Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1982-07-23
1985-05-14
Dobeck, B.
Electricity: motive power systems
Induction motor systems
Primary circuit control
318432, H02P 534
Patent
active
045175063
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a wrench provided with an alternating current drive motor.
The state of the art encompasses pneumatic wrenches (air wrenches) which have a series of disadvantages. They require an expensive preparation of the air, relatively complicated devices for a constant maintaining of pressure and for an exact switching, and offer only a relatively lower torque in the rapid motion range.
Furthermore, wrenches are in use with alternating current drive motors, which are designed as single frequency asynchronous motors. These drive motors operate with at least two stage gears which, after the occurrence of a predetermined starting moment, switches over to a lower output speed of the wrench. Such wrenches are not useable for highly exact screwed joints since the exact stopping of the drive motor is difficult after reaching the predetermined starting moment at the higher rotation of speed of the motor. Furthermore, the energy required by such wrenches is relatively high.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a wrench which is provided with an alternating current drive motor which permits to maintain an exact predetermined starting moment and which has a lower energy requirement with respect to the described state of the art.
These and other objects of the invention are attained by a drive of a wrench, comprising an asynchronous drive motor; a torque sensor; cycle control means, said torque sensor being connected to the wrench and to said cycle control means; a medium frequency line; a lower frequency line; and contactor means connected to said drive motor and to said cycle control means, said contactor means switching said motor from the medium frequency line to the lower frequency line, said torque sensor supplying signals to said cycle control means so that said contactor means is actuated by said cycle control means in dependence upon the signals from said torque sensor during the occurrence of the setting moment on the wrench, wherein the drive motor is used which has a rotor sweep frequency which is about the same as the lower line frequency.
With the inventive wrench, the drive motor is operated with a high speed until reaching the setting moment, thereafter a lowering of the speed of the drive motor occurs to such a lower value by switching over from a medium frequency, for example, 200 Hz to 50 Hz, so that after reaching the starting moment the exact stopping of the motor and thereby the total wrench is considerably facilitated. Additionally, the possibility exists to electrically slow down the drive motor by means of a three phase terminal short circuit.
Advantageous embodiments of the invention are the subject matter of the subclaims.
If, in accordance with FIG. 1, a drive motor with a rotor sweep frequency is used which is at least about the same as the lower line frequency, the sweep point, i.e., the operating point of the maximum torque when operating with the lower line frequency, for example, 50 Hz is displaced to the proximity of the starting point of the drive motor, so that the drive motor supplies an increasing torque during the tightening of the screw with a decreasing speed until stopping. Simultaneously, the copper losses of the drive motor are considerably reduced with respect to an operation with only a nominal frequency, i.e., a medium frequency.
The periodically reproducible maximum torque during the tightening of the screw is practically limited by the effective value of the motor current, averaged by means of an operational cycle. If greater torques are required, the frequency switching can be supplemented by switching a gear drive in the wrench.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view of the circuit, and
FIG. 2 are the curves of the normalized torque and the normalized motor current when fed with medium frequency or lower line frequency.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
If one at first looks at FIG. 1, it can be seen that at 1 the wrench is sc
REFERENCES:
patent: 1723528 (1929-08-01), Schneider
patent: 1810227 (1931-06-01), Shea
patent: 1970179 (1934-08-01), Miller
patent: 3892146 (1975-07-01), Yasoshima
patent: 4041361 (1977-08-01), Cornell
Heinrich Wulf-Dieter
Otte Henning
Dobeck B.
Ro Bentsu
Robert & Bosch GmbH
Striker Michael J.
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