Abstract:
A system and method of avoiding hunting behavior by a motor around disallowed speed ranges. A sensor or sensorless method determines the motor&#39;s actual speed. A memory contains lower and upper speed values that define a disallowed range. A control unit determines whether the motor is operating under a limiting condition and engaged in hunting behavior by determining whether the motor is repeatedly moving between an actual speed that is within the disallowed range and an actual speed that is at or below the lower speed value during a first period, and if so, sets a temporary speed limit for the motor at the lower speed value for a second period. The first period may be the time required for the motor to change its actual speed a few times and thereby confirm hunting behavior. The second period may be sufficiently long that the limiting condition may have changed.

Description:
FIELD 
       [0001]    The present invention relates to systems and methods for controlling the operation of electric motors. 
       BACKGROUND 
       [0002]    The operation of electric motors can be characterized by their input frequencies (measured in Hertz (Hz)) and corresponding output speeds (measured in revolutions per minute (RPM)). The input frequency to a variable speed electric motor can be varied to change the output speed, which allows the same electric motor to be used with, e.g., different fan blades rated at different speeds and load points. 
         [0003]    Fan blades may experience resonance at certain RPM. This resonance can be amplified into mechanical vibrations which may cause damage to the fan blades as well as unwanted noise. One solution is to avoid the RPM range associated with resonance by not allowing the electric motor to operate within that range. However, the electric motor may require an RPM within the disallowed RPM range in order to maintain a certain amount of power or torque, in which case disallowing the required RPM may result in even more noise or in the electric motor exhibiting “hunting” behavior, i.e., repeatedly jumping back and forth over the disallowed RPM range between a higher RPM that is not allowed and a lower RPM that is too slow. More specifically, a motor controller may command the electric motor to operate at a desired RPM that is just above the disallowed RPM range. However, load and/or temperature effects may prevent the electric motor from achieving the desired RPM, and may cause it to achieve an actual RPM that is within the disallowed RPM range, which then causes it to drop to an allowed actual RPM that is just below the disallowed RPM range. When the motor controller senses that the electric motor&#39;s actual RPM is substantially below the desired RPM, it commands the electric motor to return to the desired RPM which the electric motor cannot achieve. This cycle of jumping back and forth around the disallowed RPM range continues until the limiting operating conditions change so that the electric motor is able to achieve the desired RPM. 
         [0004]    This background discussion is intended to provide information related to the present invention which is not necessarily prior art. 
       SUMMARY 
       [0005]    Embodiments of the present invention solve the above-described and other problems and limitations by providing improved avoidance of one or more disallowed RPM ranges while still maintaining proper control over an electric motor operating in a limiting (e.g., power- or temperature-limiting) mode, including avoiding hunting behavior around the disallowed RPM ranges. 
         [0006]    An electric motor system constructed in accordance with an embodiment of the present invention may broadly comprise an electric motor having a shaft for driving a load, and a motor control subsystem operable to control operation of the electric motor. The motor control subsystem may include a speed determiner operable to measure, estimate, or otherwise determine an actual speed value of the electric motor, a memory element containing a first lower speed value and a first upper speed value, which together define a first disallowed speed range for the electric motor, and a control unit in electrical communication with the speed determiner and the memory element. The control unit may be operable to calculate a desired speed value for the electric motor, wherein the desired speed value is not within the first disallowed speed range, and to generate and send an electrical signal to cause the electric motor to attempt to achieve the desired speed value, receive the determined actual speed value of the electric motor from the speed determiner, and determine whether the electric motor is unable to achieve the desired speed value due to a limiting condition and is repeatedly changing between a first actual speed value that is at or below the first lower speed value and a second actual speed value that is within the first disallowed speed range over a first predetermined period of time (i.e., that the electric motor is engaged in hunting behavior). If the control unit determines that the electric motor is engaged in such hunting behavior, the control unit may set a temporary speed limit for the electric motor at the first lower speed value for a second predetermined period of time. At the end of the second predetermined period of time, the control unit may remove the temporary speed limit to determine whether the limiting condition still exists. 
         [0007]    In various implementations of this embodiment, the electric motor system may further include any one or more of the following additional features. The electric motor may be a three-phase, permanent magnet electric motor. The load may be a fan component of a heating and air-conditioning unit. The first predetermined time may be approximately between 20 seconds and 40 seconds, or approximately the time required for the electric motor to change between the first actual speed and the second actual speed between one time and three times. The second predetermined time may be approximately between 20 minutes and 40 minutes. The memory element may further contain a second lower speed value and a second upper speed value which together define a second disallowed speed range. In this case, the control unit may be operable to determine whether the electric motor is unable to achieve the desired speed value due to a limiting condition and is repeatedly changing between the first actual speed value that is within the second disallowed speed range and the second actual speed value that is at or below the second lower speed value over the first predetermined period of time, and if so, set the temporary speed limit for the electric motor at the second lower speed value for the second predetermined period of time. 
