Abstract:
Apparatuses, methods, and systems offering enhanced robustness, efficiency and rated voltage operability for refrigerant compressor drives are disclosed. An exemplary embodiment is a method of operating a variable frequency drive. The method includes operating the drive over a first operating range to provide at least a desired operating speed and minimize d-axis current, operating the drive over a second operating range including injecting d-axis current to provide at least the desired operating speed, operating the drive over a third operating range at a de-rated speed less than the desired operating speed. In the first operating range the drive input voltage is greater than a first value. In the second operating range the drive input voltage is lower than the first value and greater than a second value. In the third operating range the drive input voltage is lower than the second value.

Description:
BACKGROUND 
       [0001]    Variable frequency motor drives offer a number of potential benefits for applications such as driving compressors or other loads for heating, ventilation, air-conditioning, or refrigeration (HVACR) systems, including potential for enhanced efficiency, power density, and speed control precision. Such motor drives also present unique challenges with respect to robustness, service life, and tolerance to rated voltages including low input line voltages within a rated range. Heretofore, HVACR drive and motor designs have faced a number of undesirable trade-offs. For example, in selecting DC bus capacitor components there are competing needs for service life and robustness, and sufficient capacitance to meet voltage ripple and harmonic feedback mitigation goals. Traditionally, electrolytic capacitors have been utilized in the DC bus to provide the desired level of capacitance, however, their limited service life relative to the lifespan of HVACR systems has long been a source of frustration for designers and consumers. Some recent designs have utilized film capacitors which offer significantly enhanced lifespan compared to electrolytic capacitors; however, this benefit comes at a cost of lower capacitance relative to electrolytic capacitors. 
         [0002]    The aforementioned trade-offs are compounded by the need to account for rated input line voltage phenomena. Utility power lines and other power sources have a rated voltage range which is sometimes expressed as a nominal rated voltage with the range being implicit. Power electronics and motor drive systems coupled to such power sources must be configured to meet desired performance criteria over the full rated input voltage range, including the low voltage portion of the rated range, as is expected that such voltages will be encountered in normal real world operation. This presents a unique challenge to HVACR compressor drives which must be designed to maintain a desired speed to attain desired performance and efficiency. Lower capacitance drives produce lower output voltages and are more susceptible to performance variation from input voltage variation. This in turn forces system designs toward motors with lower electrical constants which require greater current to achieve functional requirements. This increases the expense both of the system itself and of operating the system. Increased current also increases losses through resistive heating which further compromises operational efficiency. Conventional attempts to address these and other challenges suffer from a number of shortcomings. There is a need for the unique and inventive apparatuses, methods and systems disclosed herein. 
       DISCLOSURE 
       [0003]    For the purposes clearly, concisely and exactly describing exemplary embodiments of the invention, the manner and process of making and using the same, and to enable the practice, making and use of the same, reference will now be made to certain exemplary embodiments, including those illustrated in the figures, and specific language will be used to describe the same. It shall be understood that no limitation of the scope of the invention is thereby created, and that the invention includes and protects such alterations, modifications, and further applications of the exemplary embodiments as would occur to one skilled in the art to which the invention relates. 
       SUMMARY 
       [0004]    Unique refrigerant compressor drive apparatuses, methods, and systems offering enhanced robustness, efficiency and rated voltage operability are disclosed. One exemplary embodiment is a method of operating a variable frequency drive. The method includes operating the drive over a first operating range to provide at least a desired operating speed and minimize d-axis current, and operating the drive over a second operating range including increasing d-axis current to provide at least the desired operating speed. In the first operating range the drive input voltage is greater than a first value. In the second operating range the drive input voltage is lower than the first value and greater than a second value. Some embodiments further include operating the drive over a third operating range at a de-rated speed less than the desired operating speed. In the third operating range the drive input voltage is lower than the second value. Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0005]      FIG. 1  is a schematic illustration of an exemplary HVACR system. 
           [0006]      FIG. 2  is a schematic illustration of an exemplary variable frequency drive and permanent magnet motor. 
           [0007]      FIG. 3  is a diagram illustrating characteristics of exemplary low input voltage compatible designs. 
           [0008]      FIG. 4  is a graph illustrating motor design characteristics. 
           [0009]      FIG. 5  is a flow diagram illustrating an exemplary controls process. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    With reference to  FIG. 1  there is illustrated an exemplary HVACR system  100  which includes a refrigerant loop comprising a compressor  110 , a condenser  120 , and an evaporator  130 . Refrigerant flows through system  100  in a closed loop from compressor  110  to condenser  120  to evaporator  130  and back to compressor  110 . Various embodiments may also include additional refrigerant loop elements including, for example, valves for controlling refrigerant flow, refrigerant filters, economizers, oil separators and/or cooling components and flow paths for various system components. 
