Abstract:
A system and method are provided for a variable speed compressor with a control module that controls an operating frequency of the variable speed compressor. The control module stores a prohibited frequency range that includes a resonant frequency of the variable speed compressor and an allowed upper frequency above the prohibited frequency range and an allowed lower frequency below the prohibited frequency range. The control module determines a requested frequency and operates the variable speed compressor at the allowed upper frequency for an upper frequency operating time and at the allowed lower frequency for a lower frequency operating time when the requested frequency is within the prohibited frequency range. The control module determines the upper frequency operating time and the lower frequency operating time such that a time-averaged frequency output over the upper frequency operating time and the lower frequency operating time corresponds to the requested frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/983,615, filed on Jan. 3, 2011, now U.S. Pat. No. 8,849,613, which is a continuation of U.S. patent application Ser. No. 12/244,528, filed on Oct. 2, 2008, now U.S. Pat. No. 7,895,003. This application claims the benefit of U.S. Provisional Application No. 60/977,859, filed on Oct. 5, 2007. The entire disclosures of each of the above applications are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to compressors, and more particularly, to vibration protection of a compressor system with a variable speed compressor. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Compressors are used in a wide variety of industrial and residential applications to circulate refrigerant within a refrigeration, heat pump, HVAC, or chiller system (generally referred to as “refrigeration systems”) to provide a desired heating and/or cooling effect. In any of the foregoing applications, the compressor should provide consistent and efficient operation to ensure that the particular refrigeration system functions properly. 
     A refrigeration system may include a series of components such as a compressor, condenser, evaporator, valves, piping and electrical components. A compressor system of the refrigeration system may include the compressor and related components that may be packaged as a unit. The compressor may be driven by a motor and the compressor system may experience vibrations. The compressor and compressor system may have one or more resonant (or natural) frequencies which may be excited at corresponding motor speeds (i.e., frequency) and result in relatively high amplitude vibrations of the compressor and compressor system. 
     For a fixed-speed compressor, a suspension system such as grommets or other such devices may be added to the compressor system such that the operating speed of the compressor does not correspond to a resonant frequency of the system. In other words, the compressor system may be designed such that its resonant frequency is at an acceptable value in relation to the compressor&#39;s operating frequency. A variable-speed compressor may operate at frequencies above, below and including the operating frequency of a typical fixed-speed compressor. Thus, a suspension system as was described with respect to the fixed-speed compressor may not be suitable for a variable-speed compressor as it would normally operate at some resonant frequency without any other preventative solutions. 
     SUMMARY 
     A method of vibration protection in a compressor system with a variable speed compressor including operating a variable speed compressor at a plurality of frequencies, measuring a plurality of vibration values associated with the plurality of frequencies, determining a frequency characteristic of the compressor system based on the plurality of vibration values, and identifying prohibited frequencies of the compressor based on the frequency characteristic. 
     The frequency characteristic may include a resonant frequency. 
     The prohibited compressor frequencies may include a range of the resonant frequency plus or minus a critical frequency difference. 
     The critical frequency difference may be at least 1 Hz. 
     The frequency characteristic may include a frequency range wherein the vibration values exceed a maximum acceleration amplitude. 
     The maximum acceleration amplitude may (A)=4π 2 ×(frequency) 2 ×(maximum allowable displacement). 
     The maximum allowable displacement amplitude may be at least 25×10 −6  meters. 
     The prohibited compressor frequencies may include the frequency range wherein the vibration values exceed the maximum acceleration amplitude. 
     The operating step may include operating a variable speed compressor at a minimum sweep frequency, increasing the frequency of the variable speed compressor by a frequency increment, and continuing the increasing until the frequency of the variable speed compressor is at least a maximum sweep frequency. 
     The measuring step may include measuring a vibration value associated with each frequency increment. 
     The vibration value may be at least one of an acceleration of the system, a velocity of the system and an amplitude of the vibration. 
     The identifying step may include storing a prohibited frequency value for each frequency wherein the vibration value exceeds a maximum allowable vibration. 
     The measuring step may include receiving a plurality of vibration values from an accelerometer and storing the vibration values in memory. 
     The method may further include operating the variable speed compressor at a first frequency (F 1 ) outside of the prohibited frequencies for a first time (T 1 ) and operating the variable speed compressor at a second frequency (F 2 ) outside of the prohibited frequencies for a second time (T 2 ), wherein the time-averaged frequency is equal to a requested frequency (T R ) within the prohibited frequencies. 
