Abstract:
A method, and a system of using the method, of controlling a motor having a rated capacity. The method includes determining the motor has been started, determining a plurality of operating parameters of the motor after the motor has started, determining a threshold from a portion of the operating parameters, comparing one of the determined operating parameters with the threshold, and operating the motor at a level corresponding to below the rated capacity when one of the determined operating parameters is greater than the threshold. When an electric motor having a control system according to the present invention lifts a load, drives a machine, or starts other motions, the control system automatically adjusts to a power level that corresponds to the load. In this way, the motor will not be allowed to exert any power or force to the load in excess of what is necessary. If the machine being driven becomes jammed, binds, or draws more power for some unexpected reason, the motor will be shut down to reduce or to limit damage to the load or the machine.

Description:
RELATED APPLICATION 
   This application claims the benefit of prior-filed, U.S. Provisional Patent Application Ser. No. 60/677,725 filed on May 4, 2005, the entire content of which is incorporated by reference herein. 

   BACKGROUND 
   The invention relates to a controller for a motor, and particularly, a controller having a load overload device. 
   To comply with the National Electrical Code, NFPA 70, electric motors larger than one horsepower are required to use overload devices. The overload devices are intended to protect electric motors and branch circuits from undue heating caused by excessive current. 
   Induction electric motors are widely used. Induction motors are generally specified by a variety of rated capacity parameters such as torque, horsepower, current, voltage, frequency, temperature, starting time, and the like. While induction motors have a simple robust physical design, induction motors rely on complex nonlinear relationships to function. For example, when an inductive motor is first turned on to drive a machine, the inductive motor can draw an additional amount of current to provide additional torque to drive the machine. The additional amount of current drawn is generally referred to as an inrush current that can last for a few seconds. The inrush current can sometimes be ten times higher than that of a normal running current. The additional torque can sometimes be three times that of a normal operating torque. After the driven machine reaches a normal operating state or speed, the current drawn will drop below the name plate current value or rated current capacity. 
   However, when the machine being driven becomes jammed or impaired in some manner, the motor will draw additional current or power to churn out additional torque in an attempt to move the machine. When the amount of the operating current drawn by the motor exceeds a certain rated amount, an overload device associated with the machine will trip. For example, an overload of about 125 percent of the rated current of the motor for about 600 seconds, or about 600 percent of the rated current of the motor for about ten seconds will trip the overload device. However, the additional torque can last for a period of time before the overload device trips. While the machine being driven is jammed or impaired and before the overload device trips, the jammed machine can destroy any jammed material or itself. 
   SUMMARY 
   While methods using sensors to detect overload in a jammed machine exist, these methods use a fixed overload set-point. In such cases, an associated overload device trips only when a current drawn exceeds the fixed overload set-point. As a result, a motor with the fixed overload set-point continues to deliver full torque when jammed during steady-state operation before the current drawn exceeds the fixed overload set-point. For example, a 10,000-lb. material lift with a fixed set-point overload exerts about 10,000 lbs. onto any associated structure when the lift is unloaded and becomes jammed. The amount of force exerted by the machine can be destructive. 
   When an electric motor having a control system according to the present invention lifts a load, drives a machine, or starts other motions, the control system automatically adjusts to a power level that corresponds to the load. In this way, the motor will not be allowed to exert any power or force to the load in excess of what is necessary. If the machine being driven becomes jammed, binds, or draws more power for some unexpected reason, the motor will be shut down to reduce or to limit damage to the load or the machine. 
   Accordingly, in one construction, the invention provides a controller for a motor, where the controller includes a load overload device. For example, the load overload device can be a jam overload device for a vertical lift. The controller variably sets a value of the load overload device each time the motor starts up, rather than having a fixed overload set point. This provides a more flexible jam load overload device. The device may or may not include any National Electric Code (“NEC”) overload protection as described. 
   In one construction, the invention provides a method of controlling a motor that has a rated capacity. The method includes determining the motor has been started, and determining a plurality of operating parameters of the motor after the motor has started. The method also includes determining a threshold from a portion of the operating parameters, and comparing one of the determined operating parameters with the threshold. The method also includes operating the motor at a level corresponding to below the rated capacity when one of the determined operating parameters is greater than the threshold. 