         [0008]    The first disallowed speed range may be associated with a first flag in the memory element, and if the first flag is not set then the control unit may ignore the first disallowed speed range. The second disallowed speed range may also be associated with the first flag in the memory element, and if the first flag is set then the control unit may ignore the second disallowed speed range. Alternatively, the first disallowed speed range may be associated with a first flag in the memory element, and if the first flag is not set then the control unit may ignore the first disallowed speed range. The second disallowed speed range may be associated with a different second flag in the memory element, and if the second flag is not set then the control unit may ignore the second disallowed speed range. 
         [0009]    Additionally, each of these implementations and embodiments may be alternatively characterized as methods based on their functionalities. 
         [0010]    This summary is not intended to identify essential features of the present invention, and is not intended to be used to limit the scope of the claims. These and other aspects of the present invention are described below in greater detail. 
     
    
     
       DRAWINGS 
         [0011]    Embodiments of the present invention are described in detail below with reference to the attached drawing figures, wherein: 
           [0012]      FIG. 1  is a schematic diagram of an electric motor system of the present invention; 
           [0013]      FIG. 2  is a block diagram of components of a motor control subsystem of the electric motor system of  FIG. 1 ; 
           [0014]      FIG. 3  is a plot of input voltage that references a speed command to an electric motor component of the motor control system of  FIG. 1  and corresponding output speed of the electric motor component; 
           [0015]      FIG. 4  is a plot of a speed command signal from the motor control subsystem and an actual speed of the electric motor of the electric motor system of  FIG. 1  over time; 
           [0016]      FIG. 5  is a flow diagram of steps involved in the operation of the electric motor system of  FIG. 1 ; 
           [0017]      FIG. 6  is a flow diagram of additional steps involved in the operation of a first implementation of the electric motor system of  FIG. 1 ; and 
           [0018]      FIG. 7  is a flow diagram of additional steps involved in the operation of a second implementation of the electric motor system of  FIG. 1 . 
       
    
    
       [0019]    The figures are not intended to limit the present invention to the specific embodiments they depict. The drawings are not necessarily to scale. 
       DETAILED DESCRIPTION 
       [0020]    The following detailed description of embodiments of the invention references the accompanying figures. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those with ordinary skill in the art to practice the invention. Other embodiments may be utilized and changes may be made without departing from the scope of the claims. The following description is, therefore, not limiting. The scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
         [0021]    In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features referred to are included in at least one embodiment of the invention. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are not mutually exclusive unless so stated. Specifically, a feature, structure, act, etc. described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, particular implementations of the present invention can include a variety of combinations and/or integrations of the embodiments described herein. 
         [0022]    Broadly characterized, the present invention provides improved avoidance of one or more disallowed RPM ranges while still maintaining proper control over an electric motor operating in a limiting (e.g., power- or temperature-limiting) mode, including avoiding hunting behavior around the disallowed RPM ranges. Referring to the figures, an electric motor system  10  constructed in accordance with the present invention is shown. Referring to  FIGS. 1 and 2 , in an embodiment of the present invention, the electric motor system  10  may broadly comprise an electric motor  12 ; a power source  14 ; and a motor control subsystem  16  including a speed sensor  20 , one or more memory elements  22 , and a control unit  24 . 
         [0023]    The electric motor  12  may be any suitable variable speed electric motor driving any appropriate load  30  via a shaft  32 . The load  30  may be, for example, a fan or a pump which may be part of a heating and air-conditioning unit, such as a commercial blower, or an appliance, such as a washing machine or a clothes dryer, which may include additional electrical or mechanical components not described herein. In one implementation, the electric motor  12  may be a variable speed, three phase, permanent magnet electric motor driving a condenser fan  30  in a heating and air-conditioning unit. The power source  14  may be any suitable residential, commercial, or other source of power. 
         [0024]    The motor control subsystem  16  may be broadly operable to control operation of the electric motor  12 , including receiving power from the power source  14  and generating a driving waveform to drive the electric motor  12 . The speed sensor  20  may be operable to measure or otherwise determine an actual speed value of the electric motor  12 . In one implementation, the speed sensor  20  may be located on the shaft  32  of the electric motor  12 , and may take any suitable form, including mechanical, electrical (e.g., a Hall effect sensor), or optical. Alternatively, the speed sensor may be replaced by a sensorless method of estimating or otherwise determining the speed of the electric motor  12 . Referring also to  FIG. 3 , the one or more memory elements  22  may contain a first lower speed value  40  and a first upper speed value  42 , which together define a first disallowed speed range  44  for the electric motor  12 . The one or more memory elements  20  may take any suitable form, including one or more electrically erasable programmable read only memories (EEPROMs). 