         [0011]    Compressor  110  is driven by a drive unit  150  including a permanent magnet electric motor  170  which is driven by a variable frequency drive  155 . In the illustrated embodiment, variable frequency drive  155  is configured to output a three-phase PWM drive signal, and motor  170  is a surface magnet permanent magnet motor. Use of other types and configurations of variable frequency drives and electric motors such as interior magnet permanent magnet motors, reluctance motors, or inductance motors are also contemplated. It shall be appreciated that the principles and techniques disclosed herein may be applied to a broad variety of drive and permanent magnet motor configurations. 
         [0012]    Condenser  120  is configured to transfer heat from compressed refrigerant received from compressor  110 . In the illustrated embodiment condenser  120  is a water cooled condenser which receives cooling water at an inlet  121 , transfers heat from the refrigerant to the cooling water, and outputs cooling water at an output  122 . It is also contemplated that other types of condensers may be utilized, for example, air cooled condensers or evaporative condensers. It shall further be appreciated that references herein to water include water solutions comprising additional constituents unless otherwise limited. 
         [0013]    Evaporator  130  is configured to receive refrigerant from condenser  120 , expand the received refrigerant to decrease its temperature and transfer heat from a cooled medium to the refrigerant. In the illustrated embodiment evaporator  130  is configured as a water chiller which receives water provided to an inlet  131 , transfers heat from the water to the refrigerant, and outputs chilled water at an outlet  132 . It is contemplated that a number of particular types of evaporators may be utilized, including dry expansion evaporators, flooded type evaporators, bare tube evaporators, plate surface evaporators, and finned evaporators among others. 
         [0014]    HVACR system  100  further includes a controller  160  which outputs control signals to variable frequency drive  155  to control operation of the motor  170  and compressor  110 . Controller  160  also receives information about the operation of drive unit  150 . In exemplary embodiments controller  160  receives information relating to motor current, motor terminal voltage, and/or other operational characteristics of the motor. It shall be appreciated that the controls, control routines, and control modules described herein may be implemented using hardware, software, firmware and various combinations thereof and may utilize executable instructions stored in a non-transitory computer readable medium or multiple non-transitory computer readable media. It shall further be understood that controller  160  may be provided in various forms and may include a number of hardware and software modules and components such as those disclosed herein. 
         [0015]    With reference to  FIG. 2  there is illustrated an exemplary circuit diagram for a variable frequency motor drive  200 . Drive  200  is connected to a power source  210 , for example, a 400/480 VAC utility power supply which provides three-phase AC power to line filter module  220 . Line filter module  220  is configured to provide harmonic damping to mitigate losses which can arise from harmonic feedback from drive components to power source  210 . Line filter module  220  outputs three-phase AC power to a rectifier  290  which converts the AC power to DC power and provides the DC power to a DC bus  291 . DC bus  291  is preferably a film capacitor-cased bus which includes one or more film capacitors electrically coupled between positive and negative bus rails. DC bus  291  is connected to inverter  280 . For clarity of illustration and description, rectifier  290 , DC bus  291 , and inverter  280  are shown as discrete elements. It shall be appreciated, however, that two or more of these components may be provided in a common module, board or board assembly which may also include a variety of additional circuitry and components. It shall be further understood that, in addition to the illustrated 6-pulse rectifier, other multiple pulse rectifiers such as 12-pulse, 18-pulse, 24-pulse or 30-pulse rectifiers may be utilized along with phase shifting transformers providing appropriate phase inputs for 6-pulse 12-pulse, 18-pulse, 24-pulse, or 30-pulse operation. 
         [0016]    Inverter module  280  includes switches  285 ,  286  and  287  which are connected to the positive and negative rails of DC bus  291 . Switches  285 ,  286  and  287  are preferably configured as IGBT and diode based switches, but may also utilize other types of power electronics switching components such as power MOSFETs or other electrical switching devices. Switches  285 ,  286  and  287  provide output to motor terminals  275 ,  276  and  277 . Current sensors  281 ,  282  and  283  are configured to detect current flowing from inverter module  280  to motor  270  and send current information to ID module  293 . Voltage sensors are also operatively coupled with motor terminals  275 ,  276  and  277  and configured to provide voltage information from the motor terminals to ID module  293 . 
         [0017]    ID module  293  includes burden resistors used in connection with current sensing to set the scaling on current signals ultimately provided to analog to digital converters for further processing. ID module  293  tells the VFD what size it is (i.e. what type of scaling to use on current post ADC) using identification bits which are set in hardware on the ID module  293 . ID module  293  also outputs current and voltage information to gate drive module  250  and also provides identification information to gate drive module  250  which identifies the type and size of the load to which gate drive module  250  is connected. ID module  293  may also provide current sensing power supply status information to gate drive module  250 . ID module  293  may also provide scaling functionality for other parameters such as voltage or flux signals in other embodiments. 
         [0018]    Gate drive module  250  provides sensed current and voltage information to analog to digital converter inputs of DSP module  260 . DSP module  260  processes the sensed current and voltage information and also provides control signals to gate drive module  250  which control signals gate drive module  250  to output voltages to boost modules  251 ,  252  and  253 , which in turn output boosted voltages to switches  285 ,  286  and  287 . The signals provided to switches  285 ,  286  and  287  in turn control the output provided to terminals  275 ,  276  and  277  of motor  270 . 
         [0019]    Motor  270  includes a stator  271 , a rotor  273 , and an air gap  272  between the rotor and the stator. Motor terminals  275 ,  276  and  277  are connected to windings provided in stator  271 . Rotor  273  includes a plurality of permanent magnets  274 . In the illustrated embodiment magnets  274  are configured as surface permanent magnets positioned about the circumference of rotor  273 . The rotor is typically constructed using the permanent magnets in such a way as essentially a constant magnetic flux is present at the surface of the rotor. In operation with rotation of the rotor, the electrical conductors forming the windings in the stator are disposed to produce a sinusoidal flux linkage. Other embodiments also contemplate the use of other magnet configurations such as interior magnet configurations as well as inductance motor configurations, reluctance motor configurations and other non-permanent magnet configurations. 
         [0020]    With reference to  FIG. 3  there is illustrated a graph  300  of maximum motor or compressor speed as a function of variable frequency drive line input voltage. Line  310  illustrates a substantially linear relationship between line input voltage and maximum speed for a motor design with a low motor back EMF constant (Kemf). Line  320  illustrates a substantially linear relationship between maximum speed and line input voltage for a motor design with a high Kemf. It shall be appreciated that the terms high and low in this context are relative and that the particular motor Kemf values will vary depending upon the needs and characteristics of a particular implementation. 
         [0021]    Graph  300  further illustrates a required speed  302  which is a system design parameter corresponding to the motor or compressor speed that an HVACR system requires to meet defined or desired performance and/or efficiency goals. For a motor conforming to the characteristics of line  310 , required speed  302  can be achieved at a defined minimum line input voltage  308  which is within the rated voltage range of the input line while simultaneously minimizing d-axis current as is indicated by the vertical axis intercept of line  310 . It shall be appreciated that minimization of d-axis current need not be absolute and may include d-axis current values which are substantially minimized while still including a some d-axis current, as well as d-axis current values that approach, approximate, or target an ideal or theoretical minimization value. For a motor conforming to the characteristics of line  320 , speed  302  cannot be achieved below threshold voltage  304  while simultaneously minimizing d-axis current. Thus, d-axis current injection may be utilized in region  330  to de-flux the motor or reduce motor back EMF to achieve the required speed  302  as is indicated by line  325 . The degree of d-axis current injection along line  325  is generally a function of the magnitude of the required speed and the distance from line  325  to line  320  over the range from voltage  308  to voltage  304 . 
         [0022]    Graph  300  further illustrates line input voltage  306  which is the nominal input line voltage at which a motor conforming to the characteristics of line  320  will operate at full torque under nominal input line voltage conditions. Such a motor will continue to operate at full torque at reduced input line voltages as low as voltage  304 . Below voltage  304  and down to voltage  308 , such a motor will operate at reduced torque while maintaining meeting or exceeding required speed  302 . Below voltage  308  such a motor will continue to operate but at a speed lower than required speed  302 . It shall be appreciated that the motor may be configured such that voltage  306  corresponds to the nominal rating of an input line source, the range between voltages  304  and  306  includes the most commonly experienced voltage variations from the nominal rating, for example, a +/− range expressly or implicitly present in the nominal rating, and that the range between voltages  304  and  308  includes lower commonly encountered voltages, for example, voltages falling outside a +/− range but which are still encountered in real world operation. 
