Abstract:
A controller for a variable frequency drive monitors electrical power from a variable frequency drive motor while a brake is maintaining a load driven or moved by the motor without requiring additional feedback components of a closed loop configuration. If excess electrical power is being generated by the motor, an undesirable condition in the brake is indicated. Support or maintenance of the load is assumed by the motor in that event. Appropriate alarms or indicators are also activated.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates generally to variable frequency driven motors and in particular to brake systems operating with variable frequency driven motors. Still more particularly, the present invention relates to a method and apparatus for responding to brake failure in open loop variable frequency driven motors. 
   2. Description of the Related Art 
   Variable frequency driven motors are utilized for a variety of lifting mechanisms, from overhead cranes and hoists to elevators. Typically, when a load supported by a variable frequency driven motor is to be held in suspension for a significant period of time, an external brake is set and the motor is disengaged from supporting the load. This allows savings in the power required for supporting the load using the motor and also saves the motor from unnecessary wear. When a brake, however, is employed to hold a load in suspension, various events may result in the inadvertent release of the load. The brake can be out of adjustment; the brake pads can be worn to the extent of reduced torque capability, or some other mechanical failure. In any case, the effect of dropping a load may be disastrous. 
   U.S. Pat. No. 6,097,165, by Herron, titled “Method and Apparatus for Handling Brake Failure in Variable Frequency Drive Motors,” which is commonly owned with the subject matter of the present application, provides techniques for handling brake failure in closed loop variable frequency drive motors. When the motor is stopped and the external brake set, pulse generator feedback is furnished from the motor and is monitored in a closed loop variable frequency drive. When the monitored feedback exceeds an alarm level, a brake problem is indicated. The motor is activated to maintain the load supported by the brake and an alarm energized. 
   So far as is known, until the present invention, brake failures have been determined with the aid of a closed loop system, with a pulse generator mounted on the motor used in conjunction with a closed loop variable frequency drive. Systems of this type, while desirable, can require the use of either shielded or fiberoptic cable to provide the feedback signals. In addition, encoder and additional control boards can be required. These items can increase the costs of such systems and also raised additional maintenance and testing considerations. 
   Recognized is that it would be desirable, therefore, to provide an apparatus and related methods for preventing a load elevated by a variable frequency driven motor and supported by a brake that could prevent dropping the load in the event of brake failure and that does not require a closed loop system. It would further be desirable if the apparatus could be incorporated into existing commercial embodiments of open loop variable frequency driven motor systems or apparatus without the introduction of numerous additional components. 
   SUMMARY OF INVENTION 
   In view of the foregoing, embodiments of the present invention advantageously provide a method and apparatus for determining and handling failures in brakes maintaining a load driven by a variable frequency drive motor. For example, in various embodiments of the present invention, a variable frequency drive is provided with the capability of monitoring an increase in voltage or power caused by the motor being in a generating state. If an increase in voltage or power is determined to be from the output of the open loop variable frequency drive (i.e. due to the motor overhauling), the drive can place the motor in a brake fail sequence. Once the brake is determined to be in a failed condition, an output alarm condition can be annunciated and the load can be automatically lowered at a safe rate of speed. Advantageously, the present invention can provide these results in an open loop configuration, without requiring a closed loop feedback mechanism. 
   Specifically, embodiments of the present invention provide an apparatus for responding to malfunctions of a brake when the brake is maintaining a load. The apparatus can include a variable frequency drive preferably in the form of an open loop variable frequency drive for driving a variable frequency drive motor which is adapted to move the load. The apparatus also includes a brake for stopping lifting or lowering of the load controlled by the variable frequency drive via a brake control switch. The variable frequency drive includes a controller adapted to monitor electrical power, such as voltage, generated by the motor when the brake is set on, excessive voltage being indicative of brake failure, to thereby determine if a brake failure exists. The variable frequency drive can include a DC bus having a voltage level and which receives power generated by the motor. The controller can monitor the voltage levels of the DC bus to thereby detect power generated by the motor to determine if the brake failure exists. The controller can be further adapted to respond to detection of excess power being generated by the motor, supporting the load with the motor. The variable frequency drive, responsive to determination of the brake failure is adapted to apply power to the motor until indications of brake failure no longer exist. 
