i
Delft University of Technology
FACULTY MECHANICAL, MARITIME AND MATERIALS ENGINEERING
Department Marine and Transport Technology Mekelweg 2 2628 CD Delft the Netherlands Phone +31 (0)15-2782889 Fax +31 (0)15-2781397 www.mtt.tudelft.nl
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Specialization: Transport Engineering and Logistics Report number: 2014.TEL.7883
Title: Case study of speed control for improving belt conveyor energy efficiency
Author: M.C.M. van Tol, BSc
Title (in Dutch) Casestudy over het verbeteren van de energie efficiëntie van bandtransporteurs door middel van snelheids regeling
Assignment: Literature
Confidential: No
Supervisor: dr. ir. Y. Pang
iii
Delft University of Technology
FACULTY OF MECHANICAL, MARITIME AND MATERIALS ENGINEERING
Department of Marine and Transport Technology Mekelweg 2 2628 CD Delft the Netherlands Phone +31 (0)15-2782889 Fax +31 (0)15-2781397 www.mtt.tudelft.nl
Student: M.C.M. van Tol Assignment type: Literature
Supervisor: Yusong Pang Report number: 2014.TEL.7883
Specialization: TEL Confidential: No
Creditpoints (EC): 10
Subject: Case study of speed control for improving belt conveyor energy efficiency Speed control has been proven feasible to improve energy efficiency of belt conveyor systems. However, speed control is not always the solution for energy savings due to the variation of conveyor operation situations and loading scenarios. In addition, some researchers (e.g. Hans Lauhoff) suggested that reducing the belt speed and maximizing the utilization of the transported material cross section may result in a certain increase of the power consumption during operation. This assignment is to study the application cases of speed control for the energy savings of belt conveyor systems. Based on case studies, the feasibility of speed control in different operation situations will be discussed. The research of this assignment should cover the following:
− To study the principle of belt conveyor speed control and relative requirements especially with respect to conveyor loading scenarios and operation situations;
− To investigate current development and applications of belt conveyor speed control with respect to energy efficiency;
− To analysis the successes (or failures) of improving energy efficiency by means of speed control, based on case studies;
− To summarize the limitations of current belt conveyor speed control and to indicate the scenarios in which speed control can be feasible;
This report should be arranged in such a way that all data is structurally presented in graphs, tables, and lists with belonging descriptions and explanations in text.
The report should comply with the guidelines of the section. Details can be found on the website. The supervisor,
S
UMMARY
Belt conveyors are widely used for transportation of bulk materials over short to medium distances because of their high efficiency of transportation compared to other transport methods [Zhang and Xia, 2010]. But, since belt conveyors are widely used on a large scale they still are large energy consumers. So there still is a large potential for energy savings. There are many ways to realize energy savings for a belt conveyor system, but one of them is belt conveyor speed control during normal operation. The material flows actually transported by belt conveyors are generally considerably smaller than the rated conveying capacity of the belt conveyors [Hiltermann et al., 2011], [Ristic et al., 2012]. The capacity of the belt conveyor can be adjusted by lowering the belt speed to maximize the material cross section on the belt based on the actual material flow. This is called belt conveyor speed control for normal operation.The theoretical background of belt conveyor speed control to realize energy savings is found in DIN 22101 [1982]. In DIN 22101 [1982] a resistance model based on Coulombs law is proposed. In DIN 22101 [1982] it is assumed that the hypothetical coefficient of friction f is constant and thus not a function of velocity and belt load. Therefor, it can be theoretically proven that energy savings can always be realised with speed control by reducing the belt speed and maximizing the belt load [Ristic and Jeftenic, 2012a], [Jeftenic et al., 2010], [Ristic et al., 2012].
One could think of various set-ups to realize belt conveyor speed control. Belt conveyors are generally driven by squirrel cage induction motors [Hiltermann et al., 2011], [Lodewijks et al., 2011], [Rocha et al., 2012]. An AC drive is a device that can be used to control the speed of an induction motor. The input for the AC drive is a reference speed generated by a controller. With respect to the controller, a distinction can be made in active/continuous and passive/discrete speed control.
The development of belt conveyor speed control can globally be divided in three stages. The first stages are characterized by further exploiting the theoretical potential for energy savings by means of simulations and real-world experiments. Lauhoff [2005] proposed an other resistance model than DIN 22101 [1982] and subjects it to various operating situations. Daus et al. [1998] and Hiltermann et al. [2011] performed power measurements during a real world experiment. Subsequently, the development is continued by implement-ing continuous as discrete forms of speed control and measurimplement-ing the efficacy. Jeftenic et al. [2010] proposed a continuous speed control algorithm and verified it by measurements performed on site. Ristic and Jeftenic [2012b] proposed a discrete speed control algorithm in the form of fuzzy logic control and also verified it by measurements.