         [0025]    The control unit  24  is in electrical communication with the speed sensor  20  and the one or more memory elements  22 , and operable to receive inputs therefrom and to generate and send command signals based thereon for controlling operation of the electric motor  12 . The control unit  24  may take any suitable form, including only or primarily hardware components, only or primarily software components, or a combination of hardware and software components. 
         [0026]    In particular, referring also to  FIG. 4 , the control unit  24  may be operable to calculate a desired speed value for the electric motor  12 , wherein the desired speed value is not within the disallowed speed range, and to generate and send a speed command signal  46  to cause the electric motor  12  to attempt to achieve the desired speed value. The control unit  24  may be further operable to receive the measured actual speed value  48  of the electric motor  12  from the speed sensor  20 , and to determine whether the electric motor  12  is unable to achieve the desired speed value due to a limiting condition and is engaging in hunting behavior, i.e., repeatedly changing between a first actual speed value that is at or below the first lower speed value and a second actual speed value that is within the first disallowed speed range  44  over a first predetermined period of time  50 . If the electric motor  12  is engaging in hunting behavior, the control unit  24  may be further operable to set a temporary speed limit for the electric motor  12  at the first lower speed value  40  for a second predetermined period of time  52 . The control unit  24  may be further operable to remove the temporary speed limit at the end of the second predetermined period of time  52  in order to determine whether the limiting condition still exists (as evidenced by the electric motor  12  continuing to engage in hunting behavior), and if so, repeat the foregoing process. 
         [0027]    As mentioned, the limiting condition may be, e.g., power or temperature effects which prevent the electric motor  12  from achieving the desired speed value. The first predetermined period of time  50  may be approximately between 20 seconds and 40 seconds, or approximately 30 seconds, or approximately the time required for the electric motor  12  to change between the first actual speed and the second actual speed between one time and three times (i.e., sufficient time to confirm that the electric motor  12  is engaging in hunting behavior). The second predetermined period of time  52  may be approximately between 20 minutes and 40 minutes, or 30 minutes (i.e., sufficient time in which a limiting condition might change). 
         [0028]    In one implementation, the electric motor system  10  may be operable to avoid multiple RPM ranges. For example, the one or more memory elements  22  may further contain a second lower speed value  54  and a second upper speed value  56  which together define a second disallowed speed range  58 . In this implementation, the control unit  24  may be further operable to determine whether the electric motor  12  is unable to achieve the desired speed value due to a limiting condition and is repeatedly changing between the first actual speed value that is within the second disallowed speed range  58  and the second actual speed value that is at or below the second lower speed value  54  over the first predetermined period of time  50 , and if so, set the temporary speed limit for the electric motor  12  at the second lower speed value  54  for the second predetermined period of time  52 . Thus, the additional disallowed RPM ranges may be treated the same or similar to the first disallowed RPM range. 
         [0029]    Furthermore, when the electric motor system  10  is operable to avoid multiple disallowed RPM ranges, an operator of the system  10  may be allowed to enable one or more of the RPM ranges  44 , 58  and disable others, wherein the control unit  24  may ignore any disabled disallowed RPM ranges  44 , 58 . For example, in a first implementation, the first disallowed speed range  44  may be associated with a first flag in the memory element  22 , and if the first flag is not set then the control unit  24  may ignore the first disallowed speed range  44 , and the second disallowed speed range  58  may also associated with the same first flag in the memory element  22 , and if the first flag is set then the control unit  24  may ignore the second disallowed speed  58  range. In a second implementation, the first disallowed speed range  44  may be associated with a first flag in the memory element  22 , and if the first flag is not set then the control unit  24  may ignore the first disallowed speed range  44 , and the second disallowed speed range  58  may be associated with its own second flag in the memory element  22 , and if the second flag is not set then the control unit  24  may ignore the second disallowed speed range  58 . For example, the first disallowed speed range  44  may be stored in EEPROM locations  52  and  54 , and the second disallowed speed range  58  may be stored in EEPROM locations  192  and  194 . The disallowed speed ranges  44 , 58  may be enabled or disabled by a high byte and a low byte in EEPROM location  190 , wherein if the high byte in location  190  equals hA5 then the first disallowed speed range  44  is disabled and ignored by the control unit  24  and if the lower byte equals hA5 then the second disallowed speed range  58  is disabled and ignored. 
