Electricity: motive power systems – With particular motor-driven load device – Tension-maintaining type of motor-control system
Reexamination Certificate
1999-11-12
2001-02-20
Dang, Khanh (Department: 2837)
Electricity: motive power systems
With particular motor-driven load device
Tension-maintaining type of motor-control system
C318S244000, C242S334000, C360S073140
Reexamination Certificate
active
06191542
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of electric motor repair and more specifically to the repair of a commutator to brush interface in a direct current motor without dismantling the motor.
In particular operations using a direct current, the commutator to brush interface can get dirty or covered with a coating commonly called a “varnish” which can cause intermittent and unreliable operation. The speed can vary causing undependable rotation. In a tape drive for a data processing system, constant motion of a tape over a transducer is compulsory. Any hindrance to a constant speed causes errors in the reading or writing of the data from the tape which can result in a failure in the data processing system. The rotational speed of the motor and thereby the linear speed of the tape must not vary. Reel to reel motion control systems in tape drives set the DC tension on the tape using calculated motor currents. When the motors are run at low currents for long periods of time the brushes get dirty and the connection to the commutator becomes unreliable. This results in data errors and tape damage when the system tries to move the tape and maintain the proper tension of the tape between the two reels. Brush motors cannot operate with small currents through the brushes for long periods of time in a servo control application such as magnetic tape drives. A high current density through the brush-commutator interface as well as high speeds are required to keep the interface clean and thereby maintain speed reliability. These are not always available especially in tape drives used in data processing systems. The tape has gotten very thin and consequently the tape drive required much lower current to maintain tape tension between the reels. the currents required for steady state motion are low once the tape is moving since the energy required to maintain constant motion is small. For a majority of the time, the motors are operating in a region, low current and low speed, where the self-cleaning action of the brush-commutator interface cannot take place.
A common method for cleaning the commutator-brush interface of a motor to spray the commutator with FREON, a chlorofluorocarbon compound, and then let the compound dry. An advantage of this cleaning method is that the FREON evaporates rapidly thereby eliminating any doubt about the armature windings having dried after being soaked by this cleaning process. Operation of the motor with wet windings must be avoided, as short-circuiting of the windings may occur. The indeterminate drying process and the environmental-unfriendliness attributed to the use of chlorofluorocarbons makes this process unusable, especially in a data processing system.
Another common solution is to dismantle the tape drive and remove and disassemble the motor for cleaning. In a small data processing system, the disconnection of the tape drive unit completely disrupts the operation of the data processing system and essentially shuts down its function. In a large data process unit, at minimum the productivity is affected.
An object of the present invention, therefore, is to provide a process for reinstating the reliability of a motor, especially a motor used in a tape drive, without dismantling the tape drive and/or the motor and without the use of unfriendly, environmentally undesirable products.
SUMMARY OF THE INVENTION
The present invention provides a method for cleaning a commutator-brush interface in a direct current motor comprising applying a very high electrical current to the motor. The polarity of the electrical current is alternated and implemented in a variable duty cycle. The motor can remain in the drive during the cleaning process. Neither the motor nor the unit driven by the motor needs to be dismantled.
Specifically, a controlled constant speed direct current motor has its commutator cleaned by applying a square wave alternating polarity direct current to the motor. The motor is particularly useful in tape drives for data processing systems or anyplace where a controlled speed motor is required. The alternating polarity square wave direct current is preferably applied in a variable duty cycle in order to control the heat buildup in the motor. The cleaning is accomplished by burning off the coating on the commutator and restoring an intimate electric contact between the commutator and the brushes of the motor.
This invention uses a method and apparatus for cleaning the brush-commutator interface of a direct current motor by delivering a maximum amount of alternating polarity direct current to the motor. The method and apparatus can be used and activated anytime the motor is inactive and the drive operated by the motor is idle. The objective of the cleaning cycle is to apply large currents to the motor while allowing the rotation in reversing directions to clean all of the commutator segments. The maximum current through the motor is obtained by reversing the direction of the applied voltage and resultant current so no appreciable rotation of the motors is achieved. The apparatus of the invention includes adding a controllable pulse width modulated motor cleaning power supply to the tape motion control unit with a motor cleaning control unit to selectively apply the output of the motor cleaning power supply to the motor when the tape motion control unit is inactive.
In the method of cleaning a commutator-brush interface of a direct current motor, the process includes the steps of connecting a cleaning power supply to a motor. The cleaning power supply is adjusted to produce a pulsed square wave current of alternating polarity and adjustable frequency. The period of the waveform and the peak current is selected based on the wattage and mass of the motor. The current waveform may be selected to provide either a slow net rotation of the motor or no net rotation. Tachometer signals from the tape motion control unit may be monitored to diagnose the cleanliness of the brush-commutator interface. The current may be varied to reverse the net rotation of the motor. The motor cleaning control unit can monitor the leaning process to monitor the output of the motor to increase the asymmetry of the duty cycle and invoke and control a longer cleaning cycle or produce a diagnostic message of a motor problem. The motors of a two reel tape drive unit may have each motor independently cleaned or both motors cleaned and monitored at the same time. The cleaning process may occur on each and every tape cartridge unload operation.
The present invention enables processes for cleaning commutators on motors while the motors are installed in the unit driven by the motor. The present invention enables cleaning processes which are fast and easy, without dismantling the unit nor the motor itself. The cleaning process according to this invention can be performed in the field where the unit is used and does not require removal of the unit or the motor for a cleaning process at the factory. In addition to other benefits realized from these improvements, significant cost advantages result from these improvements. For all of these reasons, the present invention provides substantial advantages over the prior art.
An object of the present invention, therefore is to provide an enhanced method for cleaning a commutator-brush interface in a direct current motor.
Another object of the present invention is to provide a method and process for cleaning a commutator-brush interface in a motor while the motor is installed in a unit driven by or operated in the unit.
Still another object of the present invention is to provide a process that cleans a commutator-brush interface in a direct current controlled speed motor useful in a tape drive of a data processing system.
Yet another object is to provide a method of cleaning a commutator-brush interface in a motor that can be controlled and provides information showing the results of the cleaning process.
The foregoing, and other objects, features and advantages of the invention will become more apparent to those
Dang Khanh
International Business Machines - Corporation
Pershon James A.
Sullivan Robert M.
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