   In another construction, the invention provides a method of controlling a motor that has a rated capacity. The method includes determining the motor has been started, and determining values of an initial set of operating parameters of the motor after the motor has started. The method also includes determining a statistical value of the values of the initial set of operating parameters, and adapting a set-point to the statistical value. The method also includes determining a value of a subsequent operating parameter of the motor after the values of the initial set of operating parameters have been determined, determining a difference between the value of a subsequent operating parameter and the set-point, and stopping the motor when the difference is above an overload threshold for a period of time. 
   In yet another construction, the invention provides a control system for a motor that has a rated capacity. The system includes a sensing module and a controller. The sensing module determines a plurality of operating parameters of the motor after the motor has started. The controller determines a threshold from a portion of the operating parameters, compares one of the sensed operating parameters with the threshold, and operates the motor at a level that corresponds to below the rated capacity when one of the sensed operating parameters is greater than the threshold for a period of time. 
   Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of an apparatus or system incorporating the invention. 
       FIG. 2  is a block diagram of one exemplary method of operation for the load overload. 
       FIG. 3  is a block diagram of another exemplary method of operation for the load overload. 
   

   DETAILED DESCRIPTION 
   Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
   As should also be apparent to one of ordinary skill in the art, the systems shown in the figures are models of what actual systems might be like. Many of the modules and logical structures described are capable of being implemented in software executed by a microprocessor or a similar device or of being implemented in hardware using a variety of components including, for example, application specific integrated circuits (“ASICs”). Terms like “controller” and “processor” may include or refer to both hardware and/or software. Furthermore, throughout the specification capitalized terms are used. Such terms are used to conform to common practices and to help correlate the description with the examples, and/or drawings. However, no specific meaning is implied or should be inferred simply due to the use of capitalization. Thus, unless specifically indicated otherwise, the claims should not be limited to the specific examples or terminology or to any specific hardware or software implementation or combination of software or hardware. 
     FIG. 1  schematically represents an apparatus  100  incorporating the invention. For example, the apparatus  100  can be a vertical reciprocating conveyer. Other exemplary apparatuses include water pumps, industrial mixers, air blowers, extruders, cranes, elevators, and the like. As shown in  FIG. 1 , the apparatus  100  includes a motor  105 , a controller  110  for controlling the motor  105 , and a driven machine or load  115  supported by the apparatus  110 . As will be discussed further below, the driven load  115  receives mechanical output produced by the motor  105 . 
   The motor  105 , in the construction described herein, is a three-phase induction motor. However, the invention is not limited to a three-phase induction motor. Instead, the invention can be used with almost any motor type having a relationship between the current of the motor  105  and the torque of the motor  105 . Other example motor types include, but are not limited to, single-phase induction motors, synchronous motors, direct current motors, etc. As is commonly known, the motor  105  receives electrical power from the controller  110  and produces a mechanical power in response thereto. The mechanical power is provided to the load  115  attached to the motor  105 . The load  115  can be, for example, a mechanical load having a mechanical movement and/or a material that is processed as a result of the mechanical movement (e.g., movement of a material in a plastic blow-molding machine, movement of a fluid through a pump, etc.). 
     FIG. 1  provides a representative controller  110  that can be used with the invention. The controller  110  includes an interface  120  (e.g., a switch, keyboard, key pad, or similar operator interface) to provide overall control of the motor  105 , a contactor  125  that connects the motor  105  to a power source  130 , a sensing module or a sensor  135  that senses operating parameters such as the amount of current drawn by the motor  105 , a programmable processor or device  140  and a memory  145 . The controller  110  can also include different and/or more sophisticated circuitry depending on the environment. For example, the controller  110  can include a rectifier/inverter combination or other driver for controlling the power to the motor. It is also envisioned that the controller  110  can include other circuitry not shown in the drawings that one skilled in the art would know to be present. For example, the controller  110  includes an analog-to-digital converter for converting the sensed current from an analog value to a digital value. 
   In the construction shown in  FIG. 1 , the sensor  135  is a current sensor such as SSAC current transducer TCSA 10  having a response time of about 300 ms at 90-percent span arranged between the contactor  125  and the motor  105 . However, in other constructions, outputs of the power source  130  are to fed to the sensor  135  before being fed to the contactor  125  and the motor  105 . Furthermore, although the memory  145  is shown as an external component to the processor  140 , the memory  145  can also be an internal memory integral with the processor  140 . An exemplary processor is Rockwell International programmable logic controller 1763L16BWA having analog inputs with ten bit resolution, an execution time of about 3 ms, and an update time of about 100 ms. 