It is concluded that belt conveyor speed control for realizing energy savings is proven feasible multiple times in literature. Significant energy savings are presented, either via prediction or realization. There is no unanimity about the possibilities to realize energy savings by means of speed control, Lauhoff [2005] predicts an increase in energy consumption. The discussed literature agrees that the hypothetical friction coefficient
f , that is often used in prediction models, is not constant for different conditions. Contrary to what is
as-sumed in DIN 22101 [1982]. These two findings make that it may not be asas-sumed that belt conveyor speed control will always result in energy savings. The material loading scenario is an important aspect for speed control, namely the input. The presented applications indicate that if the material loading rate varies, some kind of conversion (like discretization or averaging) of the measurements is required to use this as the input for speed control. Speed control is proven feasible with this kind of conversion [Daus et al., 1998], [Jeftenic et al., 2010], [Ristic and Jeftenic, 2012b]. The usage of a controllable feeder in combination with speed con-trol is not found in literature and thus not proven feasible (yet). A line of development from continuous speed control to discrete speed control is visible. Discrete speed control is proven feasible [Ristic and Jeftenic, 2012b]. Ristic and Jeftenic [2012b] states that the proposed discrete control algorithm is implemented on a new belt conveyor system at an OPM. Pang and Lodewijks [2011] states that the research will be followed by the implementation of the proposed fuzzy control system in a real industrial environment.
N
OMENCL ATURE
Symbol Meaning Unit
A Material cross section area m2
C Coefficient for the all-inclusive consideration of the secondary resistances –
f Hypothetical friction coefficient of carrying side and return side –
F Force N
FH Total of main resistances of carrying side and return side N
FN Total of secondary resistances N
FS Total of special resistances of carrying side and return side N
FSt Total of gradient resistance of the conveyor load N
g Acceleration due to gravity (g = 9.81m/s2) m/s2
H Lift of the conveyor (uphill conveying: H > 0, downhill conveying: H < 0) m
Im Capacity (mass flow) kg /s
kN Belt rating / minimum required rupture force N /m
L Conveyor length m
mG0 Line load resulting from the conveyor belt kg /m
mL0 Line load resulting from the load on the conveyor belt kg /m
m0
R Line load resulting from the rotating carrying idler of the carrying side and return side together kg /m
M Capacity (mass flow) kg /s
P Power W
Q Capacity (volume flow) m3/s
U00 Flexure resistance force N
v Belt speed m/s
Wspec Specific energy consumption (per meter) W s/(kg m)
δ Angle of inclination of the installation (uphill conveying:δ > 0, downhill conveying: δ < 0) ◦
² Strain –
σ Stress N /m2
φ Filling ratio or filling level –
Note: In this literature review t refers to metric tonnes (1000 kilograms)
viii 0.NOMENCLATURE
Subscript
ac s Overcome special resistance
b Beginning e Ending ec Emtpy conveyor h Horizontal i it h l Lift nom Nominal n nt h N Nominal N R Normal r Rated r e f Reference spec Specific T Total V Vertical 0 Compensation Acronym AC Alternating Current
AFD Adjustable Frequency Drives ASD Adjustable Speed Drives
DOL Direct On Line
DTC Direct Torque Control
ECS Excavator-Conveyors-Spreader
FC Frequency Converters
FLC Fuzzy Logic Control
OPM Open Pit Mine
PLC Programmable Logic Controller PWM Pulse Width Modulation VFD Variable Frequency Drives VSD Variable Speed Drives
C
ONTENTS
Summary v
Nomenclature vii
1 Introduction 1
2 Belt conveyor speed control 3
2.1 Principle of speed control . . . 3
2.2 Speed control for normal operation . . . 4
2.3 Theoretical background of speed control for normal operation . . . 6
2.3.1 DIN 22101 . . . 6
2.3.2 Energy savings based on DIN22101 . . . 8
2.3.3 Other energy calculation models. . . 8
2.4 Speed control for starting/stopping . . . 9
2.5 Components for realization of speed control . . . 11
2.5.1 Induction motor . . . 11
2.5.2 AC drive . . . 12
2.5.3 Feeder . . . 13
2.5.4 Belt weighing and volume measuring equipment . . . 14
2.5.5 Controller . . . 17
3 Applications of belt conveyor speed control 19 3.1 Development stage I . . . 19
3.2 Development stage II . . . 23
3.3 Development stage III. . . 25
4 Analysis of applications 27 4.1 Prediction of energy savings . . . 27
4.1.1 Aspect I . . . 27
4.1.2 Aspect II . . . 28
4.1.3 Aspect III. . . 28
4.2 Realized energy savings . . . 29
4.2.1 Aspect I . . . 30 4.2.2 Aspect II . . . 30 4.2.3 Aspect III. . . 30 5 Conclusions 33 Bibliography 35 ix
1
I
NTRODUCTION
