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Raw Coal Loadingand Belt ConveyerSystem at theNochten OpencastMine

Industrial Solutions and Services

mining mining mining mining mining mi

Your Success is Our Goal

Translated fromBraunkohle Surface Mining 50

No. 2 March/April 1998

Trans Tech PublicationClausthal – Zellerfeld, GermanyAuthors:Dr. Werner Daus, LAUBAG, SenftenbergS. Körber, ReichwaldeNorbert Becker, Siemens AG, Erlangen

An indispensable requirement of theLignite Planning Procedure (1992 - 1994)was the removal of the old coal loadingfacility from the town of Mühlrose. Inaccordance with the Regional PlanningAct the Lignite Plan called for this re-location in terms of a ”target” , i.e. aplanned measure with significance forregional planning which was to beimplemented by 1996.

Owing to the massive investmentsundertaken by LAUBAG a “targetamendment procedure” with respect tothe loading facility relocation was appliedfor in 1995 and approved for 1998.The investment funds were released inMay 1996, and a contract for the workwas awarded to Siemens AG (as primecontractor) in November 1996.

With regard to the construction of thefacility a maintenance contract wasentered into with Siemens. The work hadto be completed in an extremely shortperiod of time.

2.1 Funding and rationalization objectives

Based on operational experience and newtechnical capabilities, a new conveyingline and bunker complex were to bedesigned featuring state-of-the-art hand-ling technology and equipment. In viewof the cost situation and the existing plantand equipment, the design conceptincorporated a sensible combination ofthe existing technical basis and promisinginnovative technology.

The belt conveyors were taken fromexisting stock at Nochten mine. Thestacker-reclaimer (KSS 10000) requiredand the train loading facility were relocatedfrom the neighboring Reichwalde open-cast mine. Just two belt distributors hadto be built from new in the storage areain order to avoid disruptions in winter. Thisdecision allowed the investment fundsmade available to the company to beconcentrated on technical innovationswith great rationalization potential. A shortpayback period was to be achievedthrough the following effects:

• Energy savings• Rational deployment of operator's

personnel• Reduction of maintenance costs by

economical plant and equipmentoperation

• Wear-oriented maintenance on thebasis of guaranteed diagnostic values.

A New Conveying and Loading SystemBased on Drives Controlledby Frequency Converter

In November 1997, twenty-five years after production commenced

at Nochten opencast mine, a new conveying system, a new train

loading facility, and the first stage of a coal handling area were

taken into operation. The new conveyor system is only slightly

longer than the old one, but cuts rail transportation distances

by 16.5 km. This means that the newly erected part of Boxberg

power station can be supplied with coal direct by belt conveyor

(Fig. 1).

2 .The complex requirements for relocation of the Nochten coal loading plant

2

Fig. 1Old belt conveyor, 12.5 km long,installed output 17.4 MW(2-4 mining machines depending onthe customer’s quality requirements)

Mühlrose loading facility

Old railway transportation distance –21 km

New belt conveyor, 13.4 km installed output23.2 MW

New coal loading facility and stockpiles

New railway transportation distance –4.5 km

Boxberg power station

23

4

56

7

1

In order to obtain planning permission forthe facility, a 'target modification proce-dure' for the Lignite Plan for Nochten hadto be initiated. Particularly stringent require-ments were laid down with regard toemission control. The following solutionswere proposed, accepted, and eventuallyput into effect:

• Relocation of the facility near the powerstation and removal of bunkers in thepower station

• Particulate matter protection by meansof extensive cleaning installations

• Noise protection by enclosure of driveequipment, noise-reduced rollers,reduced belt speed, erection of abuilding around the loading facility,and creation of a noise-protectionembankment.

2.2 Reducing operating costs withthe new facility

2.2.1 Belt conveyor

Significant potential for cutting costs wasidentified in the shape of a variable-speedbelt conveyor.

In all the LAUBAG opencast mines, theworkings under the F60 cross-pit spreaderwere greatly oversized and serviced by anumber of small excavators. Essentially,there were three reasons for this:

• Compensation for the front-end workingmethod of the F60 by the block workingmethod of the mining machines toguarantee as far as possible the uniformand uninterrupted operation of theoverburden transporter

• Changing quality modes of operation inconjunction with a high belt conveyorutilization rate owing to excess machinecapacity

• Compensation for machine downtime.

These reserves were necessary tomaintain supplies up to 1992/1993.Declining coal sales and changingrequirements with regard to raw coalquality gave rise to new conditions.Maximum loading of the belt conveyor atall times was achieved by way of machineovercapacity.

The aim is to run the system with as littleoperator intervention as possible. Toachieve this the frequently changing worklocations must be able to go into operationcentrally without any delay. Once theoperator's team has switched from onepiece of equipment to another the neces-sary monitoring, operating and controlfunctions are to be secured. This allowsa single team to look after more than oneitem of equipment.

In this concept, the coal is to be recoveredby the KSS (when the mine has stoppedworking) and loaded without the need fora central control point. The process controlsystems required can be switched to theKSS in this case.

2.2.3 Process control system

Multimedia technology makes it easierto operate the transportation and bunkersystem with a minimum number ofpersonnel while at the same time guaran-teeing operational safety, system avail-ability, and operating performance.

The process instrumentation and controllevels have been combined on the basisof advanced communication technology.A redundant, open-protocol, broadbandmultiplex system, OTN (Open TransportNetwork), transmits all signals from theinstrumentation and control system,including emergency shutdown, plantstatus monitoring equipment (diagnosis),communications equipment (telephones,command units) and video equipmentalong fiber optic cables.

Yet the positive effects of high beltconveyor utilization met with a price in themachine sector.

Thanks to variable-speed control, which isoriented around an optimum belt conveyorload cross-section, flexible optimization isachieved for a defined control rangewithout the need to maintain reserves.The greatest benefits from this technicalmeasure are anticipated in the areas oftechnology management, wear minimi-zation, energy conservation, and thereduction of emissions. For this reason,this discussion will look at the technicaldesign, operation and the initial results ofthe measure separately and in more detailat Point 3.