     The first frequency may be a closest allowable upper frequency, the second frequency is a closest allowable lower frequency, and the first time T 1  is equal to a predetermined total time×(F R −F 2 )/(F 1 −F 2 ) and the second time T 2 =predetermined total time−T 1 . 
     The method may further include requesting operation at a first frequency, determining a first allowed frequency furthest from the first frequency, operating at said first allowed frequency for a predetermined time, determining a second allowed frequency in a direction opposite to a direction of the first allowed frequency, and operating at said second allowed frequency for a period of time substantially equal to said predetermined time. 
     The method may further include repeating the operating, measuring, determining and identifying when the compressor restarts. 
     The method may further include repeating the operating, measuring, determining and identifying steps at a predetermined interval. 
     The predetermined interval may be once a week. 
     The method may further include repeating the operating, measuring, determining and identifying steps when a heat pump system changes an operating mode between heating and cooling. 
     The method may further include repeating the operating, measuring, determining and identifying steps when a measured vibration value exceeds a predetermined sweep threshold. 
     The predetermined sweep threshold may be 110% of a maximum acceleration amplitude A=4π 2 ×(frequency) 2 ×(maximum allowable displacement). 
     The method may further include repeating the operating, measuring, determining and identifying when the ambient temperature change over a predetermined time exceeds a predetermined temperature threshold. 
     The predetermined time may be at least 24 hours and the predetermined temperature threshold is at least 40 degrees Fahrenheit. 
     A method of vibration protection in a compressor system having a variable speed compressor includes operating a variable speed compressor at a first frequency, measuring a vibration of the compressor system at the first frequency, determining whether the vibration exceeds a maximum vibration value, and operating the variable speed compressor at an average frequency equivalent to the first frequency when the vibration exceeds the maximum vibration value. Operating the variable speed compressor at an average frequency vibration value may include identifying an allowed upper frequency and an allowed lower frequency, calculating an upper operating time and a lower operating time, and operating the variable speed compressor at the allowed upper frequency for the upper operating time and the allowed lower frequency for the lower operating time. 
     The maximum vibration value may be defined by A=4π 2 ×(frequency) 2 ×(maximum allowable displacement). 
     The maximum allowable displacement may be at least 25×10 −6  meters. 
     The allowed upper frequency may be a closest frequency above the first frequency wherein a measured acceleration is less than a maximum acceleration value for the allowed upper frequency. 
     The allowed lower frequency may be a closest frequency below the first frequency wherein a measured acceleration is less than a maximum acceleration value for the allowed lower frequency. 
     The step of calculating the upper operating time and the lower operating time may include calculating an upper ratio of the difference between the first frequency and the allowed lower frequency divided by the difference between the allowed upper frequency and the allowed lower frequency, calculating the upper operating time by multiplying a predetermined operating time by the upper ratio, and calculating the lower operating time by subtracting the upper operating time from the predetermined operating time. 
     The predetermined operating time may be at least four minutes. 
     A variable speed compressor and drive system may include a compressor including a motor having a variable frequency based on a motor input, a drive in communication with the motor providing the motor input based on a drive input, a vibration measurement device operably coupled to the compressor to receive vibration from a compressor system and output vibration values based on the received vibration, and a control module in communication with the vibration measurement device and the drive, wherein the control module receives and stores the vibration values, determines frequency characteristics of the compressor based on the vibration values, and provides the drive input based on the frequency characteristics. 
     The vibration measurement device may be mounted to the shell of the compressor. 
     The vibration measurement device may be mounted to the drive. 
     The system may further include a terminal box attached to the compressor. 
     The vibration measurement device may be mounted to the terminal box. 
     The frequency characteristics may include a resonant frequency. 
     The control module may include an input to the drive that prohibits the drive from operating the motor at frequencies including the resonant frequency plus or minus a critical frequency difference. 
     The critical frequency difference may be at least 1 Hz. 
     The frequency characteristics may include a frequency range wherein the vibration values exceed a predetermined threshold. 
     The control module may include an input to the drive that prohibits the drive from operating the motor at frequencies including the frequency range wherein the vibration values exceed the predetermined threshold. 
