Research of a scraper conveyor stopping, which occurs when the drag-chain is jammed, shows that the
increase in elastic effort of tension leads to chain break, which is followed by an emergency situation on the conveyor.
If the drag-chain is jammed, then the mechanism of conveyor drive automation should protect the drive
and the drag-chain from overloading. The AUP110E device does it by turning off the voltage of an electromagnetic coupling field
coil.
However mathematic model research shows that the drag-chain will have high values of elastic effort of
tension anyway, despite turning the voltage off. It is caused by the electromagnetic coupling's field coil low induction and
small resistance, so the transitional process of current attenuation lasts longer. To avoid this additional resistance is added
to field coil's demagnetization chain. Thus the transitional process time shortens and the overload in drag-chain and the
conveyor drive decreases.
It is well-known that the transitional process' time straightly depends on the time constant volume
So by increasing resistance in field coil chain we can decrease transitional process time and by doing so we shall also decrease scraper chain's elastic effort of tension when the chain gets jammed.
Picture 1 - Dependence between time and maximum elastic effort of tension in a flight chain
A mathematic model was used to research influence of time constant on the volume of the maximum elastic effort of tension. Modeling was done for resistance ranging from 21,67 Ohm to 200 Ohm. Research results are shown on pic.1. As it is seen from the graph, when the time constant decreases the maximum elastic effort of tension decreases too, so the latter can be reduced to a level smaller than the chain's breaking strength and an emergency can be avoided.
Picture 2 - Scheme of an accelerated stopping device (OB - field coil, VD - diode, RD - additional resistance, VT - transistor wrench)
Authors have worked out an accelerated stopping device (see scheme on pic.2). The scheme works as follows: in a normal work mode the transistor wrench VT is open and the additional resistance RD is shortened. When the drag-chain is jammed, the wrench closes and additional resistance is brought into field coil demagnetization chain. The additional resistance is big, so the chain's time constant is less than if we didn't have another resistor, so the current in the field coil attenuates faster. The dynamic overloads in the drag-chain and conveyor drives lower to a safe value (see pic.3).
Usage of the given scheme in the mechanism of a conveyor drive automation will quite possibly lead to a sufficient decrease of scraper chain's elastic effort of tension, the dynamic overloads in the chain and the drive will also be decreased, so when the chain is jammed, its break will be avoided.
Picture 3 - Dependence between time and elastic effort of tension in a flight chain. (on the right - when using accelerated stopping device)
Investigations on a scraper conveyor stopping process will go on. Creation of a more effective device is planned by adding a special sensor (of a tensoresistive type, for example) reacting to the chain's elastic effort of tension. This device will be able to turn voltage off the field coils timely so the possibility of emergency situations connected with drag-chain jamming will diminish.