Patent Publication Number: US-11398794-B2

Title: Variable frequency drive thermal management

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Application No. 62/755,742, filed on Nov. 5, 2018, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Exemplary embodiments pertain to the art of variable frequency drives and more specifically to variable frequency drive thermal management for HVAC/Chiller systems. 
     Heating, ventilation, and air-conditioning (HVAC) and chiller systems, typically, utilize a variable frequency drive (VFD) to operate certain components of these systems. A VFD is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speeds and torque by varying motor input frequency and voltage. For HVAC and chiller systems, VFDs can utilize a standard space vector pulse width modulation (SVPWM) to control a motor input frequency and voltage. This standard SVPWM is utilized for acoustic and smooth operation considerations. However, VFDs can suffer from thermal trip when utilizing SVPWM and when exposed to extreme load conditions with high modulation indexes and high load conditions. 
     BRIEF DESCRIPTION 
     Disclosed is a system. The system includes a variable frequency drive, a sensor configured to collect operational data associated with the variable frequency drive, and a controller configured to receive, from the sensor, operational data, the operational data including one or more operational parameters for the variable frequency drive, compare the one or more operational parameters to a threshold, and based on the one or more operational parameters being below the threshold, operate the variable frequency drive to produce a first modulated output. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the system may include that the controller is further configured to, based on at least one of the one or more operational parameters exceeding the threshold, operate the variable frequency drive to produce a second modulated output. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the system may include that the first modulated output includes a space vector pulse width modulation (SVPWM) output. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the system may include that the second modulated output includes a discontinuous pulse width modulation (DPWM) output. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the system may include that the second modulated output is produced by the variable frequency drive for a first time period. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the system may include that the controller is further configured to operate the variable frequency drive to produce the first modulated output in response to an expiration of the first time period. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the system may include that the one or more operational parameters includes a temperature parameter (sensor) and a voltage command (modulation index) parameter. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the system may include that the controller is further configured to operate the variable frequency drive to produce the first modulated output in response to the at least one of the one or more operational parameters being below the threshold. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the system may include that the controller is further configured to monitor the operational data associated with the variable frequency drive to determine a rate of change to the one or more operational parameters, operate the variable frequency drive to produce the second modulated output for a second time period, wherein the second period of time is based at least in part on the rate of change to the one or more operational parameters, and operate the variable frequency drive to produce the first modulated output in response to an expiration of the second time period. 
     Disclosed is a method for thermal management. The method includes receiving, by a processor, operational data associated with a variable frequency drive, the operational data including one or more operational parameters for the variable frequency drive, comparing the one or more operational parameters to a threshold, and based on the one or more operational parameters being below the threshold, operating the variable frequency drive to produce a first modulated output. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the method may include based on at least one of the one or more operational parameters exceeding the threshold, operating the variable frequency drive to produce a second modulated output. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the method may include that the first modulated output includes a space vector pulse width modulation (SVPWM) output. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the method may include that the second modulated output includes a discontinuous pulse width modulation (DPWM) output. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the method may include that the second modulated output is produced by the variable frequency drive for a first time period. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the method may include operating the variable frequency drive to produce the first modulated output in response to an expiration of the first time period. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the method may include that the one or more operational parameters includes a temperature (heat sink sensor feedback) parameter and a voltage command to the motor as the output of the inverter or in other words the modulation index parameter of the PWM generation action. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the method may include that the second modulated output is produced by the variable frequency drive until the at least one or the one or more operational parameters is below the threshold. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the method may include operating the variable frequency drive to produce the first modulated output in response to the at least one of the one or more operational parameters being below the threshold. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the method may include that monitoring the operational data associated with the variable frequency drive to determine a rate of change to the one or more operational parameters, operating the variable frequency drive to produce the second modulated output for a second time period, wherein the second period of time is based at least in part on the rate of change to the one or more operational parameters, and operating the variable frequency drive to produce the first modulated output in response to an expiration of the second time period. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
         FIG. 1  depicts a block diagram of a refrigerant vapor compression system for use in implementing one or more embodiments; and 
         FIG. 2  depicts a flow diagram of a method for thermal management in a variable frequency drive according to one or more embodiments; 
         FIG. 3  depicts a flow diagram of a method for thermal management in a variable frequency drive according to one or more embodiments; and 
         FIG. 4  depicts a graphical representation of a DPWM and SPWM wave form according to one or more embodiments. 