Belt conveyors have proven themselves to be an excellent transportation method of bulk solid materials [Lauhoff, 2005]. They are widely used for transportation of bulk materials over short to medium distances be-cause of their high efficiency of transportation compared to other transport methods [Zhang and Xia, 2010]. But, belt conveyors are large energy consumers. Pang and Lodewijks [2011] states that energy consumption may form over 40% in the total conveyor operational cost. Hiltermann et al. [2011] states that belt conveyors may be responsible for 50 to 70% of the total electricity demand in dry bulk terminals. And to illustrate the significance of the material handling sector, 10% of the electricity consumption of South Africa is consumed by the material handling sector [Zhang and Xia, 2010]. So despite their relative high efficiency, there is still a large potential for energy savings. Besides reduction of operational cost, energy savings also lead to a direct reduction of carbon dioxide (CO2) emission.
Figure 1.1: Illustration of typical belt conveyor layout [Zhang and Xia, 2009]
A typical belt conveyor layout is shown in Figure 1.1 [Zhang and Xia, 2009]. The belt is made up of a (metal) carcass covered with a rubber belt cover. The endless belt turns around the pulleys. To maintain a minimum belt tension, a take up system is used. The belt is driven by an electro motor connected to the drive pulley. The bulk solid material is loaded on the belt by a loading chute. The material is transported from the tail to the head pulley. An idler is a frame with rolls and can be used to increase the carrying capacity of the belt conveyor. The belt changes from a flat profile at the pulley to for example a troughed profile. So the idlers carry the belt.
There are many ways to realize energy savings for a belt conveyor system. According to Zhang and Xia [2010], the energy efficiency can be divided into three components: equipment efficiency, operation effi-ciency and performance effieffi-ciency. Equipment effieffi-ciency can improved by for example using an energy sav-ing idler. Operation efficiency can improved by coordinatsav-ing supply and demand of two sub-systems. For example, the demand of coal by an power plant and the supply of coal from a stock pile. Performance effi-ciency can be improved by using belt conveyor speed control.
Belt conveyor speed control is a recent development with respect to the history of belt conveyors. The first developments date back from a few decades ago. These developments are preceded by developments in industrial motor control. Industrial control encompasses all methods used to control the performance of an electrical system [Wildi, 2002a]. For machinery, this involves starting, accelerating, reversal, decelerating and stopping of a motor and its load.
2 1.INTRODUCTION
Belt conveyor speed control can be used either for normal operation or for starting/stopping of the belt conveyor. The main focus of this literature review will be on speed control for normal operation. This is based on adjusting the belt speed based on the incoming material flow at the tail. Belt conveyor speed control is believed to have a high operation efficiency in the majority of literature [Zhang and Xia, 2009]. Speed control is proven feasible to realize energy savings for belt conveyors, e.g. Jeftenic et al. [2010]. However, the feasi-bility is also challenged, e.g. Lauhoff [2005]. The author of this literature report did widely survey literature for applications (simulations, real-world experiments and case studies) of belt conveyor speed control and set out the main applications. This gives an representative overview of the developments in this field. This creates the possibility to see a line of development, to make comparisons and to analyse on various aspects.
The subject of this literature survey is stated as: ‘Case study of speed control for improving belt conveyor energy efficiency’. The scope of this literature survey is limited to:
• Troughed belt conveyors • of medium and long length • transporting bulk solid materials • driven by induction motors
The outline of this report is as follows. In chapter 2 the basic principle of belt conveyor speed control will be discussed. The reader will also be introduced to various material loading scenarios. This is supple-mented with the theoretical back ground of belt conveyor speed control based on various energy calculation models. The chapter is concluded with reviewing various components that might be used for realization of belt conveyor speed control. In chapter 3 the main applications in the field of belt conveyor speed control are listed. These are either simulations, real-world experiments or case studies. The analysis of various as-pects concerning these applications is performed in chapter 4. The conclusions form the end of this literature review.