2.2.2 Coal handling plant

The new coal handling plant (Fig. 2) hadto be designed to fulfill two tasks:

• To supply the refurbished Boxberg powerstation (installed output = 2 x 500 MW)and, if applicable, other customers withraw lignite via a train loading facilitywith maintenance of separate stocks(stockpile A has a capacity of 80,000 t).This stockpile is to be served by astacker-reclaimer (KSS 10000)

• To supply the newly erected part ofBoxberg power station via a system ofpermanent stockpiling (stockpiles B andC each have a capacity of 100,000 t).Stockpile B is also to be built up by theKSS 10000; stockpile C will eventuallybe served by an extended belt conveyorand a stacker. Stockpile C and theassociated conveying equipment willgo into operation in a second stage notuntil after1999.

Stockpiles B and C are combined workingand reserve stockpiles and will be ope-rated under these conditions by thecustomer (VEAG). To this end a number ofspecial legal and organizational agree-ments are to be reached. The automaticoperation of the three major systemcomponents (KSS 1000, belt-to-trainloading facility and, at a later date, astacker) was considered an essentialmeans of optimising the process.

Fig. 2Coal storage area

3

SIMOVERT® Cabinets

Radio-based functions such as belt-stopand data radio (monitoring of miningmachines and quality assurance system)have also been set up. Complex proce-dures for conventional belt-stopmonitoring systems (push-buttons, horn,wiring) have been replaced by a simpleradio-linked structure. The open, operator-controlled, realtime systems andstandards used here allow the system tobe expanded and adapted to changingneeds and conditions without the needfor major additional expense.

The entire system is controlled fromNochten mine's central control point bya single operator. This control point alsoincludes the maintenance interface (plantand equipment diagnostics), an importantprerequisite for communications betweensystem and process engineers.

The savings achieved in this area aredifficult to quantify. They will, however,provide valuable development impetusfor operations, organization and mainte-nance in the next few years.

2.2.4 Maintenance

Owing to the very short project planningand completion time LAUBAG decidedto award the entire construction worksto a main contractor. The most impressiveconcept for this work was put forwardby Siemens. It was awarded the contract

and, together with a number of subcon-tractors, built the facility in just sevenmonths.

A major element of the contractual agree-ments was the link between plant con-struction and subsequent maintenancework. The idea was to ensure that theplant builder was interested in a lowestcost maintenance system right from theoutset.

A further consideration, which pointedin favor of a complex third-party main-tenance system, was the possibility ofcombining operating experience with theknow-how possessed by plant buildersand manufacturers. With this synthesisthe aim is to attain a new quality in thefield of professional maintenance workand sustained progress in all aspects ofmaintenance management.

In accordance with this objective newjobs are being created that reflect thisdevelopment and are backed by the latesttechnical equipment in order to maintainthe synergy between technical develop-ment and organization. An importantinstrument in this respect is the technicaldiagnostics system. In diagnosis, wedistinguish between operational andexpanded diagnosis.

In short, operational diagnosis meansthe use of product-specific softwaretools from a central point for selectivelyaccessing major, technologically complexcomponents (e.g. converter, decentralizedfield bus networks etc.) throughout thefacility in order to record, analyze and, ifnecessary, correct their current status.

Expanded diagnosis means the fore-sighted analysis of the process and statusof plant and equipment to detect anydeviations from the intended status at anearly date.

Here, time patterns (operating hours,changeover procedures, parameter

changes), capacity-related parameters(e.g. rates of utilization, torque and tem-perature curves), status data (e.g. vibrationcharacteristics, wear values) and externalinfluences are of major importance.

The principal idea behind this concept isthe gathering of data from various sources,e.g. the automation system, the statusdetection system and the results of in-spections, and putting it together with theaid of intelligent, computer-based methodsto deliver effective information. This infor-mation forms the basis for a status-orientedsystem of maintenance and the associatedreplacement parts management system.Some of the outstanding characteristicsof the expanded diagnosis system arelisted below:

• Highly detailed diagnosis for monitoringof stocks by means of error patterndetection and fuzzy logic support.

• Error detection on the basis of freely-definable error patterns.

• Automatic generation of maintenanceorders.

• Generation of freely-definablecharacteristic values.

• Universality of the information flow fromthe automation and status monitoringsystem via the intelligent diagnosticssystem to the maintenance planningsystem.

Closing the interface between the diag-nostics and maintenance systems createsthe conditions required for status-relatedmaintenance operations. Here, mainte-nance work is conducted according toactual requirements and not, as waspreviously the case, at fixed, static intervals.This new maintenance strategy will makea major contribution to savings on thebasis of wear-related maintenance,enhanced organization and an improve-ment in levels of expertise due to theintroduction of a sensible form of com-petition between the skilled minepersonnel and the service providers.

4

Workstation for maintenance

Converyor control center

3.1 Rationale

A project involving a belt conveyor systemfor the reliable transportation of definedbulk materials is planned. A major plan-ning aspect is the effective handlingcapacity of the connected plant andequipment. These capacities are addedtogether and, furnished with corres-ponding safeguards, included in theplanning of the transportation capacity.This gives an average theoretical volumeof material to be transported.

All subsequent calculations (e.g. driveoutput, number of driven drums, angle ofbelt contact, belt width, tensile strength,tension force, steel structure etc.) arebased on this value, coupled with geo-metric and climatic influences and witha constant belt speed.

This approach creates a permanentproblem: the greater the differencebetween the volume of material beingtransported at any one time and thetheoretical formulation, the less favorablethe ratio between the cost incurred (driveoutput, material) and the benefit obtained.If the system is not used to full capacity,potential is squandered; if the system isoverloaded, disruptions are often theresult. The margin above is determinedby a safety factor; if a downward deviationarises, this leads to resources beingwasted.

A fluctuation round a nominal load suchas this can be tackled using a selectivemining machine control and adjustmentsystem. The more excavators there are,the more complex this becomes, due tothe complex variety of parameters.

Example:

Total capacity breakdown as a result of:• Excavator malfunctions• Technology-related equipment

transportation during disc/blockreplacement.

Reduction in capacity due to:• Initial cutting losses• Rotary (sickle-type) cutting losses• Subgrade/footwall excavation• Quality-related capacity assignment in

accordance with the quality assurancesystem

• Special excavations (overburden,relocation etc.).