   Embodiments of the present invention provide a method of handling brake failure for a variable frequency driven motor maintaining a load. The controller for the variable frequency drive monitors electrical power from the motor, preferably voltage levels of the power generated by the motor, to thereby determine if a brake failure exists. This is accomplished without the need for additional feedback components of a closed loop configuration. The controller can determine whether excess power is being generated by the motor when the brake is set on. Responsive to such determination, the controller can maintain the provision of a selected amount of torque from the motor to support the load with the motor, typically by lowering the load at a safe speed. If it is determined that excess voltage is not being generated by the motor, the controller can reduce the reverse torque to substantially zero to allow support of the load with the brake. 
   Embodiments of the present invention can also include a computer program product in a computer usable medium. The computer program product can include instructions for monitoring electrical power from an open loop variable frequency drive motor maintaining a load while the motor is stopped, and instructions to determine whether excess power is being generated by the motor, which is indicative of brake failure. The computer program product can also include instructions responsive to the determination that excess power is being generated by the motor to support the load with the motor. In a preferred implementation, the instructions can be stored in a memory of the controller of the variable frequency drive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the features and advantages of the invention, as well as others which will become apparent, may be understood in more detail, a more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof which are illustrated in the appended drawings, which form a part of this specification. It is to be noted, however, that the drawings illustrate only various embodiments of the invention and are therefore not to be considered limiting of the invention&#39;s scope as it may include other effective embodiments as well. 
       FIGS. 1–3  are schematic diagrams of an apparatus for handling brake failure for a variable frequency driven motor illustrating various positions of a brake with respect to a motor and a transmission, according to embodiments of the present invention. 
       FIG. 4  is a high-level flowchart for a process of handling brake failure for a variable frequency driven motor, according to an embodiment of the present invention. 
       FIG. 5  is a high-level flowchart for a process of handling brake failure for the variable frequency driven motor according to an embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention will now be described more fully hereinafter with reference to the accompanying drawings which illustrate embodiments of the invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and the prime notation, if used, indicates similar elements in alternative embodiments. 
   With reference now to the figures, and in particular with reference to  FIGS. 1–3 , depicted is an apparatus for handling brake failure for a preferably open loop variable frequency driven motor  104  in accordance with embodiments of the present invention. The apparatus includes a motor  104  connected to a gearbox  106  via a common shaft  108 . Shaft  108  can be a multi-segment or unitary shaft ( FIG. 1 ). Shaft  108  can include a shaft segment  109  ( FIG. 2 ) connected to or through motor  104  or a shaft segment  110  ( FIG. 3 ) connected to or through gearbox  106 . Shaft segments  109 ,  110 , can be unitary with shaft  108  or the connected by means known to those skilled in the art. Gearbox  106  may optionally include a load brake  111  designed to retard loads from falling when approximately zero torque is applied by motor  104 . Selectively operable on shaft  108  is an electromechanical brake  112 , which may support loads when motor  104  is stopped and/or applying at or near zero torque. Brake  112  can be electrically connected to input  114  from a conventional external electrical power source (not shown) via switch  116 , which actuates brake  112 . Switch  116  is controlled by VFD  118  through line  115 . In the preferred configuration, the brake  112  is configured such that electrical power can be supplied to the brake  112  via switch  116  in order to release the brake  112 . Brake  112 , correspondingly, can be connected directly to shaft  108  ( FIG. 1 ) or to either shaft extension  109 ,  110  ( FIGS. 2 and 3 ), of shaft  108 . 
   Motor  104  preferably receives power from an open loop variable frequency drive  118 , which is also connected by input  114  to the external electrical power source. Variable frequency drive  118  has, in the past, been preferred to be that of a flux vector drive (i.e. one that has encoder feedback). However, with this invention, the variable frequency drive  118  can be an open loop vector drive employing a mathematical model for controlling the operation of motor  104 . Such drives are known in the art. Open loop enable frequency drives, such as a Model G7 available from Toshiba, for example, may be employed. 