     The control module may provide a signal to the drive to operate the motor at a minimum frequency, provide a signal to the drive to increase the motor frequency by a frequency interval, and receive and store vibration values from the accelerometer for each frequency interval, continue the increasing until the frequency of the variable speed compressor is at least a maximum compressor frequency, and calculate a prohibited frequency range based on the vibration values. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a schematic illustration of a heat pump system; 
         FIG. 2  is a schematic view of a control system for vibration protection; 
         FIG. 3  is a flow diagram of steps of a control system for vibration protection; 
         FIG. 4  is a flow diagram of steps of a control system for vibration protection; and 
         FIG. 5  is a flow diagram of steps of a control system for vibration protection. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, or other suitable components that provide the described functionality. 
     As seen in  FIG. 1 , a heat pump system  10  may include an indoor unit  12  and a compressor system  14 . A heat pump system is used for illustration purposes only, and it should be understood that the present teachings apply to any application in which a compressor may be utilized. For example, a compressor may alternatively be used in an air conditioning system, a refrigeration system, or generally in any system in which a refrigerant is compressed to provide a desired heating or cooling effect. Although compressor system  14  has been depicted as including the components described below, compressor system  14  may be any group of components that is packaged as a unit with compressor  32 . 
     Indoor unit  12  may include an indoor coil or heat exchanger  16  and a variable speed indoor fan  18  driven by a motor  20 . Indoor coil  16  and fan  18  may be enclosed in a cabinet  22  so that fan  18  forces ambient air across indoor coil  16 . Compressor system  14  may include an outdoor coil or heat exchanger  24  and a variable speed outdoor fan  26  driven by a motor  28 . Outdoor coil  24  and fan  26  may be enclosed in a protective housing  30  so that fan  26  will draw ambient outdoor air across outdoor coil  24  to improve heat transfer. 
     Compressor system  14  may further include a compressor  32  in communication with indoor coil  16  and outdoor coil  24 . Compressor  32  may include inverter drive  36  and terminal box  38 . Inverter drive  36  may be fixedly attached to a shell of compressor  32  and may provide a variable input power to a motor of compressor  32 , allowing compressor  32  to operate at a variable speed (i.e., frequency). Terminal box  38  may be fixedly attached to a shell of compressor  32  and may provide an input point for electrical, communication and other inputs to compressor  32 . 
     Accelerometer  40  and control module  42  are depicted as mounted to inverter drive  36 . Accelerometer  40  may measure acceleration and may alternatively be mounted to a shell of compressor  32 , terminal box  38 , or other locations within heat pump system  10 . Control module  42  may be integral to inverter drive  32 . Control module  42  may receive a signal from accelerometer  40  and control the output of inverter drive  36 . 
     Communication between compressor  32 , indoor coil  16 , and outdoor coil  24  may generally form a loop, wherein compressor  32 , indoor coil  16 , and outdoor coil  24  are arranged in series with one another with an expansion device  33  located between indoor coil  16  and outdoor coil  24 . The heat pump system  10  may include a reversing valve  34  disposed between compressor  32  and indoor and outdoor coils  16 ,  24 , such that the direction of flow between compressor  32 , indoor coil  16 , and outdoor coil  24  may be reversed between first and second directions. 
     In the first direction, heat pump system  10  operates in a cooling mode providing a flow in a direction indicated by the “cooling” arrow. In the cooling mode, compressor  32  provides a fluid to outdoor coil  24 . The fluid then travels to indoor coil  16  and then back to compressor  32 . In the cooling mode, indoor coil  16  functions as an evaporator coil and outdoor coil  24  functions as a condenser coil. 
     In the second direction, heat pump system  10  operates in a heating mode providing a flow in a direction indicated by the “heating” arrow. In the heating mode, flow is reversed, traveling from compressor  32  to indoor coil  16  to outdoor coil  24 , and then back to compressor  32 . In the heating mode, indoor coil  16  functions as a condenser coil and outdoor coil  24  functions as an evaporator coil. 
     Referring now to  FIG. 2 , control module  42  may include frequency control module  140 , frequency module  142 , and storage module  144 . Frequency module  142  may be in communication with an output from accelerometer  40  as well as other sensors  120  from heat pump system  10  and other control modules  122  from heat pump system  10  such as a compressor controller or system controller. Frequency module  142  may be in communication with storage module  144  and frequency control module  140 . 
