Abstract:
A braking module includes a braking load, an input terminal, an output terminal, and control logic. The input terminal is adapted to receive a motor drive signal. The control logic is adapted to receive a motor enable signal, couple the output terminal to the input terminal responsive to the motor enable signal being asserted, couple the output terminal to the braking load responsive to the motor enable signal being deasserted, and prevent the coupling of the output terminal to the input terminal responsive to a temperature of the braking module exceeding a predetermined disable set point. A method for controlling a motor includes coupling a drive lead carrying a motor drive signal to a motor lead of the motor responsive to a motor enable signal being asserted. The motor lead is coupled to a braking load responsive to the motor enable signal being deasserted. The coupling of the drive lead to the motor lead is prevented responsive to a temperature of the braking load exceeding a predetermined disable set point.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable 
   BACKGROUND OF THE INVENTION 
   The present invention relates to controlled resistive braking of non-regenerative AC drives and more particularly to a resistive braking module with thermal protection. 
   Power plants are linked to power consuming facilities (e.g., buildings, factories, etc.) via utility grids designed so as to be extremely efficient in delivering massive amounts of power. To facilitate efficient distribution, power is delivered over long distances as low frequency three-phase AC current. Despite being distributable efficiently, low frequency AC current is not suitable for end use in consuming facilities. Thus, prior to end use, power delivered by a utility is converted to a useable form. To this end, a typical power “conditioning” configuration includes an AC-to-DC rectifier that converts the utility AC power to DC across positive and negative DC buses (i.e., across a DC link) and an inverter linked to the DC link that converts the DC power back to three phase AC power having an end-useable form (e.g., three phase, relatively high frequency AC voltage). A controller controls the inverter in a manner calculated to provide voltage waveforms required by the consuming facility. 
   Motors and linked loads are one type of common inductive load employed at many consuming facilities and, while the present invention is applicable to several different load types, in order to simplify this explanation an exemplary motor and load will be assumed. To drive a motor an inverter includes a plurality of switches that can be controlled to link and delink the positive and negative DC buses to motor supply lines. The linking-delinking sequence causes voltage pulses on the motor supply lines that together define alternating voltage waveforms. When controlled correctly, the waveforms cooperate to generate a rotating magnetic field inside a motor stator core. In an induction motor, the magnetic field induces a field in motor rotor windings. The rotor field is attracted to the rotating stator field and thus the rotor rotates within the stator core. In a permanent magnet motor, one or more magnets on the rotor are attracted to the rotating magnetic field. 
   One technique for stopping a motor and linked load is to cut off power to the inverter such that the stator field is eliminated. Without power the stator and rotor fields diminish and eventually the rotor slows and stops. While this stopping solution is suitable for some applications, this solution is unacceptable in other applications where motors have to be stopped relatively quickly for safety or duty cycle concerns. 
   A technique for actively slowing the motor involves using a resistive brake circuit. The resistive brake includes braking resistors coupled across the phases of the motor and switches for enabling the braking resistors. When the switches are closed, and the motor is isolated from the drive unit (i.e., the drive signals are isolated), the motor effectively acts as a generator to provide current to the load created by the braking resistors. Hence, the energy stored in the rotor and stator fields and the inertial energy stored in the rotating motor/load are transferred to the braking resistors. The power transferred to the braking resistors is dissipated as heat. 
   Because, the energy stored in the motor is dissipated as heat, the brake unit may overheat in situations where the duty cycle between motoring and braking is short and the brake is exercised repeatedly. Typical resistive braking units employ wire-wound resistors and depend on overheating the resistor wire to the point of failure as a thermal overload protection. However, even before the point of failure, the heat may build up to a sufficient level that the temperature of the unit exceeds the Underwriters Laboratory (UL) requirements for safe touch. Moreover, the failure mechanism of the wire-wound resistors limits the range of applications in which they may be used in terms of motor size and duty cycle. 
   Another technique for braking a rotating motor involves controlling the inverter that supplies the drive signals to the motor such that the drive signals lag the motor fields (i.e., typically the drive signals lead the motor fields to drive the motor). The motor acts as a generator in this situation, and the power generated thereby can be dissipated by the inverter as heat or transferred back to the DC bus in a regenerative fashion. This technique requires more complex inverter circuitry and control logic, thereby increasing cost. Additionally, if a motor and drive unit configured to support a non-braking application is instead to be used to support an application that requires braking, the entire drive unit would have to be changed to facilitate the braking feature. 
