Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1998-03-11
2001-03-27
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S076110, C324S545000, C361S093100, C318S434000
Reexamination Certificate
active
06208159
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an overcurrent monitoring system for use with an electrical motor and, in particular, a system for determining press operating conditions based on the electrical load on an electrical press drive motor used in a machine press.
2. Description of the Related Art
As a machine press operates, there are variations in the electrical load drawn or created by the press motor. If the current load on the motor exceeds the motor's capacity, damage and loss of use may ensue. For example, current overload may cause burnout to the motor and associated portions such as the bearings, bushings, or commutators.
Load variations may be attributed to normal press operational functions. Alternatively, load variations may not be the result of normal press functions. For effective protection, it is essential to detect normal press functions which effect electrical load.
Normal press operations that effect the electrical load include initial starting of the press drive motor, engaging the press clutch, and varying the press operating speed. Other changes which effect the electrical load include lubricating oil temperature changes and die changes.
If the electrical load increase is not attributed to normal press operations, the current increase could be an indication of a potential problem in press operation. For example, when bearings or bushings overheat, the press speed tends to slow down and there is an increase in press motor drive load. Therefore, an increase in the electrical load may indicate that press bushings or bearings are overheating. In order to safeguard the press, it is advantageous to cut power to the motor when an increase in current load is not attributed to a normal press function to permit service personnel to determine the actual cause of the increased motor load.
In the art, monitoring systems have been limited to monitoring the effective power consumption of the press motor. In addition, such motor monitoring is not constant; rather, the monitoring is limited to only a portion of the press stroke.
It is advantageous to account for variations in motor electrical current draw attributed to normal press operations. Such detection would make it possible to stop the drive motor and press before potential damage may be caused to the motor, press bearings and bushings. In turn, there is a financial benefit in preventing damage to the machine press.
The present invention prevents potential damage to a machine press motor and press when the motor electrical current draw is above predetermined limits.
SUMMARY OF THE INVENTION
According to the present invention, a system accomplishes detection of steady or transient electrical current overloads on a machine press motor. The invention measures the electrical current draw or load on the press drive motor. If the electrical current draw exceeds an acceptable load limit setpoint, i.e. a predetermined or calculated limit setpoint, the new system halts the drive press motor.
The invention determines the limit setpoint by incorporating and compensating for the current load variations associated with known operational functions. Such operational functions include, but not limited to, drive motor startup, clutch engagement, and speed change commands. When the inventional system detects one or more of these known operational functions, it may accordingly adjust the limit setpoint.
The procedure for adjusting the limit setpoint after the system detects an operational function is to add a predetermined offset to the present percent of full load current. The predetermined offset is a predetermined value which is added to the present percent of full load current to account for the electric load attributable to the detected operational function.
The invention utilizes countdown or delay timers when calculating the limit setpoints. These timers allow there to be a temporary increase in electrical load without halting current to the drive motor. When an electrical load is attributable to a detected operational function, a timer is set for a finite period of time. Consequently, the electrical current will not be halted while the timer is counting down.
The invention, in one form thereof, calculates a limit setpoint after it detects the press motor startup and activates the drive motor startup delay timer. The system then sets the limit setpoint by adding a predetermined offset to the present percent of full load current. The system continues to update the limit setpoint until the drive motor startup delay timer has expired.
The invention calculates a limit setpoint after it detects the press clutch has engaged and starts the clutch engaged startup delay timer. The system then sets the limit setpoint by adding a predetermined offset to the previous limit setpoint. The system continues to update the limit setpoint until the clutch engaged startup delay timer has expired.
The invention calculates a limit setpoint after it detects a requested large press speed change and starts the range dropout delay timer. The system creates a speed range which is calculated by taking a predetermined percentage of the requested speed and adding or subtracting the calculated percentage of speed from the requested speed.
For example, if the predetermined percentage was 15 percent and the speed request was 100 spm, the speed range would be 100 spm plus or minus 15 percent, in other words, 85 spm to 115 spm.
After the timer has expired, if the press speed is not within the requested press speed range, the system sets the limit setpoint by adding a predetermined offset to the present percent of full load current. It continues to update the limit setpoint until the press speed is within the requested press speed range.
The invention calculates a limit setpoint after it detects a small press speed change request and starts the new speed holdout delay timer. The system continuously updates the limit setpoint until the new speed holdout delay timer has expired. The system sets the limit setpoint by adding a predetermined offset to the present percent of full load current.
The invention also includes a nuisance check system that prevents halting current to the drive motor when the measured electrical load increase is short lived. The invention with this system sets a fault delay timer. While this timer is running, the invention will not halt current supplied to the drive motor. This prevents halting current supplied to the motor when the electrical load exceeds the limit setpoint for a duration shorter than the time period measured by the fault delay timer.
The invention further includes an averaging function which calculates the average electrical load over a predetermined period of time, e.g. an eight second period. If the average electrical load has dropped 2.5 percent of full load current below the previous limit setpoint, the limit setpoint is set by adding a predetermined offset to the calculated average electrical load. The purpose of the averaging function is to lower the limit setpoint as the average of the measured electrical load decreases.
For example, if the average measured electrical load is 47 and the previous limit setpoint is 50, the invention will compare 47 to 2.5 percent less than the previous set point, i.e., 47.5. Since 47 is less than 47.5, the invention will update the limit setpoint by adding a predetermined offset to the calculated average electrical load.
While the invention measures short term variations of electrical load while a machine press is operating, it can easily be adopted to measuring long term variations in electrical load. The invention could store limit setpoints for various different tool sets, die combinations, machine press conditions and tolerances.
One embodiment of the present invention is an apparatus for monitoring current draw of an electric motor. It measures the electric current draw of an electric motor and checks for operational functions. The system sets a limit setpoint and compares the limit setpoint to the measured electrical draw. Th
Deb Anjan K
Knuth Randall J.
Metjahic Safet
The Minster Machine Company
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