2
B
ELT CONVEYOR SPEED CONTROL
In this chapter the principle of speed control is discussed. Speed control of a belt conveyor is concerned with both normal operation and starting/stopping of the belt conveyor. Both principles will be introduced, however the main focus is on normal operation. Various examples of loading scenarios and their efficacy on speed control are discussed in section 2.2. In the subsequent section the theoretical background of speed control for normal operation will be considered. This chapter is concluded with the description of various components that might be used for realizing a speed controlled belt conveyor.
2.1.
P
RINCIPLE OF SPEED CONTROL
The material flows actually transported by belt conveyors are generally considerably smaller than the rated conveying capacity of the belt conveyors [Hiltermann et al., 2011], [Ristic et al., 2012]. Rated (nominal) values are values guaranteed by the manufacturer, sometimes indicated with a name plate on the machine [Wildi, 2002b]. The rated value is not necessarily the maximum value. If a belt conveyors operates at less than the rated capacity, the same quantity of material can be transferred in various combinations of belt speed and cross sectional area of material. This is illustrated in Figure 2.1 [Ristic et al., 2012]. Since the belt speed of the second belt conveyor is lower than the belt speed of the first belt conveyor, a larger cross section of material is realised on the second belt conveyor.
Figure 2.1: Illustration cross section of material in relation to belt speed [Ristic et al., 2012]
v(t ) = [Q(t)/Qr]vr= [A(t )/Ar]vr (2.1)
With the illustration of Figure 2.1 [Ristic et al., 2012] in mind, one could think of lowering the belt speed to maximize the material cross section on the belt based on the actual material flow. This is called speed control for normal operation. According to DIN 22101 [1982], this results in energy savings. The belt speed could be adjusted according to the linear relationship in Equation 2.1 [Jeftenic et al., 2010]. This equation is based on the conservation of mass; the volume flow that enters a belt conveyor is the same as the volume flow that leaves the belt conveyor. The actual material flow is determined by measuring either the capacity Q(t ) or the cross sectional area A(t ) of material on the belt.
It is important to realize that viscosity and elasticity are the main characteristics in considering a dynam-ically loaded rubber belt [Jeftenic et al., 2010]. These properties of the belt indicate that it does matter how the belt conveyor is started or stopped. Speed control can be used to realize a start/stop in a controlled man-ner. This is called speed control for starting/stopping. As an example, a certain velocity trajectory could be implemented on the drive pulley during starting/stopping.
4 2.BELT CONVEYOR SPEED CONTROL
To return to speed control for normal operation; the basic idea of adjusting the belt speed during normal operation based on measurements is shown schematically in Figure 2.2 [Pang and Lodewijks, 2011]. Based on a determined material loading and a pre-defined nominal flow and reference loading, a difference with the reference loading rate is determined in the upper block of Figure 2.2 [Pang and Lodewijks, 2011]. The loading rate (filling ratio) is obtained by dividing the actual material flow by the rated material flow of the belt conveyor. In DIN 22101 [1982], filling ratio’s vary between 0.7 and 1.1. The loading rate depends on the actual belt speed and the actual material flow. The difference in loading rate is the input for adjusting of the belt speed, shown in the lower block of Figure 2.2 [Pang and Lodewijks, 2011].
Figure 2.2: Concept of belt conveyor speed control [Pang and Lodewijks, 2011]
With respect to the controller, a distinction can be made in active/continuous and passive/discrete speed control. In active speed control, the belt speed is adjusted continuously based on the actual material flow. In passive speed control, the belt speed is adjusted discretely based on a set of pre-defined values. These values are based on the expected material flow prior to conveying operation. For discrete speed control, small material flow fluctuations will not result in adjustment of the belt speed.
2.2.
S
PEED CONTROL FOR NORMAL OPERATION
From Figure 2.2 [Pang and Lodewijks, 2011] it can be seen that the material loading is the input for speed control. One could reason that belt conveyor speed control might not be applicable to all types of loading scenarios during normal operation. Above that, belt conveyor speed control might not be required for all types of loading scenarios. The loading scenario of the belt conveyor depends on how the material is delivered to the belt conveyor and how the material is loaded on the belt conveyor. One could also think of processing measurements before using them as the input for speed control.