   Therefore, there is a need for a resistive braking system that can stop a load (e.g., motor and connected load) within a given time period that requires a relatively small and inexpensive brake mechanism that can be installed with an exiting equipment base and that will maintain operating temperatures within desired operating limits. 
   This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
   BRIEF SUMMARY OF THE INVENTION 
   The present inventors have recognized that a resistive braking module may be constructed including thermal protection that reduces the likelihood that the temperatures of the braking module may exceed a desired limit due to intentional or unintentional frequent exercising of the braking module. The present inventors have also recognized that a resistive braking module may also be implemented that can be connected between a motor drive unit and a motor to provide braking capability for the motor without requiring modification to the motor or the drive unit. 
   One aspect of the present invention is seen in a braking module including a braking load, an input terminal, an output terminal, and control logic. The input terminal is adapted to receive a motor drive signal. The control logic is adapted to receive a motor enable signal, couple the output terminal to the input terminal responsive to the motor enable signal being asserted, couple the output terminal to the braking load responsive to the motor enable signal being deasserted, and prevent the coupling of the output terminal to the input terminal responsive to a temperature of the braking module exceeding a predetermined disable set point. 
   Another aspect of the present invention is seen in a braking module including a plurality of input terminals, a plurality of output terminals, a braking load coupled across the output terminals, and control logic. The input terminals are adapted to receive a motor drive signal including a plurality of phase components. The control logic is adapted to receive a motor enable signal, couple the output terminals to the input terminals responsive to the motor enable signal being asserted, couple the output terminals to the braking load responsive to the motor enable signal being deasserted, and prevent the coupling of the output terminals to the input terminals responsive to a temperature of the braking module exceeding a predetermined disable set point. 
   Still another aspect of the present invention is seen in a method for controlling a motor. The method includes coupling a drive lead carrying a motor drive signal to a motor lead of the motor responsive to a motor enable signal being asserted. The motor lead is coupled to a braking load responsive to the motor enable signal being deasserted. The coupling of the drive lead to the motor lead is prevented responsive to a temperature of the braking load exceeding a predetermined disable set point. 
   These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made, therefore, to the claims herein for interpreting the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
       FIG. 1  is a schematic diagram of a motor control system in accordance with one embodiment of the present invention; 
       FIG. 2  is a schematic diagram of a resistive braking module in the motor control system of  FIG. 1 ; 
       FIG. 3  is a simplified flow diagram of motor enabling logic implemented by the resistive braking module of  FIG. 2 ; and 
       FIG. 4  is a simplified flow diagram of a temperature warning process implemented by the resistive braking module of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   One or more specific embodiments of the present invention will be described below. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
   Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to  FIG. 1 , the present invention shall be described in the context of an exemplary motor control system  100 . The motor control system  100  includes a drive unit  110  and a motor  120  coupled to a load  130 . A resistive braking module  140  is coupled between the drive unit  110  and the motor  120 . The resistive braking module  140  receives drive leads  150  from the drive unit  110  at input terminals  155  and connects to motor leads  160  of the motor  120  at output terminals  165 . In the illustrated embodiment, the motor  120  is a three-phase permanent magnet motor and the leads  150 ,  160  provide the three phase inputs (i.e., W, V, and U) for the motor  120 . The drive unit  110  also provides a contactor coil signal for enabling the motor  120 . Although not illustrated, as is well known in the motor controls art, cable shield clamps may be used in conjunction with the connections between the drive unit  110 , the resistive braking module  140 , and the motor  120 . The application of the present invention is not limited to any particular type or size of motor. 
   Although the resistive braking module  140  is illustrated as being physically separate from the drive unit  110 , the application of the invention is not so limited. In some embodiments, the resistive braking module  140  may be integrated into the drive unit  110 . One advantage of a separate resistive braking module  140  is that it may be installed with an existing equipment base. As such, the motor  120  can be equipped with resistive braking capability without requiring a different drive unit  110 . 
   In general, the drive unit  110  produces positive and negative voltage pulses in specific sequences to generate AC voltages having controllable amplitudes and frequencies on the drive leads  150 . The construct and operation of drive units for performing this function are well known to those of ordinary skill in the art. An exemplary drive unit  110  capable of performing this function is a drive in the Kinetix 6000 drive family offered commercially by Rockwell Automation, Inc. of Milwaukee, Wis. The AC voltages provided by the drive unit  110 , cause varying currents that induce a rotating magnetic field within a stator core (not illustrated) of the motor  120 . A motor rotor (not illustrated) which is linked to a motor shaft  170  resides within the stator core. The rotor includes either permanent magnets (i.e., a permanent magnet motor) or windings (i.e., an induction motor) that interact with the magnetic field in the stator to cause the rotor to rotate within the stator core. The load  130  is attached via the shaft  170  to the rotor and therefore, when the rotor rotates, the load  130  also rotates in the same direction. 