     Storage module  144  may receive measured or determined values from frequency module  142  and may store those values. Storage module  144  may also contain predetermined values and thresholds. Frequency control module  140  may be in communication with frequency module  142  and may control inverter drive  36  to operate a motor of compressor  32  at a chosen frequency. Although control module  42  is depicted as separate from inverter drive  36 , it should be recognized that control module  42  may be integral to inverter drive  36 . 
     Compressor  32  may be driven by a motor (not shown) and compressor system  14  may experience vibrations. Vibrations experienced by compressor system  14  may be defined in different manners including, but not limited to, an amplitude of the vibration, a maximum velocity of the system  14 , or as a maximum acceleration of the system  14 . 
     Referring now to  FIG. 3 , steps in determining vibration characteristics of a compressor from accelerometer and frequency information are depicted. Control logic  200  depicts a continuous loop, but for purposes of the present disclosure description of control logic  200  will begin at block  201 . At block  201 , the compressor  32  of compressor system  14  may operate at steady state until a sweep check is initiated. As will be described in more detail in  FIG. 4  below, steady state operation may include control module  42  operating compressor  32  to avoid prohibited frequency ranges while a frequency sweep is not being performed. A sweep check may be initiated in response to a flag from an input, at a regular time interval, at an electronic clock interval, or as a regular portion of a programmed subroutine. When the sweep check is initiated, control logic  200  may continue to block  202 . 
     In the first series of steps (i.e., steps  202 ,  204 ,  220 ,  222 ,  224 ), frequency module  142  may determine whether it is necessary to run a frequency sweep to determine vibration characteristics of the compressor. At step  202 , frequency module  142  may determine whether the compressor system  14  has transitioned from OFF to ON. The ability to determine the compressor system  14  OFF or ON state may be internal to frequency module  142  or may be determined from communications with other sensors  120  or other control modules  122 . If the compressor system  14  state has changed from OFF to ON, control logic  200  may continue to block  206 . If the compressor system  14  state has not changed from OFF to ON, control logic  200  may continue to block  204 . 
     At block  204 , frequency module  142  may determine whether a change in the operating mode of heat pump system  10  has occurred. This may occur when heat pump system  10  switches from heating to cooling mode or vice versa. Frequency module  142  may communicate with other control modules  122  or other sensors  120  to determine whether the mode has changed. If there has been a change in mode, control logic  200  may continue to step  206 . If there has not been a change in mode, control logic  200  may continue to step  220 . 
     At block  220 , frequency module  142  may determine whether an input from accelerometer  40  exceeds a sweep limit. Y may be a maximum amplitude of motion that may not be exceeded at any rotational frequency of the compressor, where the frequency is represented by the variable F in hertz (Hz). An example Y value may be 25×10 −6  meters. A maximum acceleration A may be related to F and Y by the following equation in which the maximum acceptable acceleration amplitude A is equal to A=4π 2 ×F 2 ×Y. At block  220 , frequency module  142  may compare accelerometer output  40  to a maximum amplitude A for the current operating frequency of the motor of compressor  32 . If the measured accelerometer output  40  is 110 percent of the maximum amplitude A for a particular frequency, control logic  200  may continue to block  206 . When operation at a previously allowed operating frequency exhibits vibration at 110 percent of A, it is a good indication that the operation of the compressor system  14  has changed in some manner. If the measured accelerometer output  40  is less than 110 percent of the limit A, control logic may continue to step  222 . 
     At block  222 , frequency module  142  may determine the time since the last frequency sweep was performed. In many situations, a heat pump system  10  may operate for an extended period without shutting down or without other conditions that may initiate a frequency sweep. Accordingly, frequency module  142  may determine the time since the last frequency sweep and may access a predetermined time value such as one week from storage module  144 . If the elapsed time since the last frequency sweep exceeds the predetermined time, control logic  200  may continue to block  206 . If the elapsed time since the last frequency sweep does not exceed the predetermined limit, control logic  200  may continue to block  224 . 
     At block  224 , frequency module  142  may receive an ambient temperature. A temperature sensor may be integral to control module  142  or a temperature value may be read from other sensors  120 . Alternatively, frequency module  142  may communicate with other control modules  122  of heat pump system  10  which may measure an ambient temperature value. Frequency module  142  may access storage module  144  to acquire previously stored temperature values and a predetermined temperature change limit. For example, previous temperature values may be stored for 24 hours and a predetermined temperature change limit may be at least 40 degrees Fahrenheit. Frequency module  142  may compare the measured temperature with stored temperature values from the previous 24 hours and if the difference between any set of temperature readings exceeds the predetermined temperature change limit, control logic  200  may continue to block  206 . If the temperature difference does not exceed 40 degrees Fahrenheit, control logic  200  may return to block  201  to continue steady state operation. 