         [0030]    In operation, an embodiment of the electric motor system  10  of the present invention may function substantially as follows. Referring to  FIG. 5 , the first disallowed speed range  44  for the electric motor  12  is defined by storing in the memory element  22  the first lower speed value  40  and the first upper speed value  42 , as shown in step  100 . The control unit  24  calculates a desired speed value for the electric motor  12 , wherein the desired speed value is not within the first disallowed speed range  44 , and the control unit  24  generates and sends an electrical speed command signal to cause the electric motor  12  to attempt to achieve the desired speed value, as shown in step  102 . The speed sensor  20  measures the actual speed value  48  of the electric motor  12 , as shown in step  104 . The control unit  24  determines whether the electric motor  12  is unable to achieve the desired speed value due to a limiting condition and is repeatedly changing between a first actual speed value that is at or below the first lower speed value  40  to a second actual speed value that is within the first disallowed speed range  44  (i.e., is engaged in hunting behavior) over the first predetermined period of time  50 , as shown in step  106 . If the electric motor  12  is engaged in hunting behavior, then the control unit  24  sets the temporary speed limit for the electric motor  12  at the lower speed value  40  for the second predetermined period of time  52 , as shown in step  108 . The control unit  24  removes the temporary speed limit at the end of the second predetermined period of time  52 , as shown in step  110 , to determine whether the limiting condition still exists (i.e., whether the electric motor  12  returns to hunting behavior, as shown in step  106 ). If the electric motor  12  returns to hunting behavior after the second predetermined period of time  52 , then the process repeats by returning to step  108 . 
         [0031]    A second (and subsequent) disallowed speed range  58  may be defined for the electric motor by storing in the memory element  22  the second lower speed value  54  and a second upper speed value  56 , as shown in step  200 . The control unit  24  may treat this second disallowed speed range  58  substantially similar to or the same as the first disallowed speed range  44 . 
         [0032]    Referring also to  FIGS. 6 and 7 , the first disallowed speed range  44  may be associated with the first flag in the memory element  22 , and the system  10  may allow for setting the first flag, as shown in step  200 . The control unit  24  may determine whether the first flag is set or not, as shown in step  202 . If the first flag is not set, then the control unit  24  may ignore the first disallowed speed range  44 , as shown in step  204 , and if the first flag is set, then the control unit  24  may ignore the second disallowed speed range  58 , as shown in step  206 . Alternatively, the first disallowed speed range  44  may be associated with the first flag in the memory element  22 , and the system  10  may allow for setting the first flag, as shown in step  300 . Similarly, the second disallowed speed range may be associated with the second flag in the memory element  22 , and the system  10  may allow for setting the second flag, as shown in step  306 . The control unit  24  may determine whether the first flag is set, as shown in step  302 , and if the first flag is not set then the control unit may ignore the first disallowed speed range  44 , as shown in step  304 . Similarly, the control unit  24  may determine whether the second flag is set, as shown in step  308 , and if the second flag is not set then the control unit  24  may ignore the second disallowed speed range  58 , as shown in step  310 . 
         [0033]    For example, if the control unit  24  has calculated and commanded a desired electric motor speed of940 RPM, but due to a limiting condition the electric motor  12  actually operates at 930 RPM, and a disallowed RPM range has been established between 900 and 935 RPM such that the electric motor  12  begins several cycles of hunting across the disallowed range, then the control unit  24  may set the maximum RPM at 900 RPM for 30 minutes. At the end of the 30 minute period, the control unit  24  may reset the maximum RPM and reassert the command signal for 940 RPM. If the electric motor  12  begins hunting again, then the process repeats. 
         [0034]    It will be understood that references in the foregoing description to “avoiding” or “skipping” a disallowed speed range  44 , 58  mean not commanding the electric motor  12  to achieve and hold a particular speed value within the disallowed speed range and, when necessary, moving relatively quickly through those disallowed speed ranges  44 , 58 . 
         [0035]    Thus, the present invention provides advantages over the prior art, including that it provides improved avoidance of one or more disallowed RPM ranges  44 , 58  while still maintaining proper control over the electric motor  12  operating in a limiting (e.g., power- or temperature-limiting) mode, including avoiding hunting behavior around the disallowed ranges  44 , 58 . 
         [0036]    Although the invention has been described with reference to the one or more embodiments illustrated in the figures, it is understood that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims. 
         [0037]    Having thus described one or more embodiments of the invention, what is claimed as new and desired to be protected by Letters Patent includes the following: