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I. General description of the work.
Conveyor transport is the integral technological process of the mining enterprise. Its efficiency renders significant influence on coal output of all enterprise as a whole. Automation of conveyor transport provides increasing of delivery process efficiency by reduction of expenses for service and repairs, and also due to reduction of an idle time and increasing in productivity. A start-up problem is one of the actual questions that concern operating modes of the conveyor and its effective operation. Start-up process is characterized by presence of transients both in a mechanical part of a design, and in the electric drive. Transients are accompanied by sharp changes of various condition parameters in time: mechanical parameters (acceleration forces, belt fabric speed and its tensions), electromagnetic parameters in AC induction drive (stator and rotor currents, EMF, MMF). Thus values of the given variables can vary substantially, falling outside the admissible limits or even going critical. This can lead to inefficient work or damages and destructions of all design as a whole. The overload of a belt fabric during the conveyor’s start-up can lead to dangerous belt safety factor reduction if its durability is chosen without taking into account characteristics of an applied drive of the conveyor or its brake device. Other task of the transients analysis is definition of such tension of a belt fabric contour which would provide conveyor start-up and braking without belt slipping on drive pulley and belt stability losses (in case where the drive provides smooth conveyor start-up the rated force of the tensioning gear can be accordingly reduced and the tape of smaller longitudinal durability can be applied on the conveyor).
The purpose of the work is increasing of the electric drive operation efficiency. This can be done on basis of scientific rational management parameters substantiation in a start-up and steady state running modes and by automation means development.
For the purpose achievement it is necessary to solve following tasks:
- mathematical model of the belt conveyor electric drive development and researching at a variation of speed diagram formation laws in a view of an admissible load fluctuations range;
- technical decision of the indirect conveyor belt loading control development;
- technical decisions development, which means an automatic control of asynchronous drive by a variable frequency voltage in function of loading stabilization.
Let's examine more in detail conveyor start-up with the description of dynamic processes and let’s reveal weaknesses, which reduce efficiency and operation safety in this mode.
After excessive torque being applied to a drive pulley of the conveyor with the rigid tensioning gear, the wave, which increases an initial tension, is passing round along the top branch. And the wave, which decreases an initial tension, is passing round along the bottom branch.
In cases when the initial tensions diagram differs from the diagram in the steady run mode, direct waves move on branches with speeds c1, c2. At the same time the reflected waves, which caused by friction forces reorientation, move from moving wave front points and have speeds c'1, c'2, signs on these reflected waves coincide with signs on direct waves.
Each direct wave is partially reflected when reached the border of loaded and empty sites of the belt fabric. When transition of a wave from loaded to empty site takes place, reflected wave has inverted sign against to a direct wave. The factor of reflection is defined by expression:
(1.1)
Signs on the direct and reflected waves coincide at a wave transition from empty to loaded site. The factor of reflection is equal to
(1.2)
where 
Direct wave changes its amplitude by a factor of (l + kот). This means that its amplitude decreases at the transition from loaded to empty site and it increases at the transition from empty to loaded site. After a colliding of direct waves during the moment of time t1 all tape is involved in movement. The further occurrence of the reflected waves caused by reorientation of friction forces will stop, but last elements of these waves continue to extend to a drive, and their influence on a belt fabric tension at a drive will stop only when fronts of the direct waves (they are moving after their meeting with speeds c1, c2) will bypass all contour of a belt. Then direct waves are reflected from a drive, and signs on the reflected waves correspond to signs on direct waves. The further distribution of waves will cause oscillatory process in each section of a belt, and if intensity of drive revolting force does not increase, then amplitudes will fade due to internal friction in system.
In the conveyor with a rectilinear line structure dynamic tension extreme values take place at a drive.
Figure 1.1 - the Scheme of belt acting forces.
According to the design scheme (fig. 1.1), we have differential equations that describe the processes in the top branch sites of a belt:
(1.3)
And the processes in the bottom branch:
(1.4)
Initial belt fabric tension depends on a final phase of previous braking mode and on a conveyor inclination corner.
If conveyor inclination corner and ξ0 value are constants then initial tension on the top branch of the conveyor is defined as:
(1.5)
And on the bottom branch of the conveyor it is defined as:
(1.6)
After substitution of the initial tension defining derivatives to (1.3), (1.4) we will have:
(1.7)
(1.8)
On moving belt sites near to a drive (for t < τ2) dynamic tensions are formed by direct waves and the friction forces reflected waves. As signs and distribution speeds of these waves inside the sites are coinciding, next parity is true:
(1.9)
Considering this definition in points, where belt fabric approaches and leaves the drive pulley, we will have:
(1.10)
(1.11)
After integration we will receive solutions of the given equations:
(1.12)
(1.13)
Speed of the reduced mass of a drive is defined from the drive movement equation:
(1.14)
After the given equation solution we will receive value of the maximal tension:
(1.15)
Fig. 1.2 represents diagrams of dynamic forces distribution on a belt length during the various moments in time.
Figure 1.2 - Dynamic tensions distribution on a tape contour at the conveyor’s start-up with the rigid tensioning gear during the moments of time:
a)
t < τ1; b)
;
c) 
; d)
To realize such an effective start-up mode we need to use some technical devices providing smooth drive torque increasing during time up to value that corresponds belt start-up moment. Then it is necessary to switch prime mover to a nominal operating mode. Modeling processes in a conveyor belt at start-up allows to determine rational values of drive acceleration time, providing as much as possible fast start-up at the minimal dynamic forces in a belt. The given device will be designed on the basis of the frequency converter. This means that regulation of the speed rotation will be made by a frequency method.
III. Results and Perspectives
Effective conveyor operation will be achieved by development of automated control system allowing to realize the set algorithm of the start-up both the separate belt conveyor, and system of the conveyors connected in a uniform transport line.
It is necessary to provide an opportunity of a conveyor line productivity regulation with the purpose of an effective mode of its work maintenance, as on technical, and to economic parameters.
Thus, the developed system should carry out following functions:
- start of a conveyor line in the set sequence against movement of a freight traffic;
- soft start of each conveyor entering into system;
- part of the conveyor line start-up;
- start-up of the separate conveyor for repair and adjustment works;
- an opportunity to start-up parts of a conveyor line due to other working conveyors;
- the automatic belt speed control;
- permission to start-up of each subsequent conveyor only after nominal speed achievement by the previous conveyor;
- switching-off of the conveyor due to belt descent;
- switching-off of the conveyor due to speed set point top level excess, or at decreasing below the fixed level;
- switching-off of the conveyor at absence of a cargo;
- switching-off of the conveyor due to overload;
- simultaneous automatic switching-off of all conveyors transporting a cargo on a stopped conveyor;
- automatic line productivity control - reduction in speed due to underload and increasing (up to rated drive rated speed) while excess of the load set point;
- blocking to start overloaded or clogged conveyor;
- blocking to start the conveyor with the descended belt;
- diagnostics of a communication link condition;
- diagnostics of the control units functionability;
- monitoring of working and not working conveyors quantity in a line;
- ability to signal about emergency operation;
- monitoring of a stopped conveyor number and the pipeline stall reason;
- line productivity registration;
- transfering to a surface the information about conveyor line condition.
The block diagram of automation system which is planned to develop, is shown on fig. 3.1
Figure 3.1 - the Block diagram of the automated system
Fig. 3.1 shows the transport network consisting of three conveyors (for the figure simplification) 1, 2, and 3. The maximal number of conveyors is limited by drivers RS485 loading ability designated on the scheme as repeaters R1 and R2. The system consists of the frequency converters FC connected to the drive motor; control units CU; control panel CP which carries out functions of gathering and processing of the information. It transfers control instructions to control units, and also transmits the information about line condition to the surface.
The information from the speed sensor SS and strain gage (conveyor weight scales) SG are transferred to the control unit CU. Block CU by himself provides soft start-up of conveyor’s drive motor.
Control panel CP carries out conveyor line start-up control, implements speed regulation of all line depending on loading, carries out pipeline stalling or its parts in emergency operation and due to underload condition, transmits line condition information to the surface.
Data exchange between CP and CU is implemented by RS485 driver unit, allowing to create high extended networks. CU transfers separate conveyor condition parameters data to CP and the CP carries out the coordinated control by all pipeline.
The block diagram of the separate conveyor automation principle is shown on fig. 2.2. The set algorithm of start-up is provided with a control unit CU system. Its output operating signal carries out the task to assign PWM parameters of the frequency converter.
Figure 2.2 - the Scheme of the separate conveyor automation principle.
Signals from speed sensor and weighting scales Uс and Uв transfers to the control unit CU which according to the set parameters produces an operating control voltage U which is entered into control logic of the frequency converter (it changes frequency of the motor’s supply voltage) and there is a speed change of the drive and belt occurs.
During the further work it is planned to obtain adequate model of the belt conveyor and its drive with the purpose of the start-up rational diagram definition. This will allow realization of start-up with the minimal duration in time and the minimal dynamic efforts both in a tape, and in a drive.
CONCLUSIONS
Following results are received during this work. It is determined, that the most critical operation mode of the belt conveyor is start-up. Mathematical analysis of the direct start-up process have led to a conclusion about necessity of reduction in dynamic forces in a belt by performing an control action to a drive.
The review of the technical decisions of the drive control methods is made. Various ways of regulation comparison and their parameters of quality have allowed drawing a conclusion on an opportunity to perform variation of speed parameter by the motor supply voltage frequency regulation.
Technical realization of automated system is carried out, allowing to increase an overall performance of conveyor transport and to lower expenses (operational and power) on underground coal transport process.
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