A pattern of feeding rate by a belt conveyor is shown in Figure 2.3 [Zhang and Xia, 2010]. Zhang considers a conveying system in a coal-fired power plant. The boilers from the power plant are fed with coal from coal bins. The level of the remaining coal in the coal bins should maintain above a lower limit and below an upper limit. In the current applied strategy for feeding the bins, if the level in the bins is below the lower limit, the bins are filled with coal by the belt conveyors until an upper limit is reached. This takes place at rated capacity (1500 t /h, 2.5 m/s). The possibilities for application of speed control in this case are obvious. Due to the steady flow, the belt speed would have to be selected only once. The selected belt speed should be slightly higher than the belt speed in order to reduce the risk of overloading in the case of small material flow fluctuations [Hiltermann et al., 2011]. Besides this attempt to improve performance efficiency, Zhang and Xia [2010] proposes a method to improve operation efficiency (as discussed in chapter 1. It is stated that in this situation the coal consumption of the power plant can be forecasted. Based on this forecast, speed control can be combined with a control strategy. This gives possibilities to spread the transportation of coal over a longer time span or to shift the transportation to hours of the day with lower energy tariffs.
2.2.SPEED CONTROL FOR NORMAL OPERATION 5
Figure 2.3: Feeding rate for power plant [Zhang and Xia, 2010]
Measurements of a loading rate of an other belt conveyor are shown in Figure 2.5 [Daus et al., 1998]. The measurements are performed at an opencast mine while several excavators were in service. The possibilities for application of speed control to this type of loading scenario are not as obvious as they are for the steady flow. Due to the excavator way of operation, the instantaneous capacity of transported material cannot be forecasted and may vary from 0 to 100% [Jeftenic et al., 2010] and thus the feeding is not controlled. Pang and Lodewijks [2011] states that practically the direct measurement of material flow on the belt is not suitable for the input of speed control, since the risks of belt overload and material spillage in the case of short-term peaks in material loading rate. Based on the short-term peaks in Figure 2.4 [Daus et al., 1998], Daus states that continuously moving the maximum quantity of material that can be handled, according to the available belt cross section, over the entire length of a belt is proven unsuccessful. Hiltermann et al. [2011] states that the (stressful) continuous acceleration-deceleration of the belt conveyor with small material flow fluctuations should be avoided. Similar statements are made by Jeftenic et al. [2010], Pang and Lodewijks [2011] and Ristic and Jeftenic [2012b]. Ristic and Jeftenic [2012b] adds to this that it could lead to increased energy consumption. Above that, detrimental vibrations of the belt and belt conveyor construction may occur at certain belt speeds and belt load combinations [Hiltermann et al., 2011], [Pang and Lodewijks, 2011]. The controller should avoid the belt speeds at which this occurs.
Figure 2.4: Loading rate for 1s mean value [Daus et al., 1998]
Pang and Lodewijks [2011] states that in order to avoid unnecessary/harmful acceleration and decelera-tion of a belt conveyor when short-term extreme loads occur, the data from the material weighing measure-ment can be represented by standardized mean values over a pre-defined moving average sample interval. In essence, this is the same as the conversion from Figure 2.4 [Daus et al., 1998] to Figure 2.5 [Daus et al., 1998]. The difference in plot pattern occurs due to different time pattern averaging (1s vs. 8h). The represented material flow may be used as input for speed control. Depending on material flow fluctuation, the sample interval is selected. As an example, a mine is considered with considerable and frequent material flow fluctu-ations. The material flow via an outlet valve controlled hopper will have very small material flow fluctufluctu-ations. In this case the sample interval will be shorter in comparison with a small fluctuating material flow.
Figure 2.5: Loading rate for 8h mean value [Daus et al., 1998]
Pang and Lodewijks [2011] also states that the loading rate can be discretized to avoid the influence of short-term extreme loading situation to the control system. An example is shown in Figure 2.6 [Pang and Lodewijks, 2011]. The material loading scenario is generated in a simulation with a bulk generator. The discretized loading rate is shown with the horizontal lines.
6 2.BELT CONVEYOR SPEED CONTROL
Figure 2.6: Discretization of material loading rate [Pang and Lodewijks, 2011]
Jeftenic et al. [2010] states that when the capacity increases, the speed should be increased to avoid spillage of material. On the other hand, the speed should be decreased slowly, not strictly following the ca-pacity decrease, in order to avoid frequent speed variations and to protect the equipment from unnecessary stress and wear. The belt is divided into a number of segments of equal length. Based on sampling of the measurements of the instantaneous material cross section, one filling rate value is assigned to a segment. So a discrete distribution of the material cross section along the belt as a function of time is developed. An ex-ample is shown in Figure 2.7 [Jeftenic et al., 2010] for a 36 meter long belt conveyor. This offers an advantage for belt conveyor systems consisting of multiple belt conveyors in sequence, since the attached value can “be passed trough” and converted if necessary for the next belt conveyor. So the cross section is measured once and for every subsequent belt conveyor no measuring device is used.
