Method and apparatus for handling brake failure in variable frequency drive motors

The variable frequency drive of a motor monitors pulse generator feedback while the motor is stopped and an electromechanical brake is set. When the pulse generator feedback exceeds a predetermined level indicative of brake failure, tie controller actuates the motor to operate in zero servo mode and maintain the load. An alarm is also sounded, allowing an operator to safely lower the load.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates generally to variable frequency driven motors 
and in particular to brake systems within variable frequency driven 
motors. Still more particularly, the present invention relates to a method 
and apparatus for responding to brake failure in variable frequency driven 
motors. 
2. Description of the Related Art 
Variable frequency driven motors are utilized for a variety ot 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 to supporting the load using the motor and also saves 
the motor from unnecessary wear. 
When a brake is employed to hold a load in suspension, various events may 
result in release of the load, including brake failure and inadvertent 
release. The brake may fail as a result of a broken spring or a failed 
brake shoe. Inadvertent release of the load may result from accidental 
closing of contacts during maintenance. In either case, the effect of 
dropping the load may be disastrous. 
It would be desirable, therefore, to provide a mechanism for preventing a 
load elevated by a variable frequency driven motor and supported by a 
brake from dropping n the event of brake failure. It would further be 
advantageous if the mechanism could be incorporated into existing 
commercial embodiments of variable frequency driven motors without the 
introduction of additional components. 
SUMMARY OF THE INVENTION 
The variable frequency drive of a motor monitors pulse generator feedback 
while the motor is stopped and an electromechanical brake is set. When the 
pulse generator feedback exceeds a predetermined level indicative of brake 
failure, tie controller actuates the motor to operate in zero servo mode 
and maintain the load. An alarm is also sounded, allowing an operator to 
safely lower the load.

DETAILED DESCRIPTION 
With reference now to the figures, and in particular with reference to FIG. 
1, a variable frequency driven motor in accordance with a preferred 
embodiment of the present invention is depicted. The variable frequency 
driven motor 102 includes a motor 104 connected to a gear box 106 via a 
common shaft 108. Gear box 106 may optionally include a Load brake 110 
designed to retard loads from falling when zero torque is applied by motor 
104. Selectively operable on shaft 108 is an electromechanical brake 112, 
which may support loads when motor 102 is stopped and/or applying zero 
torque. Brake 112 is electrically connected to an input 114 from an 
external power source (not shown) via switch 116, which actuates brake 
112. 
Motor 104 receives power from variable frequency drive controller 118, 
which is also connected(to input 114 from the external power source. 
Variable frequency drive 118 is preferably a flux vector technology drive 
employing a mathematical model followed by the drive in controlling the 
operation of motor 104. Such drives are known in the art, and drives 
manufactured by Saftronics, Inc., for example, may be employed for 
variable frequency drive 118. Variable frequency drive 118 includes a 
memory 120 which is selectively programmable to control operation of 
variable frequency drive 118. 
Variable frequency drive 118 receives feedback from pulse generator 122 
(also sometimes called a "motor encoder" or "motor position encoder") 
attached to or forming a part of motor 104. Pulse generator 122 is 
preferably a 1024 pulse-per-revolution (ppr) pulse generator. Feedback 
from pulse generator 122 allows variable frequency drive 118 to operate 
rotor 104 in zero servo (or "load float") mode, in which motor 104 applies 
torque to a load at zero speed. This is a known advantage of closed loop 
drives over open-loop controllers. 
Through instructions stored in memory 120, variable frequency drive 118 is 
programmed to operate as described below for handling of brake failure. 
Referring to FIG. 2, a high level flowchart for a process of handling brake 
failure in a variable frequency driven motor in accordance with a 
preferred embodiment of the present invention is illustrated. The process 
begins at step 202, which depicts the motor being stopped, which 
conventionally means that the motor is run at zero speed for at least one 
second, and the electromechanical brake applied and proven. At this point, 
power to the motor is typically discontinued. 
The process then passes to step 204, which illustrates monitoring of the 
pulse generator ("PG") feedback. As long as no pulse generator feedback is 
received, conditions are deemed satisfactory and the process loops back to 
step 204 to continue monitoring pulse generator feedback. When nonzero 
pulse generator feedback is detected, indicating movement of the load 
causing, in turn, movement of the motor, the process passes to step 206, 
which illustrates starting the pulse generator counter, in which a 
measurement of the pulse feedback from the pulse generator is stored. In 
some commercial embodiments of variable frequency drives, the register in 
which this value is stored is not normally accessible when the motor is 
stopped. However, the register may be located and suitable modifications 
made to permit the pulse generator count value stored in the register to 
be read while the motor is stopped. 
The process then passes to step 208, which depicts a determination of 
whether the pulse generator feedback exceeds a predetermined alarm level. 
The alarm level may be selected based on the sensitivity of the pulse 
generator and the desired allowance for load shifting due to external 
influences. For a 1024 ppr pulse generator, a suitable alarm level would 
be 10 pulses or more within a 50 millisecond period. As long as the pulse 
generator feedback does not exceed the selected alarm level, the process 
continues to simply monitor pulse generator feedback. Optionally, should 
the pulse generator feedback fall to zero, the process may return to step 
204, described above. 
If the pulse generator feedback exceeds the predetermined alarm level, the 
process proceeds from step 208 to step 210, which illustrates activating 
the Run and Zero Servo commands (or equivalents) for the variable 
frequency drive. The Run command may be employed to generate sufficient 
torque to return the motor to its position prior to the pulse generator 
exceeding the alarm limit, or merely to generate sufficient torque to 
maintain the motor in zero servo mode. Step 210 also illustrates releasing 
the electromechanical brake, so that the motor is independently holding 
the load. 
The process then passes to step 212, which depicts maintaining the zero 
servo value (ZSV) of the motor position and outputting an alarm. An 
operator may then take control of the device in which the motor and 
variable frequency drive are utilized and safely lower the load. The 
process then passes to step 214, which illustrates a determination of 
whether a Start command has been received from the operator controls. If 
not, the process returns to step 212 and continues maintaining the load 
and outputting an alarm. If so, however, the process passes instead to 
step 216, which depicts run the motor in the chosen direction in response 
to operator command. 
The process next passes to step 218, which depicts a determination of 
whether a Stop command has been received. If not, the process returns to 
step 216 and continues running the motor in the direction chosen. Once the 
stop command is received, the process proceeds instead to step 220, which 
illustrates a determination of whether the brake has been set and 
verified. If not, the process returns to step 212, maintaining the zero 
servo position of the motor and outputting an alarm to indicate continued 
brake failure. If the brake has been verified, however, the process 
returns instead to step 204, continuing monitoring the pulse generator 
feedback. 
The present invention provides a mechanism for handling brake failure or 
inadvertent release in variable frequency drive motors. When movement of a 
suspended load is detected, indicating brake failure or release, the motor 
is actuated to provide sufficient torque to independently support the 
load. An alarm is then sounded to allow an operator to safely lower the 
load. 
Various features may be desirable in specific implementations of the 
present 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 by the present 
invention. That is, it may be preferably to allow the motor to burn itself 
out supporting the load rather than permit the load to be dropped due to 
brake failure. 
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. 
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. 
Many modifications and variations will be apparent to those of ordinary 
skill in the art. The embodiment was 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 for 
various embodiments with various modifications as are suited to the 
particular use contemplated.