         [0023]    With reference to  FIG. 4  there is illustrated a diagram  400  of two exemplary design optimizations for a variable frequency drive and motor system. In the 460 V design shown in the upper portion of  FIG. 4 , unit rating voltage  402  is the rated range of the voltage provided by the input line which powers the drive and extends from approximately 440 V to 520 V or 480 V +/−10%. Unit utilization voltage  404  is the rated range of voltages that the design is configured to use as an input. In the illustrated embodiment unit utilization voltage  404  also extends from approximately 440 V to 520 V or 480 V +/−10%, however, it shall be appreciated the range could vary in other embodiments. VFD output voltage  406  is the output voltage range that can be provided by the variable frequency drive. VFD output voltage  406  ranges from approximately 447 V to 378 V due to variable frequency drive and line impedance  407  which cause a voltage drop (in this embodiment approximately 14%, though the value may vary in other embodiments) relative to the unit utilization voltage  404 . The 460 V design is optimized to a 460 V line input voltage and 396 V drive output voltage as indicated by optimized output voltage  408 . Accordingly efficiency is maximized and d-axis current is minimized at these voltages. Voltage  410  is the minimum voltage at which the drive and motor system must meet speed requirements while permitting reduced performance attributable to d-axis current injection. 
         [0024]    In the 360 V design shown in the lower portion of  FIG. 4 , unit rating voltage  412  is the rated range of the voltage provided by the input line which powers the drive and extends from approximately 360 V to 440 V or 400 V +/31 10%. Unit utilization voltage  414  is the rated range of voltages that the design is configured to use as an input. In the illustrated embodiment unit utilization voltage  414  also extends from approximately 360 V to 440 V or 400 V +/−10%, however, it shall be appreciated the range could vary in other embodiments. VFD output voltage  416  is the output voltage range that can be provided by the variable frequency drive. VFD output voltage  406  ranges from approximately 378 V to 310 V due to variable frequency drive and line impedance  417  which cause a voltage drop (in this embodiment approximately 14% though the value may vary in other embodiments) relative to the unit utilization voltage  404 . The 380 V design is optimized to a 380 V line input voltage and 327 V drive output voltage as indicated by optimized output voltage  418 . Accordingly efficiency is maximized and d-axis current is minimized at these voltages. Voltage  420  is the minimum voltage at which the drive and motor system must meet speed requirements while permitting reduced performance attributable to d-axis current injection. 
         [0025]    With reference to  FIG. 5  there is illustrated an exemplary control process  500 . Process  500  begins at operation  510  which determines a magnitude of voltage Vin. From operation  510  process  500  proceeds to conditional  520 . Conditional  520  determines whether the magnitude of voltage Vin is less than magnitude of voltage Vth 1  which is a threshold corresponding to an operating point below which d-axis current injection is performed, such as voltage  304 , voltage  408 , or voltage  418 , for example. If conditional  520  determines that Vin is not less than Vth 1 , process  500  proceeds to operation  525  which operates the drive at or above speed Smin which corresponds to system performance and/or efficiency requirements, such as speed  302 , for example. From operation  525  process  500  returns to operation  510 . If conditional  520  determines that Vin is less than Vth 1 , process  500  proceeds to operation  530  which determines injection current Iinj. From operation  530  process  500  proceeds to conditional  540  which determines whether the magnitude of voltage Vin is less than the magnitude of voltage Vth 2  which is a threshold corresponding to an operating point below which a desired speed can no longer be maintained such as voltage  308 , voltage  410  or voltage  420 , for example. If Vin is not less than Vth 2 , process  500  proceeds to operation  545  which provides injection current Iinj and operates the drive at or above speed Smin. From operation  545  process  500  returns to operation  510 . If conditional  540  determines that the N is less than Vth 2 , process  500  proceeds to operation  550  which provides injection current Iinj and operates the drive below speed Smin. From operation  550  process  500  returns to operation  510 . 
         [0026]    It shall be understood that the exemplary embodiments summarized and described in detail above and illustrated in the figures are illustrative and not limiting or restrictive. Only the presently preferred embodiments have been shown and described and all changes and modifications that come within the scope of the invention are to be protected. It shall be appreciated that the embodiments and forms described below may be combined in certain instances and may be exclusive of one another in other instances. Likewise, it shall be appreciated that the embodiments and forms described below may or may not be combined with other aspects and features disclosed elsewhere herein. It should be understood that various features and aspects of the embodiments described above may not be necessary and embodiments lacking the same are also protected. In reading the claims, it is intended that when words such as “a,” “an,” “at least one,” or “at least one portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language “at least a portion” and/or “a portion” is used the item can include a portion and/or the entire item unless specifically stated to the contrary.