   Turning now to  FIG. 2 , a schematic diagram of the resistive braking module  140  in the motor control system  100  of  FIG. 1  is illustrated. The resistive braking module  140  receives a contactor coil signal from the drive unit  110  indicating the desired operating state (i.e., driven or decelerating) of the motor  120 . When the contactor coil signal is asserted to activate the motor  120 , the drive unit  110  provides drive voltages on the drive leads  150  for operating the motor. The resistive braking module  140  includes a contactor  200  that is activated by the contactor coil signal. The contactor  200  includes normally open load contacts  205 ,  210 ,  215  coupled in a “Y” configuration between the drive leads  150  and a common node  241 . When the contactor coil signal is asserted, the load contacts  205 ,  210 ,  215  close, and the drive voltages are applied via the drive unit  110  to drive the motor  120 . An exemplary contactor  200  suitable for use in the resistive braking module  110  is an Allen-Bradley 100S series contactor offered commercially by Rockwell Automation, Inc. 
   When the contactor coil signal is deasserted to deactivate the motor  120 , the contactor  200  is deactivated, and the load contacts  205 ,  210 ,  215  return to their normally open state and cut off the drive voltages to the motor  120 . However, when the motor  120  and load  130  are still rotating, the stored field energy and inertial energy must be dissipated to stop the motor  120 . 
   The contactor  200  also includes normally closed auxiliary contacts  220 ,  225 ,  230 ,  235  mechanically linked to the load contacts  205 ,  210 ,  220  that close when the contactor coil signal is removed and the contactor  200  is deactivated. The auxiliary contacts  220 ,  225 ,  230  couple braking resistors  240 ,  245 ,  250  across the phases of the motor leads  160 . The motor  120  acts as a generator in this state that drives the electrical load created by the braking resistors  240 ,  245 ,  250  to dissipate the energy stored in the motor/load combination. The energy of the motor/load is dissipated as heat in by the braking resistors  240 ,  245 ,  250 . 
   In the illustrated embodiment, the braking resistors  240 ,  245 ,  250  are ceramic bar style resistors mounted to a fiberglass insulator. Of course, other types of resistors may be used depending on the particular implementation. The capacity and resistance values of the braking resistors  240 ,  245 ,  250  are implementation specific and depend on factors such as the RMS current and instantaneous peak current generated by the largest motor  120  intended to be used with the resistive braking module  140 , the speed range and inertial mismatch, the intended duty cycle, etc. 
   The auxiliary contact  235  provides a contactor status signal (ConStat). The resistive braking module  140  receives an external IO_PWR signal, which is provided to the auxiliary contact  235 . When the contactor  200  is open (i.e., the motor  120  is off), the auxiliary contact  235  is closed and the IO_PWR signal is passed to a contactor status output terminal  255 . When the contactor  200  is closed (i.e., the motor  120  is operating), the auxiliary contact  235  is open. The IO_PWR signal is interrupted and no voltage appears at the contactor status output terminal  255 . Hence, the ConStat signal is at a high logic state when the motor  120  is off and at a low logic state when the motor  120  is operating. The ConStat signal may be passed to a programmable logic controller (PLC) (not illustrated) or other control circuitry, depending on the particular implementation. The contactor status output terminal  255  may also be coupled to a local indication device, such as an LED to indicate the contactor status. 
   As the duty cycle of the resistive braking module  140  increases, the temperature of the braking resistors  240 ,  245 ,  250  rises as heat dissipated therein does not have time to transfer to the ambient environment. Accordingly, the temperature of the resistive braking module  140  itself rises, and, if unchecked, the temperature may rise above established Underwriters Laboratory (UL) safe touch standards. To reduce the likelihood of the resistive braking module  140  exceeding safe touch temperature limits, thermal protection is provided through normally closed thermal limit switches  260 ,  265 . 
   The thermal limit switch  260  provides an elevated temperature warning (Temp_Warn) if the temperature exceeds a temperature warning set point of approximately 65 degrees Celsius. The external IO_PWR signal is also provided to the thermal limit switch  260 . If the temperature is below its activation point, the thermal limit switch  260  is closed and it passes the IO_PWR signal to a temperature warning output terminal  270 . If the temperature of the resistive braking module  140  exceeds the set point of the thermal limit switch  260 , the switch  260  opens and interrupts the IO_PWR signal. The temperature warning output terminal  270  may be connected to a PLC or other circuitry to indicate the temperature warning. Automatic or manual corrective actions may be taken to avoid a further temperature increase. For example, an operator or controller may increase the interval between subsequent contactor coil signal assertions to allow the braking resistors  240 ,  245 ,  250  to cool. 