A number of investigations and testsrevealed that under the prevailing con-ditions at Nochten mine a linked open- orclosed-loop control system for four miningmachines with the aim of achieving auniform volume of material being movedis not feasible. The solution is to be foundin keeping the belt speed under constantcontrol, which will be explained at Point3.3.2. Economic efficiency constitutes amajor factor when employing a belt-speedcontrol.

by a consortium made up of the followingcompanies:Siemens (ATD Division), STAMAG(mechanical supply of the belt conveyorsystem), FAM Magdeburg (KSS stacker-reclaimer), BEA Technische Dienste Lausitz(KSS electrical assembly) and theconstruction company Schwarze Pumpe(construction supply), with Siemens (ATDDivision) in the role of prime contractor.

From the preparation to the trial stage,the definition of tasks was accompaniedby a research project run by the Facultyof Mechanical Engineering at SenftenbergTechnical College. Details of the technicalperformance can be seen at Figure 3.

The belt is tensioned by motor-drivencable winch. Depending on the capacityrequirements the belts are equipped witha maximum of four three-phase squirrelcage motors each driving a lower andupper belt drum via gear units.

Fig. 3Conveyor routing

As the electrical equipment had to bereplaced in full after 25 years of operation,a large number of synergies arose. Theadded expense of the converter solutionamounted to some 25% in comparisonwith conventional systems. Besides thequestion of costs, additional investigationsare to be carried out with regard to thesystem's endurance properties. Thecorrelation between variable speed,increased system utilization and thewearing of major system components isespecially worthy of study. The kineticresistance properties of the belt underthese conditions are also in need of re-evaluation. The start-up and run operationsare of particular importance here.

3.2 Design of the drive systems

The definition of tasks for the conceptwas drafted by an interdisciplinary LAUBAGteam. The project was implemented

3. The concept behind a variable-speed belt conveyor

Figur ?:Dies ist ein Blindtextund steht in keinemZusammenhang

900 kW M3~

1000 kW

1000 kW

900 kW M3~

1000 kW

1000 kW

900 kW M3~

1000 kW

1000 kW

900 kW M3~

1000 kW

2) 2) 2) 2)

1000 kW

4200 kVA6/0,69/0,69 kV

1)

M

3 AC 6 kV

1) NTSCGEWöu 3,6/6 kV2x(3x95+3x50/3)

2) NSSHöu-O 4x(3x95)

Fig. 4Drive configuration

Conveyor drive 4 x 900 kWwith SIMOVERT Master DrivesConveyor K 63, K 66

5

Bd. K66

Bd. K67

Distributor 1

+128

2 Hs-S VEAG Feeder conveyor

Bd. K72

VEAG Building

Distributor 2

As-G

Train loadingConveyorsS1 to S4

Ks-SBd. K71

+128

+126

+124,8

+80,7 +78,5+49

+50

+54

Bd. K61

Bd. K62

Bd. K63Bd. K64

Bd. K65

Belt conveyor

Bd. K61

Bd. K62

Bd. K63

Bd. K64

Bd. K65

Bd. K66

Bd. K67

Bd. K71

Bd. K72

Phase one IBN 11/1997

Phase two IBN 05/2000

Bd. K73

Bd. K74

AA(m)

1870

1720

2045

1735

870

2525

1755

665

220

114

690

BB(mm)

2000

2000

2000

2000

2000

2000

2000

2000

2000

2000

2000

Belt

P3150

P3150

St3150

St3150

St3150

St3150

St3150

St3159

St3150

St3150

St3150

P(kW)

3x900

3x900

4x900

3x900

3x900

4x900

3x900

2x630

2x630

1x630

2x630

Phase onePhase two

The motors have the followingparameters:

PN = 900 kW (630 kW)MN = 8665 Nm (6060 Nm);

MK/MN = 2.8 (2.6)UN = 690 VIN = 2 x 455 A (630 A)nN = 992 min-1

Speed adjustment range:500 - 1000 min-1 with constant torque

Cooling system:self-cooling/integrated cooling system

Connection to two separate 3-phasewinding systems.

A SIMOVERT® MASTERDRIVES inverteris connected to each of the motor.The inverters are supplied by a commondirect-current link via two feed unit orfeed/feedback units connected in parallelwith each other.

The converter transformer is a three-winding transformer, which means thata 12-pulse system perturbation isachieved. The converter transformer issupplied in the primary circuit by a 6 kVmedium voltage unit (Fig. 4).

The drives are designed for:Starting torque = 1.3 x MnMaximum running torque:approx. 1.6 x MnStart-up time = 60 sBraking time = 18 sCreep speed = 200 min-1 at 0.8 MN for

S1 operation

The inverters adjust the speed of thedrives by means of a TRANSVEKTORcontrol. They are equipped with fullydigital SIMOVERT MASTERDRIVESsystem components with IGBT powertransistors for connection to a motor withtwo separate winding systems. Coolingis provided by a closed-circuit coolingsystem.

The braking energy occurring on shutdownis fed back to the line supply with the aidof a second thyristor bridge.

The technological controls consist of asoft ramp generator for the setpoint speedof a drive station. Each motor has its ownspeed controller with adjustable loadregulation. Tachometer speed controllersare not employed, the actual speed istaken from the motor model.

To cushion the effect of speed fluctuationsin conjunction with two motors workingat one drive drum a speed differentialcontrol is provided. By storage of themost recent torque value prior to driveshutdown and precontrol of the setpointrate of acceleration the torque controlleris provided with a defined precontrol valuefor the next start-up operation. Unevenload distribution caused, for example, byunequal drive drum diameters is preventedby a load sharing control.

All electrical equipment (6kV, 500/400 V,converter equipment, control and monito-ring equipment) is housed in a closedcontainer (9500 mm x 6300 mm L x B).In the case of movable equipment (K61,K62) the containers are mounted on thedrive station. At permanent items ofequipment (K63 to K66) the containersare positioned alongside the equipment.

The technical design of the Nochtencoal belt conveyer system is documentedat [1].

3.3.1 The load-dependent belt-speedadjustment system

Electricity consumption represents a majorcost factor when operating belt conveyorsystems and is influenced by a largenumber of parameters. The effect of theseinfluencing factors has been looked at indetail in the relevant specialist literature,including [5, 6, 7, 10].

In the case of the belt conveyor systemat Nochten the task of adjusting the beltspeed according to the various quantitiesto be transported so as to exploit theavailable load cross-section to the greatestpossible degree was turned into reality.This solution will be described in relationto power consumption below.

The required drive output Pel of a beltconveyor, which has to be transmitted tothe materials being transported by oneor, simultaneously, by more than one drivedrum, is arrived at based on the kineticresistance F according to (4):

Pel = (1/η) F · v (1)

v = belt speedη = efficiency of gearing and motor

The kinetic resistance F is calculatedaccording to DIN 22 101 in equation (2).

F = C fRT L(m'R + (2m'G + m'L) cos _) g + H m'L g (2)

If we put equation (2) in equation (1),we obtain Pel with cos δ = 1:

Pel = (1/η) C fRT L (m'R + 2m'G) g v + (1/η) (C fRT L g + H g) · (m'L v) (3)

whereL = distance between drum centersm'G = mass of material being

transported in kg/mm'R = mass of idlers in kg/mm'L = mass of material being

transported in kg/m(distance load)

δ = conveyor gradientH = difference in elevationC = coefficient of lengthfRT = coefficient of friction and

temperatureIm = handling capacity in kg/s

or t/h (after conversion)g = acceleration due to gravityKO = (1/η) C fRT L (m'R + 2m'G) gKL = (1/η) (C fRT L g + H g)Im = -mL v

Using the abbreviations KO, KL and Im itis possible to write in simplification:

Pel = KO v + KL and Im (4)

The lifting work (H · m'L · g) is independentof the speed and no longer taken intoconsideration below.

According to [11] and [5] fRT also becomesa function of belt speed. This correlationwas demonstrated during the practicaltrials.

By breaking down the drive output into ano-load element (KO v) and a load element(KI Im) the following facts becomeapparent:

• The no-load element is proportional tothe speed v

• The load element is proportional to thedistance load m'L and speed v.

3.3.2 Evaluation and representation of the initial test results

The trials delivered the correlations shownat Figure 5.

Essentially, the theoretical principles inaccordance with equations (3) and (4)were confirmed, see also [2, 3].

First, sufficiently significant evidence wasgathered to demonstrate a proportionaldependence between the electrical driveoutput and variable load at a constant beltspeed in each case. This means that inpractice fRT can be assumed to be virtuallyconstant at a fixed temperature andconstant belt speed in each case. Incontrast, fRT decreases as the belt speeddrops, i.e. specific energy consumptionalso decreases due to the diminishingload KL.

The parameterization for the envelopecurves (1, v = vnom) and (2, v = 0.5 vnom)was done on the basis of a performancetest conducted on the Nochten beltconveyor system in the period betweenDecember 11, 1997 (10 p.m.) andDecember 12, 1997 (2 p.m.).

The data obtained was analyzed usinglinear regression; the results for thepermanent conveyor lines K63 to K71were grouped together to give standar-dized mean values.

PelP el=PN1,0

0,9

0,8

0,7

0,6

0,5

0,4

0,3

0,2

0,11 P02 Pn

0,26v`=0,5

(2)

(3)

0,90,80,70,6

v`=1

(1)

0,51,0 I´m = Im / ImN

6

Fig. 5

h

The stock of values available for theenvelope curve (2, v = 0.5 vnom) will haveto be increased by further trials. The curvesection (3) is determined by the thresholdcapacities of the conveyor lines fordownward adjusted belt speeds(v - 0.5 ... vnom).

A second result is obtained from theformation of handling capacity meanvalues over various time patterns(see Fig. 6).

During the performance test fourexcavators (2 SRs I300 in the high cut,2 ERs 710 in the low cut) were in service.They were configured to run at maximumoutput by mine manage-ment. Themaximum eight-hour output achieved inthe period between 4 a.m. and 12 noonamounted to 40,223 t (a very high figurein comparison).

In evaluation of the results the slidingmean handling capacity value wasobtained by 5, 15, 30 min and 1, 2, 4, 8hour averaging (see Fig 6.). The maximum8 hour mean value was 4643 t/h.

The following conclusions can be drawnfrom this series of measurements.

1. Attempts to continuously move alongthe length of a belt the maximum quantity of material that can be handledaccording to the available belt cross-section proved unsuccessful. The integration time of five minutes (5 minmaximum value = 84.36%) correspondsto the running time of a longer conveyor(1800 m at 6.0 m/s).

These findings are to be consolidatedduring long-term testing. There exists a genuine basis for ”re-optimizing”the drive output in accordance withDIN 22 101 for similar coal belt conveyorsystems.

Protection against short-term peak loadscan be provided by intermittent overloadoperation (operating mode S6 in accor-dance with EN 6000341).

2. With an integration time of less than four hours the integrated maximum handling capacity value lies below 50%of potential handling capacity. As a minimum requirement this necessitatesa belt speed adjustment range of50 to 100% vnom.

Theoretically, the control range wouldneed to be extended downwards. Asimple co-ordination of the machines soas not to drop below the 50% mark wouldappear to make sense.

Fig. 6

100%7550250

-25

Belt scale K65 t/h 1s Mean value

100%7550250

-25

Belt scale K65 t/h 5min Mean value

100%7550250

-25

Belt scale K65 t/h 15min Mean value

100%7550250

-25

Belt scale K65 t/h 30min Mean value

100%7550250

-25

Belt scale K65 t/h 1h Mean value

100%7550250

-25

Belt scale K65 t/h 2h Mean value

100%7550250

-25

Belt scale K65 t/h 4h Mean value

100%7550250

-25

Belt scale K65 t/h 8h Mean value

Test results 11/12/97 21:30 - 12/12/97 13:00

22:00 0:00 02:00 04:00 06:00 08:00 10:00 12:00

11/12/97 12/12/97

100

75

50

25

0

100%

75%

50%

25%

Belt scale K65 (%) V (%)

4:3012.12.1997

4:35 4:40 4:45 4:50 4:55time (sec

5:00 5:05 5:10 5:15 5:20 5:25

Readings

100

75

50

25

0

100%

75%

50%

25%

Belt scale K65 (%) V (%)

6:3012.12.1997

6:35 6:40 6:45 6:50 6:55time (sec

7:00 7:05 7:10 7:15 7:20 7:25

Readings Namemax. load K 62(%)Belt scale K65(%)

max96.7899.02

min34.96310.210

mean72.6280.61

0

0

Namemax. load K 62(%)Belt scale K65(%)

max91.65383.59

min27.9328.301

mean64.82545.5

Fig. 7

7

3.3.3 Implementation of thebelt speed adjustment

Loading of the K61 and K62 face con-veyors is controlled from the control pointaccording to the machines in use.

K61 e.g.:1 excavator 50% nominal belt speed2 excavators 75% nominal belt speed3 - 4 excavators 100% nominal belt speed

The lower base value for the K62 belt is75% of the nominal belt speed.

From belt K63 to the loading facility allbelt lines are controlled from 0.5 - 1 vnomaccording to the quantity of material beingtransported.

The actual quantity of material beingtransported is determined 100 m beforethe point of transfer from belt K62 to beltK63 (transfer to the permanent head belt)by a Vegasonde sensor (made by VEGA,measuring principle: ultrasound). Thedistribution function of the data recordedwith regard to the quantity of materialtransported is shown in Figure 7.

In the process the control system doesnot compensate for every peak load. Shortoverload peaks can be absorbed by theconveyor system (peripheral loading andstorage in transfer chutes) [2]. Load-dependent control is currently beingachieved as follows: the data obtained issent via the Vegasonde sensor to a shiftregister, which is a pulse-clocked device,and maintained as the setpoint in accor-dance with the running time of the belt(e.g. 15 s at vnom) up to the K62/K63transfer point; smaller values aresuppressed during this time. The setpointvalue is adjusted upwards only by a lateroccurring larger value.

A downward speed adjustment isachieved by means of a gradual decele-ration ramp (vnom - vmin) in five to tenminutes. The complete loading controlsystem is much more extensive andspecific than described here. Furtherdetails can be obtained from thedocumentation [1].

Figure 7 shows two distribution functionsfor different load runs. For the first loadrun the average quantity amplitude is45.5%, the control attains a mean speedof 60% vnom, the minimum downwardspeed is limited to 50%. The limit speedlines (red) correspond to the decelerationramp described above.

For the second load run the average load-dependent belt speed is approximately75% vnom.

During long-term testing mean beltspeeds of 0.68 vnom were achieved bythis method. With these settings up to25 control operations occur in an hour.Even better speed adjustment can beachieved with steeper deceleration ramps.However, the number of control opera-tions increases.

At present work is being undertaken inconjunction with Senftenberg TechnicalCollege and the ATD Division of Siemensto qualify these operations.

3.3.4 Energy conservation resultsto date

On completion of a long-term studyelectricity savings of 20% have hithertobeen demonstrated.

This figure already takes into accountconverter equipment (semiconductorvalves, ventilation) losses totalling 3.5%.The average belt speed has becomeestablished at vr - 0.68 vnom during con-trolled operation. The monthly energy billsincurred so far have more than confirmedthese figures.

The 20% savings can be broken down asfollows:

• Load-dependent belt speed adjustment

With an average haulage capacity of IM/INbelow 50% and an average belt speed of0.68 vnom the savings achieved accordingto Figure 5 amount to 16%. The potential25% savings (difference between curves(1) and (2)) have been demonstrated duringtesting under practice conditions onseveral occasions (Siemens).

• Savings achieved by removal of slipresistors

Permanent slip resistors on conventionalmachines are designed for approximately4% belt slippage (s) in order to achieveload compensation for the driving torques.

Saving:

∆P = s Pnom = 3%

• Removal of starting resistors onslipring motors

∆A = 3 x I2start · Rstarter ∆t · z

With z - 2700 start-up processes/∂ theenergy saving amounts to 0.5%.

• Acceleration energy reduction

Starting up only at v = 0.68 vnom yieldsacceleration energy reductions ofapproximately 45% (see also Point 3.5'Effect of wear') with a drop in kineticenergy:

(mred + (m'R + 2m'G + m'L) · L) · v2/2mred – reduced driving massm'L – see equation (2, 3)

Related to the annual energy consumptionof all the equipment this gives a savingof approximately 0.4%.

• Start-up losses due to simultaneousstart-up of converter-controlled beltconveyor systems

Conventional machines(wound rotor motors consecutively withtime delay)

8

Stacker / reclaimer

Nine conveyors, each with a start-up timeof 30 s, are started up with a 20% timeoverlap. This gives:

(9 x 30) · 0.8 = 3.6 min

Converter-driven conveyor systemswith 5 s time delay

9 x 5 s = 0.75 min∆A = 2.85 min. · Pφ · hour/a · zz = number of starting operations per year

Resultant energy saving per annum =approx. 3% due to enhanced equipmentcapacity utilization on start-up (excavatorsand conveyors).

• Braking energy recovery

To date the braking energy has beenconverted to heat; now the braking energyis fed back to the line supply by the feed-back power converters of the converterdrives. The braking energy is calculatedaccording to:

(mred + (m'R + 2m'G + m'L) · L) · v2nom/2

mred = rotating driving masses, otherdesignations as at equation (2, 3).

Savings through energy recovery perannum approx. 0.5%.

• The remaining factors of influencewhen using converter drives in com-parison with slipring motors roughlycancel each other out

.These include:

• Line power factor converter ≥ 0.98 cos ϕ slipring rotor ≥ 0.70• Higher transformer losses in conjunction with converters.• Lower deterioration of motor efficiency in conjunction with converter drives.

The additional losses due to cooling (fans,air-conditioning units) and semiconductorlosses amount to approximately 3.5%.

3.4 Effects of load-dependent speed adjustment on equipment status

In light of the operating experience gainedto date any initial analysis can only pointtowards possible answers.

Precise findings will be obtained only on eva-luation of long-term wear measurements.As already demonstrated at Point 3.3, thespecific conveyor belt load per unit of length(distance load m'L) increases as the beltspeed falls; see also equations (2), (3):

m'L = Im/v see also equations (5, 10).

Thus, according to equation (2) there isalso an increase in kinetic resistance F.