   Although  FIG. 1  shows a current sensor, the controller  110  can include other sensors such as speed sensors, temperature sensors, torque sensors, pressure sensors, and the like, that determine other operating parameters of the motor  105 . For example, a sensor  135  that includes speed sensors can determine a speed exhibited by the motor  105 . A sensor  135  that includes temperature sensors can determine a temperature adjacent to windings of the motor  105 . A sensor  135  that includes torque sensors can determine a torque value generated by the motor  105 . As such, the controller  110  is not limited to using operating parameters such as current drawn by the motor  105  to provide control to the motor  105 . Rather, the controller  110  can also use other operating parameters of the motor  105  such as speed, temperature, pressure, and torque to provide control to the motor  105 . 
   Furthermore, the sensor  135  is configured to detect and monitor a condition of the motor  105  that is indicative of the operating parameters exhibited or produced by the motor  105 . Collectively, values of signals output by the sensor  135  are referred to as sensed values, or values hereinafter. In some constructions, the sensor  135  is equipped with calibration circuitry or microprocessors therein, the amount of current can be converted internally to a calibrated form. Otherwise, the conditions can be converted into calibrated signals by other external processes in a manner known in the art. The sensor  135  can also include multiple internal sensors or sensing elements in a plurality of sensor arrays, for example, that may be coupled to the processor  140 . 
   In the shown construction, the controller  110  includes one or more programmable devices  140  (e.g., one or more microprocessors, one or more microcontrollers, etc.) and the memory  145 . The memory  145 , which can include multiple memory devices, includes program storage memory and data storage memory. The programmable device  140  receives instructions and data from the memory  145 , receives information (either directly or indirectly) from attached devices (e.g., the sensor  135 ) in communication with the programmable device  140 , executes the received instructions and data, processes the received information, and communicates outputs to the attached devices (e.g., the contactor  125 ). It is envisioned that the programmable device  140  and memory  145  can be replaced by, for example, an application specific integrated circuit (“ASIC”) and/or analog circuitry that performs the function of the programmable device  140  and memory  145  discussed herein. Other variations known to those skilled in the art are possible. 
     FIG. 2  includes a flow chart that further illustrates an automatic set-point jam overload detection process  190  that occurs in some constructions including processes that may be carried out by software, firmware, or hardware. The programmable device  140  receives an input from the interface  120  indicating a request to start the motor  105 . In response, the programmable device  140  closes the contactor  125  and enters the process  190 . In other constructions, however, a second control system (not shown) is configured to receive the input from the interface  120 , and to start the motor  105  in response to the input. At block  200 , the programmable device  140  resets a timer, an overload set point threshold, and other related parameters. At block  205 , the programmable device  140  determines whether the motor  105  is running. If the programmable device  140  determines that the motor  105  is running (“Yes” path of block  205 ), the programmable device  140  proceeds to block  210 . Otherwise, if the programmable device  140  determines that the motor  105  is not running (“No” path of block  205 ), the programmable device  140  returns to block  200 . 
   At block  210 , the programmable device  140  acquires a timer value (e.g., from the memory  145 ) after the motor  105  has started for an initial period of time such as 0.5 seconds. In some constructions, the timer value varies from about 1 second to about 10 seconds. At block  215 , the programmable device  140  starts or increments the timer based on the timer value. The programmable device  140  then repeats blocks  220  and  225  until either when the timer has not run out (“No” path of block  220 ) and the motor  105  is no longer running (“No” path of block  225 ), or when the timer times out (“Yes” path of block  220 ). At block  230 , the programmable device  140  reads a plurality of operating parameters of the motor  105  such as an amount of current drawn or a steady state current value from the sensor  135 , and at block  235 , determines and writes a set point threshold for the overload device  140  based on a portion of the determined operating parameters. In some constructions, the set point threshold can be a percentage (e.g., from about +0.5 percent to about +10 percent of a statistical value) above the statistical value derived from the portion of the operating parameters determined at block  230 . Exemplary statistical values include averages, means, variances, standard deviations, and the like. 
   At block  240 , the programmable device  140  determines whether the motor  105  is running. If the programmable device  140  determines that the motor  105  is running (“Yes” path of block  240 ), the programmable device  140  proceeds to block  245 . Otherwise, if the programmable device  140  determines that the motor  105  is not running (“No” path of block  240 ), the automatic set-point jam overload detection process  190  terminates. In other constructions, if the programmable device  140  determines that the motor  105  is not running (“No” path of block  240 ), the programmable device  140  returns to block  200 . 