     At block  206 , the frequency sweep routine may begin. Frequency module  142  may receive a minimum sweep frequency and a maximum sweep frequency from storage module  144 . Frequency module  142  may communicate with frequency control module  140  to operate inverter drive  36  to operate a motor of compressor  32  at the minimum sweep frequency. Control logic  200  may continue to block  208 . At block  208 , frequency module  142  may receive an accelerometer output  40  associated with the commanded frequency. Frequency module  142  may store the accelerometer and frequency values at storage module  144 . Control logic  200  may continue to block  210 . 
     At block  210 , frequency module  142  may determine whether the present operating frequency is at least the maximum sweep frequency. If the present operating frequency is not at least the maximum sweep frequency, control logic  200  may continue to block  212 . At block  212 , frequency module  142  may increment the current frequency of operation of the motor to a higher value to continue the frequency sweep. Frequency control module  140  may control inverter drive  36  such that the motor of compressor  32  operates at the incremented frequency. In this manner, blocks  208 ,  210 , and  212  may loop until the frequency sweep is complete and store the frequency values and associated accelerometer  40  acceleration readings. Once the operating frequency reaches the maximum sweep frequency, control logic  200  may continue to block  214 . 
     At block  214 , frequency module  142  may determine the resonant frequencies from the stored acceleration and frequency values in storage module  144 . A resonant frequency may be found in any given frequency range when a local maximum of displacement, or velocity, or acceleration amplitude occurs. In other words, within each frequency range at which the measured acceleration exceeds maximum acceleration amplitude A=4π 2 ×F 2 ×Y, a resonant or natural frequency is found where a local maximum acceleration amplitude occurs within that range. A resonant or natural frequency may also be found within a given frequency range where a local maximum displacement or velocity amplitude occurs within the given range. Frequency module  142  may store the resonant frequencies in storage module  144 . 
     At block  216 , frequency module  142  may determine a prohibited frequency range such that frequency control module  140  will not operate inverter drive  36  to operate the motor of compressor  32  at any frequency within the prohibited frequency range in steady-state mode. The prohibited frequency range may be a range defined by the resonant frequency plus or minus a critical frequency difference (CFD), which for a typical compressor may be at least 1 hertz (Hz). This CFD may be approximately 1.5 percent of the particular resonant frequency encountered based on the following relationship. A frequency ratio may be represented by the equation R=f o /f n , where f o  is the operating frequency and f n  is the natural frequency. Undesirable vibrations may occur when the f o  value is within 1.5 percent of f n  as represented by the following equation of |R−1|&lt;Δ R , where Δ R  is equal to 0.015. From these equations it can also be recognized that the critical frequency difference (in Hz) may increase at higher natural frequencies. 
     Alternatively, the prohibited frequency range can be the actual ranges where the measured acceleration exceeds the maximum A. Frequency module  142  may store the prohibited frequency ranges in storage module  144 . It should be noted that the steps described in blocks  214  and  216  may also be performed as part of the loop of blocks  208 ,  210  and  212 , wherein blocks  214  and  216  may calculate the resonant frequencies and forbidden frequency ranges during the frequency sweep. Control logic  200  may return to block  201  to operate at steady state. 
     Referring now to  FIG. 4 , control logic for operating a motor of compressor  32  at a requested frequency is depicted. A requested frequency may be based on an input from a user such as to change the heating or cooling effect of the heat pump system  10  or may be based on an output of other control modules  122  of heat pump system  10  such as a thermostat. Although control logic  300  depicts a continuous loop, control logic  300  is a subloop of overall steady state operation demonstrating a response to a changed requested frequency. Description of control logic  300  will begin at block  301 . 
     At block  301 , frequency module  142 , upon receiving a command from other controllers  122 , may command frequency control module  140  to operate inverter drive  36  to operate a motor of compressor  32  at a previously requested frequency. Otherwise, steady state operation may continue until the requested frequency changes. When the requested frequency changes, as is determined by frequency module  142  at block  302 , control logic  300  may continue to block  304 . 
     At block  304 , frequency module  142  may compare the requested frequency to the prohibited frequency values stored in storage module  144 . If the requested frequency value is not within the prohibited frequency ranges, control logic  300  may return to steady state operation at block  301 . If the requested frequency value is within the prohibited frequency ranges, control logic  300  may continue to block  306 . 
     At block  306 , frequency module  142  may receive an allowed upper frequency from storage module  144 . This allowed upper frequency may be a first frequency above the requested frequency but outside of the prohibited frequency range. The allowed upper frequency may also include a safety factor above this first frequency. Once the allowed upper frequency is determined, control logic  300  may continue to block  308 . 
     At block  308 , frequency module  142  may receive an allowed lower frequency from storage module  144 . This allowed lower frequency may be a first frequency below the requested frequency but outside of the prohibited frequency range. The allowed lower frequency may also include a safety factor below this first frequency. Once the allowed lower frequency is determined, control logic  300  may continue to block  310 . 
     At block  310 , frequency module  142  may access a predetermined time value from storage module  144 . The predetermined time value may correspond to a total time during which the frequency averaging routine described below may be run. For example, the total time may be four minutes. Frequency module  142  may then determine an upper frequency operating ratio based on the following: upper ratio=(requested frequency−lower frequency)÷(upper frequency−lower frequency). The upper frequency operating time may be equivalent to the predetermined time multiplied by the upper ratio. Once the upper frequency operating time is determined, control logic  300  may continue to block  312 . 
     At block  312 , frequency module  142  may use the predetermined time and calculated upper frequency operating time to determine the lower frequency operating time. The lower frequency operating time may simply be equal to the predetermined time minus the upper frequency operating time. It is also possible to calculate the lower frequency operating time first using a lower ratio=(upper frequency−requested frequency)÷(upper frequency−lower frequency). The lower frequency operating time and upper frequency operating time could then be calculated from the lower ratio. Once the allowed lower frequency operating time is determined, control logic  300  may continue to block  314 . 
     At block  314 , frequency module  142  may command frequency control module  140  to operate inverter drive  36  and the motor of compressor  32  at the allowed upper frequency and continue to operate at that frequency for the upper frequency operating time. Once the upper frequency operating time has elapsed, control logic  300  may continue to block  316 . At block  316 , frequency module  142  may command frequency control module  140  to operate inverter drive  36  and the motor of compressor  32  at the allowed lower frequency for the lower frequency operating time. Once the lower frequency operating time is complete, the time-averaged frequency output of the compressor over the total predetermined time may be equal to the requested frequency. Control logic  300  may then continue to block  318 . 
     At block  318 , frequency module  142  may determine whether there has been change in the requested frequency. If there has not been a change in the requested frequency, control logic  300  may return to block  314  and continue to loop through operating at the allowed upper frequency and allowed lower frequency such that the average frequency is equivalent to the requested frequency. If there has been a change in the requested frequency, control logic  300  may continue to block  304  to determine whether the requested frequency is prohibited. 
     Although the operation at an average frequency equivalent to a requested frequency within the prohibited frequency range has been described in a certain manner above, it should be recognized that such operation may also be done in other manners. For example, frequency module  142  may determine the allowed frequency furthest from the requested frequency. The motor of compressor  32  may be operated by inverter drive  36  and frequency control module  140  at this frequency for a predetermined time. Frequency module  142  may then determine a second operating frequency in the opposite (greater than or less than) direction from the first operating frequency. The second operating frequency may be at a same frequency difference from the requested frequency as the first operating frequency. The motor of compressor  32  may then be operated by inverter drive  36  and frequency module  140  at the second frequency for the same predetermined time as the first operating frequency because the differences between the requested frequency and the two operating frequencies are the same. 
     Referring now to  FIG. 5 , alternative steps for variable speed compressor vibration protection are depicted. The steps described in  FIG. 3  and  FIG. 4  show performing a frequency sweep based on certain conditions, storing prohibited frequency ranges, and avoiding these frequency ranges during normal operation. The steps described in  FIG. 5  simply measure the acceleration and avoid frequencies where the measured acceleration exceeds the limit A=4π 2 ×F 2 ×Y. 
     Although control logic  400  depicts operation in a continuous loop, the description of control logic  400  may begin with block  401 . At block  401 , frequency module  142  may command frequency control module  140  to operate inverter drive  36  to operate the motor of compressor  32  at a requested frequency. Operation may continue in this manner until a check of an acceleration value from accelerometer  40  is initiated. A check may be initiated in a number of ways, such as with each change in requested frequency, at a predetermined time interval, or whenever an acceleration value is received. Control logic  400  may continue to block  402 . 