This clearly shows that the correlationbetween the change in belt speed andthe associated effects on requiredmaintenance effort is to be consideredcomplex and not merely proportional.

Generally speaking, the degree of wearon a belt conveyor due to friction can beworked out on the basis of the energybalance because:

1. Kinetic energy (friction, acceleration) accounts for more than 90% of energyconsumption and

2.The energy savings achieved by the reduction in speed stem exclusively from the kinetic energy.

In qualitative terms this statement isbased on the following assumptions:

The wear caused by friction is proportionalto the generated frictional work (AR).

AR = µ · K · F · ds = µ · K · F · v · dt (5) ARun = µU · K · FU · vU · ∆tU (5.1) ARger = µg · K · Fg · vg · ∆tg (5.2)

whereARun = uncontrolledARger = controlled

According to Point 3.3.2 (Fig. 5) thefollowing can be treated as equivalent(both occurrences take place in the sametime):

vg = 0.68; vU = 0.68 · vnom andFg = 1.25 · FU; µU = µg and ∆tU = ∆tg

Thus, the product of (F · v), which corre-sponds to the drive output according toPoint 3.3.1, becomes an evaluationcriterion:

(F · v)ger = 1.25 · 0.68 (F · v)un = 0.85 (F · v)unARger = 0.85 ARun (6)

Using this approach we can calculatethat the wear to the moving componentsof a belt conveyor system is reducedon average by approximately 15% whenthe average belt speed is reduced to68% of vnom.

The effects of wear on components willbe dealt with below. We appreciate thatwear processes unfold in a more com-plicated manner than that described here.

The extent and significance of belt wearis dealt with, in particular, at [9].

3.4.1 Wear on conveyor belts and idlers

The coal belt conveyor system wasequipped with used belts and idlers. Forthis reason it is difficult to make and, moreespecially, prove any forecast in respectof wear.

The kinetic resistances are of decisiveimportance for belt and idler wear [10]:• Indentation rolling resistance• Vibration resistance between idler

catenaries• Grinding resistance of the material being

moved and resultant friction on beltand, if applicable, idlers

• Charging and stripping resistance• Damage caused by foreign bodies.

As already shown above, not all influencingvariables exhibit a proportional relationshipto speed.

The dependence relationships are in somecases extremely complicated. To permittheir better estimation it is planned toconvert a part of the mine conveyorsystem into a test section.

The results obtained are to be subject offuture discussion.

The basis for a 15 to 20% saving bya reduction in speed is thus to bedemonstrated.

3.4.2 Changes in wear on the belt drums

The wear to the surface of belt drums inrelation to the speed of the belt is to beassessed with regard to creep (elongationof the belt due to tensile forces) andfrictional work due to contamination bydirt. In accordance with Point 3.4 thiselement of wear ought to be reduced byapproximately 15%.

The slide slip is dealt with at Point 3.5.

It is safe to say at this point that whereconverter drives are used there is hardlyany slide slip worthy of note. This can betaken as a basis of support for the wearanalysis and its projected saving ofapproximately 15%.

3.4.3 Other subassemblies

• Wear to the material stripping devices

In accordance with the product of(FNr · V) the reduction in wear is likely tobe directly proportional to the reductionin belt speed. A 30% reduction is consi-dered feasible.

• Wear behavior on baffle flaps and wearplates

The reduction in belt speed in conjunctionwith an inversely proportional increase inthe specific volume of material beingmoved m'L (equation (2, 3)) causes a linearreduction of the kinetic energy impartedon the baffle flaps.

m · (v2/2) - (m'Lnom/0.68) · (0.68 vnom)_ =0.68 · m'Lnom (v2

nom/2)

The impact face is enlarged by a varyingparabola of trajectory. In conjunction withmore abrasive materials (sand, gravel etc.)this can lead to additional wear. For coalsavings in the range of 20% are antici-pated.

9

6 KV-switchgear

slip resistance (normally 3 to 4%) and aresultant drive operation with naturalmotor slack (0.5 to 0.7%) is to be rejected.This also applies, to the same or to alarger extent, to an alignment of thepermanent slip resistance to varying drumdiameters at nominal load.

In the case of converter drives, such loadtransfer because of different drum dia-meters cannot arise; the load compen-sation equation between the drumsalways maintains the torque at the sameload proportion.

3.5.2 The belt slip problem

Belt slide slip is caused by varying loaddistribution on the drive drum (see aboveand [3]) and mainly occurs at the lowerdrum during the start stage when themaximum belt tension-force relationshipT1/T2 (Eytelweiner Equation) is exceeded:

T1u/T2u < e µα (7)

µ = active friction value between belt and drumα = drum looping angle

The reasons why slide slip occurs can beclearly recognized in Figure 9 (arrow atlower belt tension path). Due to the sharpincrease in starting torque (dM/dt is veryhigh with conventional drives), thevibratory conveyor belt system causestransient recovery to take place.

3.5 Reduction in dynamic forces through use of converter-fed drives

Using converter-controlled drives producedby Siemens (ATD Division), a completelynew drive system for largescale beltconveyor systems has been developedand constructed. The effects on equip-ment system dynamics are outlinedbelow:

For a general understanding of the subjectthe major drive system control functionswill be named first:

• Speed controller (via the frequency)for all motors

• Active current differential control andspeed differential control for all themotors working on a drive shaft

• Load sharing control between upperand lower drive drums

• Load torque and acceleration precontrolwith load torque storage on belt shut-down and

• Rate setter with rate of change limitation(first derivative of acceleration).

3.5.1 Drive characteristics

The power output of the front drivestations is generally transmitted to thebelt by two driven belt drums. The drumdrives are provided with a semi-rigidconnection by the link formed by motor-gear-shaft-gear. A minor amount of slipcaused by the elasticity of the gearcomponents (backlash and spring effect)is contained by the active currentdifferential control (conventionally bymeans of slip resistors). Providing abalance between the upper and lowerdrive drums is a much more complicatedaffair. Stretching of the belts and diffe-rences in belt diameter lead to con-siderable differences in speed and thusdifferences in load between the drums.This problem is outlined at Figures 8and 9.