   The open loop variable frequency drive  118  includes a controller  119 , which is selectively programmable to control operation of the variable frequency drive  118  and the switch  116 , and includes a memory  120  for storing various programming instructions, which can be entered using various forms of machine readable medium (not shown). The variable frequency drive  118  also can include a DC bus  121 . A preferably internal voltage regulator  122  can be electrically connected to the DC bus  121  to regulate voltage on the DC bus  121 . In the preferred configuration, the internal voltage regulator  122  includes an internal breaking transistor (not shown) and resistor (not shown) to dissipate any excess voltage or power generated by the motor  104 . The controller  119  can monitor the voltage on the DC bus  121 . In the preferred configuration, the controller  119  can control voltage suppression of the DC bus  121  through control of the voltage regulator  122 . An alarm or other similar device (not shown), electrically connected to the controller  119 , can be further provided to annunciate the existence of a failure of brake  112 . Advantageously, through the instructions stored in memory  120 , the open loop vector variable frequency drive  118  can be programmed to operate as described below for handling of a brake failure. 
   Referring to  FIGS. 4 and 5 , illustrated is a high level flowchart for a process of handling a brake failure in a open loop vector variable frequency driven motor  104  in accordance with a preferred embodiment of the present invention. There are generally two different scenarios possible in the brake check sequence, these being: a forward/raising sequence and a reverse/lowering operation. 
   Referring to  FIG. 4 , shown is the flowchart for determining and handling brake failure during the reverse/lowering operation. The process begins at step  200 , which depicts the motor running in the reverse/lowering direction at some desired speed. The process then continues to step  210 , which indicates the drive  118  has received a stop command from the operator. When the stop is initiated, the drive  118 , under control of instructions stored in memory  120 , proceeds to line  220  and gradually decreases or ramps from the previous speed (frequency) down to a brake set frequency, typically approximately 2 Hz. Upon reaching the brake set frequency, the drive  118 , through control line  115 , commands the brake  112  to set by removing power from the brake  112  (line  230 ) and reduces the internal reverse torque limit to some suitable lower limit, such as 10% of rated torque, for example, as outlined in line  240 . Operation at the suitable torque then continues at the brake set frequency for a brake set time, the reverse torque being applied with the brake  112  set on. Note, torque, brake set frequency, and brake set time preferably can be preselected and stored in memory  120  for access by controller  119 . Note also, though having the brake  112  set by removing power provides a fail safe control, the brake  112  can alternatively be implemented such that it is set by application of power rather than removal of power. 
   The next step in the operating sequence of  FIG. 4 , line  250 , is to determine if the brake  112  is functioning correctly. To do this, the variable frequency drive  118  monitors the DC bus level to detect if the motor  104  is in a generating state, to thereby determine if a brake failure exists. In the preferred configuration, if the controller  119  of the variable frequency drive  118  detects a noticeable voltage increase, indicative of brake failure, the drive  118  can proceed to line  260  on the flow chart, which is the brake fail sequence. The controller  119  of the variable frequency drive  118  can monitor the DC bus  121  and compare the voltage level of the DC bus  121  to a preselected or predetermined brake failure voltage level, an overvoltage or increased level of which is indicative of brake failure. Thus, the DC bus  121  can provide the controller  119  indications of overhauling (generating) by the motor  104 . Note, the above described reference voltage level need not be a fixed value but maybe relative to that expected according to relative environmental conditions. 
   Responsive to the detection of the increased voltage in the DC bus  121 , the controller  119  can further control and monitor the status of the voltage regulator  122 . Further, in the preferred configuration, the controller  119  can control an internal braking transistor or voltage dissipater circuit (not shown) of the voltage regulator  122 . Advantageously, control of the voltage regulator  122 , by the controller  119 , provides intelligent power regulation. More specifically, as a result of such provisions for monitoring the voltage on the DC bus  121  and control of the voltage regulator  122 , even though the voltage applied to the DC bus  121  may not appear to an outside observer to be excessive due to voltage suppression by the voltage regulator  122 , such implementation advantageously allows the controller  119  to determine and signal the existence of a brake failure, brake failure being indicated by application of an increasing voltage or excessive voltage applied to the DC bus  121  by the motor  104  as a result of the motor  104  being in a generating or overhauling state. 
   The brake fail sequence (line  260 ) starts an infinite loop until the regeneration condition is removed. Operation of the motor  104  is continued (line  270 ) in the lower/reverse direction at the preselected suitable lower limit, such as 10% of rated torque, at preferably the brake set frequency, so that the load can be automatically lowered at a safe rate of speed. Ultimately, this may continue until the load is placed on the ground. If, however, the drive  118  detects no noticeable voltage increase for the brake set time, the drive  118  then shuts down (line  280 ) and waits for the next command. 