   At block  245 , the programmable device  140  determines from one value of the determined operating parameters such as the drawn motor current whether the one value of the determined operating parameters is greater than the set point threshold for a time period (e.g., about 8 ms). In some constructions, the one value of the determined operating parameters is a value from of the portion of the determined operating parameters. In other constructions, the one value of the determined operating parameters is a value of the operating parameters determined at block  230 . 
   Thereafter, the controller  110  operates the motor  105  based on decisions generated at block  245 . In some constructions, if the programmable device  140  determines that the one value of the determined operating parameters is greater than the set point threshold for the time period (“Yes” path of block  245 ), the motor  105  shuts down and an error is indicated at block  250  in a stop mode or stop level. In other constructions, if the programmable device  140  determines that the one value of the determined operating parameters is greater than the set point threshold for the time period (“Yes” path of block  245 ), the motor  105  runs at a level that corresponds to a portion of the rated capacity such as ten percent of the rated torque before shutting down and displaying the error at block  250  in a run mode or run level. In this way, the programmable device  140  initially slows down the motor  105  before shutting down the motor  105 . If the programmable device  140  determines that the one value of the determined operating parameters is less than the set point threshold for the time period (“No” path of block  245 ), the programmable device  140  repeats block  240 . 
   In still other constructions, after the programmable device  140  has determined the statistical value of the portion of the determined operating parameters and the percentage that can be used as the set-point threshold at block  235 , the programmable device  140  proceeds to determine a first difference between the one value of the determined operating parameters and the statistical value also at block  235 . At block  245 , the programmable device  140  determines a second difference by comparing the first difference with the percentage. If the programmable device  140  determines that the first difference is greater than the percentage (“Yes” path of block  245 ), the programmable device  140  repeats block  250 , as described earlier. Otherwise, if the first difference is not greater than the percentage (“No” path of block  245 ), the programmable device  140  repeats block  240 , as described earlier. 
   Although  FIG. 2  shows that the programmable device  140  executes operations at blocks  210 - 250  only once, the automatic set-point jam overload detection process  190  can also configure the programmable device  140  to execute operations at blocks  210 - 250  repeatedly. As an example,  FIG. 3  shows a second automatic set-point jam overload detection process  190 ′ which repeats a portion of the automatic set-point jam overload detection process  190  to adapt the set-point thresholds and the percentage (as determined at block  235 ) to the statistical value at various periods of operating time, wherein like blocks are referenced with like numerals. 
   Particularly, as shown in  FIG. 3 , the programmable device  140  acquires a first timer value (such as about 1 second) at block  210 , and executes the operations at blocks  215  through  250  as described below. At block  230 , the programmable device  140  reads a plurality of operating parameters of the motor  105  from the sensor  135  as described. At block  235 , the programmable device  140  determines and writes a set point threshold for the overload device  140  based on a portion of the determined operating parameters for a period of time that corresponds to the first timer value, as described above (with an exemplary overload set-point of about 3 percent above the statistical value.) The programmable device  140  repeats any operations necessary at blocks  240 - 250 , as described above (with the portion having ten of the operating parameters, and an exemplary operating parameter sampling period of about 2 ms). The programmable device  140  then determines whether the timer has expired at block  220 ′. 
   If the programmable device  140  determines that the timer has expired (“Yes” path of block  220 ′), the programmable device  140  determines whether a next timer is needed at block  254 . Otherwise, if the programmable device  140  determines that the timer has not expired (“No” path of block  220 ′), the programmable device  140  continues to read additional operating parameters. 
   If the programmable device  140  determines that a next timer is needed (“Yes” path of block  254 ), the programmable device  140  repeats block  210  to set up other timer values (such as 2 seconds), and adapts other overload set-point thresholds (such as about 1 percent and 0.5 percent above the statistical value) with the portion having about twenty of the operating parameters and different operating parameter sampling periods (such as about 2 ms and 100 ms). Otherwise, if the programmable device  140  determines that a next timer is not needed (“No” path of block  254 ), the programmable device  140  terminates the automatic set-point jam overload detection process  190 ′. 
   Therefore, the invention provides a new and useful load overload device. While numerous aspects of the apparatus  100  were discussed above, not all of the aspects and features discussed above are required for the invention. Additionally, other aspects and features can be added to the apparatus  100  shown in the figures. The constructions described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the invention. 
   Various features and advantages of the invention are set forth in the following claims.