Figure 8 shows two motor characteristiccurves with 4% nominal slip (slip resistors)and a difference in drum speed of 2%(drum diameter). As the speed of the beltdrums is the same due to the impressedbelt speed, different load distributionsarise for different drum radii.

Example according to Figure 8:

Load occurrence 1:upper drum MO = 1.0 Mnomlower drum MU = 0.5 Mnom

Load occurrence 2:upper drum MO = 1.0 Mnomlower drum MU = 0 · Mnom

Similar load occurrences were recordedby Senftenberg Technical College at theNochten conveyor line prior to the con-version work (see Fig. 9).

The consequences and effects of thefacts described here regarding thekinematics are considerable, cannothowever be further treated at this point.

From the point of view of the plantcapacity (maintenance), above all in partialload operation, removal of the permanent

Fig. 8

10

1501251007550250M

otor

cur

rent

[A

]

50403020100

-10

Torq

ue [k

Nm

]

60

50

40

30

5 5,5 6 10 15Time [min]

5 5,5 6 10 15Time [min]

5 5,5 6 10 15Time [min]Motor current, OLMotor current, ORMotor current, UL

Load run 30% Load run 100%

Torque, ULBelt tension at the KMD [kN]

Idle

upper drive (1)

lower drive (2)

T2u

T1u

ν=ϖ.r=2π.n.r

ν1>ν2>ν3 ; caused by strain slip, see (13)

ν1=ν2(1+ε1–ε2)

ε1,ε2 ; strain coefficient

2π.rνn=

M/Mn

1.0

0.5

0.25

-0.25

-0.5

1% 2% 3% 4%s%slip

lowerdrum

upperdrum

3

2

1

∆nTrnTr

= 2%

Fig. 9Starting and steady operation on ATS

ε1,ε2 ; strain coefficient

Multi-mechanical drives for conveyors

Mot

or c

urre

nt [A

] 250Te

nsio

n [k

N] 150

Bel

t sp

eed

[m/s

] 10

2251359

2001208

1751057

150906

125755

100604

75453

50302

25151

0002555 2560 2565 2570 2575time [s]

Motor 2Motor 1

According to Figure 9 (arrows), theminimum belt tension has the effect ofminimal T2u, and the maximum drive torqueas T1u. Consequently, T1u/T2u becomesextremely large, facilitating slide slip inthe starting process.

The relevant correlations are also illustratedin [8] and [11].

This fact can be compensated with leveland adjusted torque increase during thestarting process. This cannot be achievedwith conventional drives, pre-tension stagesonly serve to reduce the problem. Converterdrives fulfill this requirement by means ofan adapted torque control; torque transientrecovery is avoided as much as possible.A further advantage of the implementationof converter drives is the considerablereduction in friction work when belt slipoccurs. With conventional units, thisgenerates considerable frictional heat AR.

AR = 2 πµ - M anf - ∆n - ∆t (8)

The torque relief (slip) causes the entirespeed range from 0 to nnom to run upimmediately. With a controlled converterdrive this acceleration is limited to the pre-set range (in Nochten it is 5%).

As a result, possible slip friction withconverter drives amounts to a maximumof 5% of the slip-free work, which canoccur with slipring armatures. It thereforebecomes apparent that belt slide slip in thestarting stage of regulated converter drivesis reduced immensely.

Considerable transient belt tension, whichputs strain on the belt tightness andpositioning, also occurs during the shut-down phase with conventional drivesystems (Fig. 10).

These processes are minimized by meansof adjusted rundown procedures withregulated belt drives.

3.5.3 Explanation of the starting and load characteristic curves

Converter plants are selectively started upvia predetermined start inclines (speedover time, for Nochten 60 s:dv/dt = constant). The starting torque istherefore adjusted as required in accordancewith the load state. As a result theacceleration torque

Mb = Jges (dω/dt) - Jges (1/r) - (dv/dt)

in total is reduced considerably.

With conventional drives, the starting torqueis determined by the motor characteristicsset for maximum load. In the case ofminimum load, the starting torque duringstart-up is far too high, thereby putting anunnecessary strain on the plant systems.

∆T = Jges/M Manf - 1/r - ∆v

M Manf= motor starting torque is a set value

Previous test results bear testimony tothis fact.

Of all the starting processes which havebeen evaluated, the maximum startingtorque achieved was 1.07 x Mnom. However,starting peak values of 1.35 x Mnom are thebeing aimed at. Starting processes withconventional drives, in full load operation,produce starting peak values of 1.5 x Mnom,the remaining starting peaks (partial load)lie insignificantly under this value. In practice,the difference between the torque startingpeak values (old/new) is 28%. This eva-luation is based on the highest quarter-hourly value for transport performanceattained during the performance test ofDecember 11-12, 1997:

Belt K63 0.75 MnomBelt K64 0.75 MnomBelt K65 0.85 MnomBelt K66 0.69 MnomBelt K67 0.70 Mnom

The average daily temperature was 8.5°C.If these values are converted to -20°Caccording to (4), the projected parametersare moreorless confirmed. Taking thecorrelations into account (see also 3.3.2),it must be concluded that some passagesof the evaluation procedure for beltcalculation in line with DIN 22 101 mustbe reconsidered for the implementationof converter drives. This especially appliesto the determining of the safety factors(12).

3.5.4 Effects on the maintenance of the electrical units

Savings on electrical maintenance areachieved with the following components:

• SF6 switchgear instead of air-insulatedunits

• Use of encapsulated low voltage units(MCC technology) instead of half-openunits

• Use of decentralized relays instead ofcentralized low voltage units

• Use of sensor/actuator bus or field businstallations instead of conventionalstar-shaped wiring layouts

• Use of converter technology instead ofconventional resistance control

• Use of low-maintenance three-phaseinduction motors instead of slipringarmatures

• Use of handheld radios for deactivatingbelt/stop instead of the morecomplicated and conventional belt/stopinfrastructure (release cords or buttons)

• Installation of switching buildings incontainer construction next to the drivestation in the case of stationary beltconveyor units (avoiding vibratory strain)

• Implementation of on-site inspectionsusing a centrally automated diagnosissystem.

Additional expense is only incurred by thecooling system for the converter units(fan, cooling aggregate). In total it hasbeen calculated that a considerable savingin electrotechnical maintenance require-ments can be achieved by the concent-rated implementation of the measuresnamed above, whereby the savings factorafter the introductory phase will be in theupper half of the savings range.