   If the temperature of the resistive braking module  140  continues to increase beyond a disable set point, the thermal limit switch  265  opens and interrupts the contactor coil signal to the contactor  200 . In the illustrated embodiment, the nominal disable set point of the thermal limit switch  265  is about 80 degrees Celsius. If the motor  120  is in a deactivated state when the thermal limit switch  265  opens, the contactor  200  is prevented from activating. Hence, the motor  120  cannot be energized until after the temperature of the resistive braking module  140  has cooled down below the set point and the thermal limit switch  265  closes. Because the braking resistors  240 ,  245 ,  250  are exercised upon a motor shutdown, it is likely that the temperature limit switch  265  will open between cycles of the motor  120  as the temperature increases from braking operation. However, in the event that the motor  120  is currently operating when the thermal limit switch  265  opens, the contactor  200  is deactivated, which opens the load contacts  205 ,  210 ,  215  and closes the auxiliary contacts  220 ,  225 ,  230  to enable the braking resistors  240 ,  245 ,  250  and stop the motor  120 . The auxiliary contact  235  also closes and generates the ConStat signal. A mismatch between the Contactor Coil signal and the ConStat signal indicates that the motor  120  has been shut down during operation due to a temperature limit violation in the resistive braking module  140 . 
   The temperature warning and disabling set points described above are exemplary, and may vary depending on the particular implementation. The thermal limit switches  260 ,  265  may be located in various positions on the resistive braking module  140 . For example, they may be located near or in contact with an outer housing of the resistive braking module  140  to monitor the contact temperature of the resistive braking module  140 . Alternatively, the thermal limit switches  260 ,  265  may be located near the braking resistors  240 ,  245 ,  250 . The set points of the thermal limit switches  260 ,  265  will depend, in part, on where they are positioned within the resistive braking module  140 . 
   The logic implemented above by the thermal limit switches  260 ,  265  and/or contacts  205 ,  210 ,  215 ,  220 ,  225 ,  230 ,  235  is exemplary. Equivalent arrangements may be implemented using different arrangements of normally open or normally closed thermal limit switches and/or contacts. 
   A simplified flow diagram of the logic implemented by the resistive braking module  140  is illustrated in  FIG. 3 . The method initiates in block  300 . In block  310 , the resistive braking module  140  monitors the status of the ContactCoil signal. If the ContactCoil signal is deasserted, the drive unit  110  seeks to stop the motor  120  and the motor leads  160  are isolated in block  320 , and the braking load (e.g., the braking resistors  240 ,  245 ,  250 ) is enabled in block  330 . The motor leads  160  remain isolated and the braking load remains enabled until a subsequent assertion of the ContactCoil signal is identified in block  310 . 
   If the ContactCoil signal is asserted in block  310 , the drive unit  110  seeks to operate the motor  120 . In block  340 , the resistive braking module  140  determines if the temperature is above the disable set point (e.g., 80 degrees C.). If the temperature is below the disable set point, the braking load is isolated in block  350  and the drive leads  150  are coupled to the motor leads  160  in block  360 . If the temperature is above the disable set point in block  340 , the resistive braking module  140  isolates the motor leads  160  in block  320  and enables the braking load in block  330 . 
   The steps of isolating or enabling the motor leads  160  and/or braking load do not require changes of state, but rather, if the logic state does not change, the state of the connection does not change. For example, if the motor leads  160  are coupled to the drive leads  150  and the temperature remains below the disable set point, the leads  150  remain coupled. 
   As illustrated in  FIG. 4 , the resistive braking module  140  implements temperature warning logic starting at block  400  in parallel with the motor enabling logic of  FIG. 3 . If the temperature is above the warning set point in block  410 , the resistive braking module  140  issues a temperature warning in block  420 . If the temperature is below the warning set point in block  410 , the resistive braking module  140  clears any existing temperature warning in block  430  and continues monitoring the temperature in block  410 . 
   The resistive braking module  140  described herein has numerous advantages. Because it can be connected between the drive unit  110  and the motor  120 , it may be used with an installed equipment base. Also, the thermal protection provided by the resistive braking module  140  reduces the likelihood that its temperature will exceed safe touch limits through intentional or unintentional frequent cycling of the braking load. 
   The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.