ARRANGEMENT IN SCREW CONVEYERS FOR BULK GOODS

Abstract: An arrangement in a screw conveyer (1) for vertical transport of bulk material has an intake device (2) which, like the screw conveyer has its own electric driving motor (5). A derivation circuit (20) is provided for generating a signal representing the time derivate of the instantaneous value of the load on the screw conveyer motor (3). The said time derivative signal is utilised for reducing the speed of and, optionally, stopping the motor (5) of the intake device when said motor (5) exceeds a predetermined limit value indicating that a slide has occurred in a fluidisable material around the intake device (2).

Description: ARRANGEMENT IN SCREW CONVEYERS FOR BULK GOODS The present invention relates generally to the vertical transport of bulk goods, such as cement and coal. More particularly, the invention relates to an arrangement in a screw conveyer for the vertical transport - of bulk goods, said screw conveyer having.an intake device which, like the screw conveyer, has its own electric driving motor. For the unloading of, for example, cement from ships, it is advantageous to use a screw conveyer having an intake device of the type disclosed in, for example, US specifications 3,596,751 and 3,685,638. During unloading, however, the fact that the cement is a fluidisable bulk material causes difficulties. During unloading, the lower screw conveyer end, with the intake device, is introduced into the cement mass and digs a hole therein from which the cement is elevated. Soon after unloading has begun, there occurs inevitably a slide when one or more cement blocks slide down into the hole. During this sliding movement, however, the cement in the block or blocks suddenly passes into a fluidised state and thus behaves primarily as a liquid. From having formed, before the slide, an angle of slope of about 50 , the cement which after the slide behaves as a liquid material, will form a pool with an angle of slope of 6-8 . As a consequence, the intake device will be filled completely, and the load on the motor of the screw conveyer will be increased to 150-170% of the nominal load. This means that the unloader compri¬ sing the screw conveyer and the intake device must be stopped, lifted out of the cement and then restarted to resume unloading. The repeated stopping of the intake device which thus occurs during unloading of, for example, cement will considerably reduce the average capacity of the O P! - unloader . Furthermore, the screw conveyer may have to be stopped because a plug is formed therein. Such plug formation which quickly increases the load on the motor of the screw conveyer may occur, for example, when the screw conveyer is made to receive, at low filling degree and high rotational speed, a relatively incompressible bulk material, such as coal. Also in this instance, the average capacity will be reduced considerably. It therefore is the object of the present invention to provide an arrangement in a screw conveyer intended for vertical transport of bulk goods and comprising an intake device, by which arrangement the above-mentioned routine of stopping, raising and restarting the unloader is eliminated, and which arrangement makes it possible to avoid plug formation or at least stopping of the motor in connection therewith. The arrangement thus is intended to simplify the work of the operator of the unloader and to increase the average capacity of the unloader. According to the invention, this object is achieved by means of a load detector for sensing the load on the screw conveyer motor and generating a signal corre¬ sponding to the instantaneous load value, a derivation circuit connected to said load detector and adapted to receive the signal corresponding to the instantaneous load value and providing a signal representing the time derivative of said instantaneous value signal, as well as a threshold circuit receiving said last-mentioned signal and adapted, when said time derivative exceeds a predetermined limit value, to considerably reduce the speed of the motor of the intake device and, if necessary, to stop it. The threshold circuit may control a switch connected between the output of the derivation circuit and a control circuit for the motor of the intake device, such that the switch is closed when the time derivative exceeds the said limit value. - UREA The load detector preferably consists of an actual value transducer in a control equipment for driving the intake device substantially inversely proportional to the load on the screw conveyer motor. Thus, the speed of the motor of the intake device is controlled by means of the conventional control equip¬ ment of the unloader in a manner essentially inversely proportional to the load on the screw conveyer motor. However, this control does not suffice to eliminate the difficulties outlined above. By utilising an arrange¬ ment according to the present invention, it is possible, during unloading of fluidisable bulk material, to prevent overflowing of the unloader due to self-suction when such material is set in sliding motion. This is acco - pushed in that the increased load on the screw conveyer motor provides for a relatively rapid indication. Since this makes it possible to reduce the speed of the motor of the intake device, possibly to a standstill, at a very early stage of the slide, the overflowing which would otherwise occur may be prevented, and instead the load on the screw conveyer motor will be increased by some ten per-cent for a short period of time during which the pool of fluidised material formed by the slide is emptied. After that, the load decreases to approxi- mately the same value as before the slide. The invention will now be described in more detail below, reference being had to the accompanying drawing which illustrates a block diagram of a control equipment for a screw conveyer having an intake device, in which control equipment use is made of the arrangement according to the present invention. The drawing illustrates schematically a vertical screw conveyer 1 which is provided at its lower end with an intake device 2 which at a later stage gives an essentially increased filling degree in the screw conveyer and, thus, considerably increases its feeding capacity. The screw conveyer 1 has a driving motor 3 which, in this instance, is an A.C. motor supplied from the mains via a switch 4. The intake device 2 is operated by a variable speed D.C. motor 5 supplied from the mains via a control circuit 6. The control input of the control circuit 6 is connected to the output of a summation circuit 7 which, in conventional manner, has an input from a setting means 8 for setting a desired value for the speed on the motor 5, an input from a -overload protection circuit comprising amplifiers 9, 10 and 11 and a further summation circuit 12, as well as an input from a load sensing unit comprising a load detector in the form of a current transformer 13 connected in the supply line of the motor 3 and a converter 14 connec¬ ted to said current transformer and also directly to the supply line, a proportional amplifier 15, and a sample and hold circuit 16. The outputs from the setting means 8 and the load detector 13, 14 are connected to the inputs of a third summation circuit 17, the output of which is connected to the input of the amplifier 15. Furthermore, the output of the setting means 8 is connected to the input of the amplifier 9, and the output of the load detector 13, 14 is connected to the input of the amplifier 10, the outputs of said amplifiers 9, 10 being connected, via the summation circuit 12, to the input of the amplifier 11, the output of which is connected to an input of the summation circuit 7. For displaying the instantaneous load exerted on the motor 3 of the vertical conveyer 1, a pointer instru¬ ment 18 is connected to the output of the load detectors 13, 14 via an amplifier 19. The above-mentioned circuits are conventional in a bulk material unloader comprising a screw conveyer and an intake device, and therefore no detailed descrip- tion of these circuits is necessary. It should be pointed out, however, that the circuits 15 and 16 may be formed to control the speed of the motor by about + 25% at a sampling frequency of some ten seconds. Furthermore, the control of the overload protection circuits 9-11 is of the on-off type. According to the present invention, there is connect¬ ed to the output of the amplifier 19, at which output a signal occurs which corresponds to the instantaneous value of the load on the screw conveyer motor 3, a deri¬ vation circuit 20 having its output connected on the one hand to a threshold circuit 21 and, on the other • hand, via a switch 22 to an input of the summation circuit 7. The switch 22 which may be a relay, is controlled by the threshold circuit 21. More particularly, the derivation circuit 20 will generate a signal which repre¬ sents the time derivative of the said instantaneous value signal and which is received by the threshold circuit 21. The threshold circuit is adapted, when the value of the time derivative exceeds a predetermined value, to provide at least a speed reduction or even an interruption of the driving of the motor 5 of the intake device 2, which is done by closing the switch 22, the output signal of the derivation circuit 20 being supplied via the summation circuit 7 to the control circuit 6, whereby the speed of the motor 5 is drastically reduced, optionally to standstill. The magnitude of the reduction depends upon the instantaneous value of the output signal of the derivation circuit 20 when the switch 22 is closed and immediately afterwards. OMPI