11

Fig. 10 Stopping process for belt conveyor system Stopping with masses (7|25|97 10:39)

Motor 4Motor 3

mining mining mmining mining mining mining mining mimining mining mmining mining mining mining mining mi

The investment project "Relocation ofCoal Loading, Nochten" was realized inthe LAUBAG in 1997. This relocation wasnecessary in order to comply with legalpollution control regulations. At the sametime, shortening the rail transport distancebrought a considerable economicaladvantage.

The investment requirements were to bekept to a minimum by the use of existingtechnical equipment, and attention wasto be concentrated on a promisinginnovation potential. In particular, the aimwas to achieve results in

• Energy saving• Efficient use of operator personnel• Reduction in maintenance requirements.

This was to be realized in four ways:

1. Energy saving by means of a speedcontrol or regulator throughout theentire coal belt plant

2. Automatic stacking, reclaiming and train-loading of coal, with low operatorrequirements and safe operation.

3. Use of an advanced process control system with low operator require- ments, with integrated communicationsstructure and multimedia applications

4. Realization of a new maintenance management system by means of situation-relevant, diagnosis-supportedmaintenance, using contractors in a new complex manner.

These objectives were achieved to thelargest degree possible.

In particular, this article has attempted toillustrate the technical concept and theregulation principle, as well as the resultswhich have been achieved and which areexpected, of a speed-regulated belt plant.After the theoretical investigations onhand and initial practical experience, thefollowing effects of the use of convertercontrolled large belt plants compared toconventional plants are consideredpossible:

• Electrical power consumption:savings of between 15% and 38%

• Mechanical and electrical maintenanceexpenditure:savings in the main components ofbetween 10% and 30%, in the specialcomponents savings of up to 50%

• Plant construction:reduction in the projected driveperformance by 15% to 30%, and upto 30% of dynamic belt tension andbearing strain.

Siemens Aktiengesellschaft

© Siemens AG. All Rights Reserved

Bestell-Nr. E10001-P11-A3-V1-7600

Printed in Germany

Dispo-No.: 21662 K-No.: 28700

11D6677 TA15312X07 SD 12021.

Subject to change without prior notice

Contact

Siemens AGIndustrial Solutions and ServicesMining TechnologiesP.O.Box 32 4091050 Erlangen, GermanyTel.: +49-9131-74 26 17Fax: +49-9131-72 22 33E-mail: mining@erl9.siemens.de

www.siemens.com/mining

The initial experience gained is beingimplemented in the LAUBAG specificationfor other projects (conveyor Tgb. Jänsch-walde, conveyor ATS 306 Tgb. Welzow-Süd). The total completion costs forconverter controlled drive solutions willnot be greater than those for conventionalsolutions. The costs for the electrical unitswill rise by 15% to 20% compared tothose for mechanical components (driveperformance, output, belt connections,etc.).

The reports that have been presented,and the results that have been achieved,are to be consolidated by the accompani-ment of further technical measurementsand back-up interpretation, in order todevelop further potential in the interestof high economy and decisive cost-savingsin lignite mining.

This article has made concrete referenceto belt conveyor plants with raw coal; thecorrelations are similar in the case ofbarren rock belt conveyor plants; thereare however differences in detail. Forexample the speed adjustment is morebalanced, the energy saving from the loaddependence is somewhat less (thespecific weight of overburden is greaterthan that of raw coal).

Bibliography[1] WEBER, D., KELLER, C., HALKOW, K., a.o.: Pflichtenheft

zum Project Kohlebandanlage und KohleverladungNochten, Siemens AG, ATD/NPI field, April 1997.

[2] BIEGEL, P., METZING, a.o.: Untersuchungen undForschungsergebnisse zum Aufbau der Kohle-bandanlage Nochten Oktober bis Dezember 1997;Technical College Lausitz, Senftenberg, MachineEngineering Faculty.

[3] WUSCHEK, V.: Neuartige Antriebslösung verringertEnergieaufwand bei der Kohleförderung;Siemens Special Edition, Issue 3/1997.

(4] ALLES, R.: Fördergut Berechnung: ContinentalGummiwerke AG Hannover, 1st Edition 1979.

[5] HAGER, M.: Entwicklungsmöglichkeiten derGurtfördertechnik in Tagebaubetrieben;Braunkohle/Tagebautechnik Vol. 43 (1991),Nos. 1-2, pp 26-31.

[6] HAGER, M. and HINTZ, A.: Einfluß des Gurtlaufesauf den Energieverbrauch von Gurtförderanlagen;Braunkohle/Tagebautechnik Vol. 45 (1993),No. 9, pp 5-13.

[7] GÖHRING, H. and NEUGEBAUER, H.:Bewegungswiderstände an Gurtfördern mitB = 2500mm; Braunkohle/TagebautechnikVol. 46 (1994), No, 4, pp 4-12.

[8] ZIEGLER, M:: Einige Aspekte zur Verminderung vonStörungen und Erhöhung der Verfügbarkeit derGroßbandanlagen im Tagebau Hambach;Braunkohle/Tagebautechnik Vol. 47 (1995),No. 1\2, pp 12-22.

[9] EICKEMEIER, J.: Stand der Gurtfördertechnik beiRheinbraun; Braunkohle/Tagebautechnik Vol. 43(1991), No. 1\2, pp 17-25.

[10] GREUNE, A.: Energiesparende Auslegung vonGurtförderbandanlagen; Dissertation at theFaculty of Engineering at the Technical University,Hannover 1989.

[11] FUNKE, H.: Dynamisches Verhalten vonFörderbandanlagen beim Anfahren und Stillsetzen;Dissertation at the Faculty of Engineering at theTechnical University, Hannover 1989.

[12] LODEWIJKS, G.: Non-Linear Dynamics of BeltConveyor Systems: Bulk Solids Handling,Vol. 17 (1997), No. 1, pp 57-67.

[13] GRIMMER, K-J.: Die Verteilung der Umfangskräftebei Zweitrommelantrieben von Gurtbandförderern;BAM, 142nd year (1997), Issue 2.

4. Summary