     
    
    
     The diagrams depicted herein are illustrative. There can be many variations to the diagram or the operations described therein without departing from the spirit of the disclosure. For instance, the actions can be performed in a differing order or actions can be added, deleted or modified. Also, the term “coupled” and variations thereof describes having a communications path between two elements and does not imply a direct connection between the elements with no intervening elements/connections between them. All of these variations are considered a part of the specification. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is shown a refrigerant vapor compression system  50  having a variable speed compressor  52  driven by a variable speed motor  68  according to one or more embodiments. The system  50  includes refrigerant vapor from compressor  52  that is delivered to a condenser  54  where the refrigerant vapor is liquefied at high pressure, thereby rejecting heat to the outside air. The liquid refrigerant exiting condenser  54  is delivered to an evaporator  58  through an expansion valve  56 . In embodiments, the expansion valve  56  may be a thermostatic expansion valve or an electronic expansion valve for controlling super heat of the refrigerant. The refrigerant passes through the expansion valve  56  where a pressure drop causes the high-pressure liquid refrigerant to achieve a lower pressure combination of liquid and vapor. As the indoor air passes across evaporator  58 , the low-pressure liquid refrigerant absorbs heat from the indoor air, thereby cooling the air and evaporating the refrigerant. The low-pressure refrigerant is again delivered to compressor  52  where it is compressed to a high-pressure, high temperature gas, and delivered to condenser  54  to start the refrigeration cycle again. It is to be appreciated that while a specific refrigeration system is shown, the present teachings are applicable to any heating or cooling system, including a heat pump, HVAC, and chiller systems. In a heat pump, during cooling mode, the process is identical to that as described hereinabove, while in the heating mode, the cycle is reversed with the condenser and evaporator of the cooling mode acting as an evaporator and condenser, respectively. 
     Turning now to an overview of technologies that are more specifically relevant to aspects of the disclosure, the system  50  includes a compressor  52  driven by an inverter drive  62 . In embodiments, the inverter drive  62  may be a variable frequency drive (VFD) or a brushless DC motor (BLDC) drive. Particularly, inverter drive  62  is operably coupled to compressor  52 , and receives an alternating current (AC) electrical power (for example, electrical power is a single-phase AC line power at 230V/60 Hz) from a power supply  60  and outputs electrical power on line  66  to a variable speed motor  68 . The variable speed motor  68  provides mechanical power to drive a crankshaft of the compressor  62 . In an embodiment, the variable speed motor  68  may be integrated inside the exterior shell of the compressor  62 . Inverter drive  62  includes solid-state electronics to modulate the frequency of electrical power on line  66 . In an embodiment, inverter drive  62  converts the AC electrical power, received from supply  60 , from AC to direct current (DC) using a rectifier, and then converts the electrical power from DC back to a pulse width modulated (PWM) signal, using an inverter, at a desired PWM frequency in order to drive the motor  68  at a motor speed associated with the PWM DC frequency. For example, inverter drive  62  may directly rectify electrical power with a full-wave rectifier bridge, and may then chop the electrical power using insulated gate bipolar transistors (IGBT&#39;s) or thyristors to achieve the desired PWM frequency. In embodiments, other suitable electronic components may be used to modulate the frequency of electrical power from power supply  60 . Further, control unit  64  includes a processor for executing an algorithm used control the PWM frequency that is delivered on line  66  to the motor  68 . By modulating the PWM frequency of the electrical power delivered on line  66  to the electric motor  68 , control unit  64  thereby controls the torque applied by motor  68  on compressor  52  there by controlling its speed, and consequently the capacity, of compressor  52 . Also shown, the control unit (controller)  64  includes a computer readable medium for storing data in a memory unit related to estimating compressor discharge pressure from compressor and refrigeration system parameters. In embodiments, the control unit  64  stores information related to compressor torque as well as line voltages, compressor motor current, and compressor speed obtained from inverter drive  62 . 