Claims: CLAIMS 1. An arrangement in a screw conveyer for the vertical transport of bulk goods, said screw conveyer having an infeed device which, like the screw conveyer, has its own electric driving motor, comprising a load detector (13, 14) for sensing the load on the motor (-3) of the screw conveyer (1) and generating a signal corresponding to the instantaneous load value, a derivation circuit (20) connected to said load detector and adapted to receive the signal corresponding to the instantaneous load value and providing a signal representing the time derivative of said instantaneous value signal, as well as a threshold circuit (21) receiving said last-mentioned signal and adapted, when said time derivative exceeds a predetermined limit value, to considerably reduce the speed of the motor (5) of the intake device (2) and, if necessary, to stop it, c h a r a c t e r i s e d in that said threshold circuit (21) controls a switch (22) connected between the output of the derivation circuit (20) and a control circuit (6) for the motor (5) of the intake device, such that the switch is closed when the time derivative exceeds the said limit value. 2. An arrangement as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the load detector (13, 14) preferably consists of an actual value transducer in a control equip- ment (6-17) for driving the intake device (2) substantially inversely proportional to the load on the screw conveyer motor (3) . 3. An arrangement as claimed in claim 2, c h a r a c ¬ t e r i s e d by a summation circuit (7) for summation of the control signal of the control equipment (6-17) and the output signal of the derivation circuit (20) via the switch (22) , said summation circuit having an output connected to the control circuit (6) of the motor (5) of the intake device. f _OMPI

Inventors: SANDBERG KNUT ARNE, SANDBERG, KNUT, ARNE
Applicants: SIWERTELL AB, AB SIWERTELL
Application year: 1984
Main classification: B65G43
IPC codes: B65G33, B65G43, B65G65