and slot  68  and an outer coil of coil set  14   b  is inserted in slots  70 ,  72  with inner coils being inserted in selected slots between slot  70  and slot  72 . 
     As an example, first winding  12  consists of a single strand of #17.25 copper wire having 16-23-32-46-46 turns wound over 3, 5, 7, 9 and 11 teeth, respectively, and second winding  14  consists of a single strand of #18.00 copper wire having 15-25-25-31 turns wound over 5, 7, 9 and 11 teeth, respectively. Motor  10  constructed in this manner with a 3.500 inch stack height has clockwise lead end rotation and a synchronous speed of 3600 rpm and puts out approximately 2.75 horsepower when energized by single phase, 60 Hz, alternating current. It is to be understood that motor  10  may be constructed with a variety of internal coil connections. 
     In the embodiment of FIG. 3, motor  10  uses four leads L 1 -L 4  for electrical connections. As shown, leads L 1  and L 2  are located on opposite sides of first winding  12  and leads L 3  and L 4  are located on opposite sides of second winding  14 . It is to be understood that an additional lead is used if motor  10  includes a protector circuit P (see FIG.  1 ). If motor  10  includes the protector P, then L 1  is moved to the input of protector P and an additional lead L 5  (see FIG. 1) provides an electrical connection to motor  10  at switch  24 . 
     As described above, motor  10  is particularly applicable to, but not limited to, hermetically sealed compressor units. Leads L 1 -L 4  are adapted to pass through the compressor shell (or similar structure) in which a hermetic motor of this invention is housed. Those skilled in the art recognize that switching circuit  18  may be connected to leads L 1 -L 4  outside the compressor shell. 
     In motor design terms, the motor  10  of the present invention provides a variable “A-ratio” to provide optimum performance at both heavy and lightly loaded conditions. 
     Generally, the A-ratio is the ratio of turns in the main winding to the turns in the auxiliary winding. In addition, the present invention may be applied to a multiple speed motor (e.g., a two speed PSC blower motor) where a reduction of breakdown torque changes the speed at which the fan or blower rotates. The following tables provide exemplary efficiency and breakdown torque data under various operating conditions with respect to A-ratio. In Tables I and II, the first load point (Load PT#1) represents expected load conditions (e.g., CHEER) and the second load point (Load PT#2) represents maximum load conditions (e.g., ARI) conditions. The data exemplifies improved efficiency at lighter load conditions, otherwise unavailable, by operating in the modulated mode. At higher load conditions, when greater torque is required, operating in the full mode provides optimum efficiency. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 2 Pole Motor 
                   
                   
                   
                 Breakdown 
               
               
                 with 45 μF 
                 A- 
                 Load PT #1 
                 Load PT #2 
                 Torque 
               
               
                 Run Capacitor 
                 Ratio 
                 Efficiency (%) 
                 Efficiency (%) 
                 (oz-ft) 
               
               
                   
               
             
             
               
                 Full Mode Run 
                 1.239 
                 82.88 
                 87.73 
                 250.9 
               
               
                 Winding 
               
               
                 Modulated Mode 
                 0.807 
                 87.84 
                 87.76 
                 133.6 
               
               
                 Run Winding 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 2 Pole Motor 
                   
                   
                   
                 Breakdown 
               
               
                 with 50 μF 
                 A- 
                 Load PT #1 
                 Load PT #2 
                 Torque 
               
               
                 Run Capacitor 
                 Ratio 
                 Efficiency (%) 
                 Efficiency (%) 
                 (oz-ft) 
               
               
                   
               
             
             
               
                 Full Mode Run 
                 1.103 
                 83.18 
                 87.85 
                 253.2 
               
               
                 Winding 
               
               
                 Modulated Mode 
                 0.907 
                 86.15 
                 88.39 
                 171.5 
               
               
                 Run Winding 
               
               
                   
               
             
          
         
       
     
     The following Table III provides further exemplary efficiency and breakdown torque data under various operating conditions. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE III 
               
               
                   
               
               
                   
                 Unloaded 
                 Light Load 
                 Heavy Load 
                 Breakdown 
               
               
                 2 Pole Motor 
                 (36 oz-ft) 
                 (58 oz-ft) 
                 (80 oz-ft) 
                 Torque 
               
               
                 with 45 μF 
                 Efficiency 
                 Efficiency 
                 Efficiency 
                 (@ 3000 rpm) 
               
               
                 Run Capacitor 
                 (%) 
                 (%) 
                 (%) 
                 (oz-ft) 
               
               
                   
               
             
             
               
                 Full Mode Run 
                 78 (est.) 
                 86.8 
                 88.3 
                 244.0 
               
               
                 Winding 
               
               
                 Modulated 
                 88.9 
                 88.5 
                 84.4 
                 135.1 
               
               
                 Mode Run 
               
               
                 Winding 
               
               
                   
               
             
          
         
       
     
     Referring now to FIGS. 4 and 5, exemplary motor dynamometer test results illustrate the efficiency benefits of dual-mode, modulated motor operation. The graph of FIG. 4 reveals an exemplary crossover point of approximately 3.75 lb-ft of torque. At loads below 3.75 lb-ft, motor  10  optimally operates at modulated capacity while, at loads above 3.75 lb-ft, motor  10  optimally operates at full capacity. FIG. 5 illustrates that speed and power characteristics for the two modes are substantially similar. 
     As an example, motor control circuit  20  generates a signal representative of the amount of torque demanded by the motor load conditions relative to a torque threshold. For example, if the motor load conditions demand torque of 3.75 lb-ft or less, the motor control signal causes switching circuit  18  to energize the windings  12 ,  14  in the modulated capacity mode. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.