     At block  402 , frequency module  142  may receive an acceleration reading from accelerometer  40 . Control logic  400  may continue to block  404 . At block  404 , frequency module  142  may then determine whether the acceleration reading from the accelerometer exceeds a limit A for the particular frequency F based on the equation A=4π 2 ×F 2 ×Y. If the acceleration does not exceed the limit A, control logic  400  may return to block  401 . If the acceleration does exceed the limit A, control logic  400  may continue to block  406 . 
     At block  406 , frequency module  142  may command frequency control module  140  to operate inverter drive  36  such that a motor of compressor  32  operates at a higher frequency. Frequency module  142  may receive measurements from accelerometer  40  and may continue to command frequency control module  140  to increase the frequency of inverter drive  36  and the motor of compressor  32  until an acceleration reading from accelerometer  40  is less than A=4π 2 ×F 2 ×Y for the given frequency. This first frequency at which the measured acceleration does not exceed the acceleration limit may be the allowed upper frequency. The allowed upper frequency may also be this first measured frequency plus a safety factor. Once the allowed upper frequency is determined, control logic  400  may continue to block  408 . 
     At block  408 , frequency module  142  may command frequency control module  140  to operate inverter drive  36  such that a motor of compressor  32  operates at a frequency less than the requested frequency. Frequency module  142  may receive measurements from accelerometer  40  and may continue to command frequency control module  140  to decrease the frequency of inverter drive  36  and the motor of compressor  32  until an acceleration reading from accelerometer  40  is less than A=4π 2 ×F 2 ×Y for the given frequency. This first frequency at which the measured acceleration does not exceed the acceleration limit may be the allowed lower frequency. The allowed lower frequency may also be this first measured frequency minus a safety factor. Once the allowed lower frequency is determined control logic  400  may continue to block  410 . 
     At block  410 , frequency module  142  may access a predetermined time value from storage module  144 . The predetermined time value may correspond to a total time during which the frequency averaging routine described below may be run. For example, the total time may be four minutes. Frequency module  142  may then determine an upper frequency operating ratio based on the following: upper ratio=(requested frequency−lower frequency)÷(upper frequency−lower frequency). The upper frequency operating time may be equivalent to the predetermined time multiplied by the upper ratio. Once the upper frequency operating time is determined, control logic  400  may continue to block  412 . 
     At block  412 , frequency module  142  may use the predetermined time and calculated upper operating time to determine the lower frequency operating time. The lower frequency operating time may simply be equal to the predetermined time minus the upper frequency operating time. It is also possible to calculate the lower frequency operating time first using a lower ratio=(upper frequency−requested frequency)÷(upper frequency−lower frequency). The lower frequency operating time and upper frequency operating time could then be calculated from the lower ratio. Once the lower frequency operating time is determined, control logic  400  may continue to block  414 . 
     At block  414 , frequency module  142  may command frequency control module  140  to operate inverter drive  36  and the motor of compressor  32  at the allowed upper frequency and continue to operate at that frequency for the upper frequency operating time. Once the upper frequency operating time has elapsed, control logic  400  may continue to block  416 . At block  416 , frequency module  142  may command frequency control module  140  to operate inverter drive  36  and the motor of compressor  32  at the allowed lower frequency for the lower frequency operating time. Once the lower frequency operating time is complete, control logic  400  may continue to block  418 . 
     At block  418 , frequency module  142  may determine whether there has been change in the requested frequency. If there has not been a change in the requested frequency, control logic  400  may return to block  414  and continue to loop through operating at the allowed upper frequency and allowed lower frequency such that the average frequency is equivalent to the requested frequency. If a change in the requested frequency has occurred, control logic  400  may continue to block  420 . At block  420 , frequency module  142  may command frequency control module  140  to operate inverter drive  36  and the motor of compressor  32  at the new requested frequency. Control logic  400  may continue to block  401  to operate at steady state until the next check of the accelerometer. 
     Those skilled in the art may now appreciate from the foregoing that the broad teachings of the present disclosure may be implemented in a variety of forms. Therefore, while this disclosure has been described in connection with particular examples thereof, the true scope of the disclosure should no be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.