     In one or more embodiments, the variable speed drive (inverter)  62  can utilize a standard space vector PWM (SVPWM) for acoustic and smooth operation reasons when driving the variable speed motor  68  in the system  50 . However, as mentioned above, operating with a SVPWM under extreme load conditions can cause thermal trip within the VFD  62 . In one or more embodiments, a heat sink and a sensor can be utilized to address thermal trip by the control unit  64  monitoring the heat sink temperature using the sensor. When the temperature approaches to thermal trip level (e.g., threshold temperature, for example, 90 degrees Celsius), the VFD can switch to a discontinuous PWM (DPWM) for a set or variable time period or until the heat sink temperature returns to below the threshold temperature. After the expiration of this time period in DPWM mode of operation, the VFD can resume back to the SVPWM mode if the margin from the thermal trip is large. In one or more embodiments, the reason for remaining at the DPWM mode for a set period of time is to avoid chattering of DPWM and standard SVPWM causing irregular and noticeable acoustic noise. 
     In one or more embodiments, a second condition, besides thermal trip, can be utilized when switching to DPWM mode in the VFD. This second condition includes a voltage modulation index. For example, when the modulation index is above 85%-90%, is added to the decision for switching to DPWM. The reason for this second condition in the decision is to avoid excessive PWM ripple in the variable speed motor  68  which can cause increased noise level from the variable speed motor  68  and also increases motor losses. 
     In one or more embodiments, when switching to the DPWM mode, the control unit  64  can keep the VFD  62  in this DPWM mode until the operational conditions (e.g., temperature and voltage command or modulation index) are below the threshold temperature and threshold voltage modulation. Once the operational conditions return to below the threshold levels, the control unit  64  can operation the VFD  62  to produce the SVPWM output. In one or more embodiments, the VFD  62  is operated in the DPWM mode for a time period. Also, the control unit  64  can monitor the operational conditions, through the sensor, to determine a rate of change to the operational conditions. For example, if the temperature is slowly falling, the control unit  64  can determine the rate of change and set a time period for when to switch back to SVPWM mode. 
       FIG. 2  depicts a flow diagram of a method for thermal management in a variable frequency drive according to one or more embodiments. The method  200  includes receiving, by a processor, operational data associated with a variable frequency drive, the operational data comprising one or more operational parameters for the variable frequency drive, as shown in block  202 . At block  204 , the method  200  includes comparing the one or more operational parameters to a threshold. And based on the one or more operational parameters being below the threshold, the method  200  includes operating the variable frequency drive to produce a first modulated output, as shown at block  206 . And at block  208 , the method  200  includes, based on at least one of the one or more operational parameters exceeding the threshold, operating the variable frequency drive to produce a second modulated output. 
     Additional processes may also be included. It should be understood that the processes depicted in  FIG. 2  represent illustrations and that other processes may be added or existing processes may be removed, modified, or rearranged without departing from the scope and spirit of the present disclosure. 
       FIG. 3  depicts a flow diagram of a method for thermal management in a variable frequency drive according to one or more embodiments. In one or more embodiments, the method  300  begins at decision block  302  that is monitoring operational conditions to determine that the operational conditions exceed a threshold operating condition. The operating conditions being monitored include, but are not limited to, the inverter heat sink temperature  312  and the inverter phase voltage  314 . For example, if the heat sink temperature exceeds 90 degrees C. (threshold temperature) then the decision block moves to process step  304 . At process step  304 , the method  300  switches the operational frequency of the inverter to the DPWM mode. While in DPWM mode, a timer can be set and the operational conditions can be continued to be monitored. When the timer expires, at decision block  306 , the operational conditions are analyzed. In one or more embodiments, the timer can be set to a longer time period than needed for the VFD heat sink cool down time. Having a set time longer than needed for the heat sink to cool down avoids a cyclic nature of entering and exiting the DPWM mode which can cause customer discomfort with the HVAC system due to noise. In one or more embodiments, the timer can have varying minimum time periods when entering DPWM mode to avoid a cyclic change. At decision block  308 , when both the timer has expired and the operational conditions are below the threshold, the method  300  proceeds to process block  310  and switches back to SVPWM for the inverter. If the timer is not expired or the operational conditions are above the threshold, the inverter remains in DPWM mode. 
       FIG. 4  depicts a graphical representation of a DPWM and SPWM wave form according to one or more embodiments. The graphical representation  400  includes a wave form for the SVPWM mode  402  and the DPWM mode  404 . 
     A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
     The term “about” is intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof. 
     While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.