   Referring to  FIG. 5 , shown is the flowchart for determining and handling brake failure during the forward/raising direction scenario, which begins on line  300 . The process continues to step  310 , which indicates the drive  118  has received a stop command from the operator. When the stop is initiated, the drive  118  proceeds to line  320  and gradually decreases or ramps from the previous speed (frequency) down to the selected brake set frequency. Upon reaching the brake set frequency, the drive  118 , through control line  115  and switch  116 , commands the brake  112  to set (line  330 ). Operations then will continue at a suitable torque at the brake set frequency for a brake set time, the forward torque being applied with the brake  112  set on. Note, torque, brake set frequency, and brake set time preferably can be preselected and stored in memory  120  for access by processor  119 . Note also, these programmable parameters need not be the same as those used for the reverse/lowering direction scenario. 
   As shown in line  340 , once the brake set time has timed out, the drive  118  changes the drive direction to the reverse/lower direction and can simultaneously change the reverse torque limit to some suitable lower limit, such as 10% of rated torque. From this step, the drive  118  then follows the same steps (beginning at line  240 ) as shown in the reverse/lowering flow chart ( FIG. 4 ). If it is determined (line  350 ) the brake  112  is not functioning correctly, the brake fail sequence (line  360 ) is entered. The brake fail sequence (line  360 ) starts an infinite loop until the regeneration condition is removed. Operation of the motor  104  is continued (line  370 ) in the lower/reverse direction at the preselected suitable lower limit, such as 10% of rated torque, at preferably the brake set frequency, and the load can be automatically lowered at a safe rate of speed. If, however, the drive  118  detects no noticeable voltage increase for the brake set time, the drive  118  then shuts down (line  380 ) and waits for the next command. 
   Regardless of which scenario is implemented, once the brake  112  is determined to have failed or is failing, the variable speed drive  118  can annunciate or activate an output to be interfaced with a horn of some suitable type, or other suitable alarm indicator or indicators (not shown), to indicate a brake failure. Additionally, the forward/hoisting speed can also be limited for additional annunciation to the operator. 
   The invention has significant advantages. Embodiments of the present invention provide an apparatus and method for handling brake failure or inadvertent release of a load carried by variable frequency drive motors. It can be seen that embodiments of the present invention provide an open loop variable frequency drive with the capability of detecting movement of a suspended load. The variable frequency drive can monitor for, or determine the existence of, an increase in voltage due to the motor in the generating state, indicating brake failure or release. If such an increase in voltage is determined to be from the variable frequency drive motor (i.e. due to the motor overhauling) the motor is placed in a brake fail sequence. The motor can provide sufficient torque to allow a controlled descent of the load. Additionally, an alarm can be sounded to allow an operator to safely lower the load. Until now, brake failures were only determined with the aid of a closed loop system, a pulse generator mounted on the motor and used in conjunction with a closed loop variable frequency drive. 
   In the drawings and specification, there have been disclosed a typical preferred embodiment of the invention, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation. The description of the preferred embodiment of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limit the invention in the form disclosed. The illustrated embodiments were chosen and described in order to best explain the principles of the invention and the practical application to enable others of ordinary skill in the art to understand the invention. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the invention as described in the foregoing specification. Various changes in the size, shape, materials, components, circuit elements, wiring connections and contacts, as well as in the details of the illustrated circuitry and construction and method of operation may be made without departing from the spirit of the invention. For example, most controllers provide a motor overload fault condition, in which a brake is applied and the motor stopped when motor overcurrent is detected. It may be desirable to disable this control when a brake failure is detected and being handled in accordance with embodiment of the present invention. That is, it may be preferable to allow the motor to burn itself out supporting the load rather than permit the load to be dropped due to brake failure. Also for example, though the illustrated example described lowering the load once the brake was determined to have failed, alternative operation can instead include activating an alternative breaking means responsive to detection of brake failure, rather than lowering the load. 
   Also, it is important to note that while the present invention has been described in the context of a fully functional variable frequency driven motor, those skilled in the art will appreciate that the mechanism of the present invention is capable of being distributed in the form of a computer readable medium of instructions in a variety of forms, and that the present invention applies equally regardless of the particular type of signal bearing media used to actually carry out the distribution. Examples of computer readable media include: recordable type media such as floppy disks and CD-ROMs and transmission type media such as digital and analog communication links.