Figure 2.7: Visual representation of discrete discrete distribution of material cross section [Jeftenic et al., 2010]
As indicated in the first paragraph of this section, the loading scenario of the belt conveyor also depends on how the material is loaded on the belt conveyor. This is discussed in detail in subsection 2.5.3.
2.3.
T
HEORETICAL BACKGROUND OF SPEED CONTROL FOR NORMAL OPERA
-TION
2.3.1.
DIN 22101
The DIN 22101 [1982] standard applies to belt conveyors for the conveying of bulk materials and it contains the basic principles for the calculation and design of such conveyors. In view of the scope of this literature review, only the sections of this standard concerning resistances to motion and power requirements in steady state operating condition will be considered. These sections can be used to design the drive system of a belt conveyor. The equations and definitions that will follow in this section are according to DIN 22101 [1982], unless otherwise indicated.
The power P required at the periphery of the driving pulley(s) by a uniformly loaded belt conveyor instal-lation is:
2.3.THEORETICAL BACKGROUND OF SPEED CONTROL FOR NORMAL OPERATION 7
Where F is the total of resistances to motion of the carrying side and the return side in steady state oper-ation and v is the belt velocity. For the power required at the AC drive, mechanical and electrical efficiencies have to be incorporated. F is subdivided into primary/main resistances FH, secondary resistances FN,
gradi-ent resistances FStand special resistances FS.
F = FH+ FN+ FSt+ FS (2.3)
FH= L f g [m0R+ (2mG0 + m0L) cosδ] (2.4)
FN= (C − 1)FH (2.5)
FSt= H g mL0 (2.6)
As can be seen from Equation 2.4, Coulomb’s law is used for the determination of the primary resistance, where f is the hypothetical coefficient of friction. According to DIN 22101 [1982], the primary resistance is the sum of all friction-related resistances along the belt:
• Running resistance of idlers • Flexure resistances:
– belt flexure resistance
– transport load flexure resistance – indentation resistance of belt
The magnitude of the secondary resistances is independent of belt conveyor length. However, if the length of the belt conveyor L becomes larger, the secondary resistances represent a lower percentage of the total resistance, see Figure 2.8 [DIN 22101, 1982]. According to Hiltermann et al. [2011], the secondary resistances are friction and inertia resistances, which only occur at certain parts of the belt conveyor:
• Feed resistance of transported bulk material
• Friction between the bulk material and the loading chute • Friction of the belt cleaners
• Deflection resistance of the belt at the pulleys
Figure 2.8: Standard values for coefficient C for belt conveyors with filling ratiosφ between 0.7 and 1.1 [DIN 22101, 1982] The gradient resistance is the resistance of the conveyor load due to a difference in altitude between the loading and unloading of a belt conveyor. For uphill conveying H > 0 and for downhill conveying H < 0. Compared to the other resistances, overcoming this resistance may require a relatively large amount of power [Hiltermann et al., 2011]. This is illustrated in Figure 2.9 [DIN 22101, 1982].
The special resistance includes (if applicable) camber resistance (arises at an individual side carrying idler), frictional resistance between the material conveyed and the lateral chutes outside the feeder points and resistances of devices for the delivery of goods along the conveying path (e.g. stripping or scraping de-vices) [DIN 22101, 1982]. The special resistance is generally of relatively little influence on the total motional resistance (± 1%) [Hiltermann et al., 2011].
Figure 2.9 [DIN 22101, 1982] is shown in appendix A of DIN 22101 [1982]. This figure illustrates an example of the portions of the primary, secondary, gradient and special resistances for a long belt conveyor (over 1000
m). The left column represents a horizontal belt conveyor, the right column represents a belt conveyor with
5% inclination. It appears that the indentation resistance of the belt is the most significant resistance for a long horizontal belt conveyor.
8 2.BELT CONVEYOR SPEED CONTROL
Figure 2.9: Comparison of two long belt conveyor of identical design, with different inclination [DIN 22101, 1982]
2.3.2.
ENERGY SAVINGS BASED ON
DIN22101
In DIN 22101 [1982] it is assumed that f is constant and thus not a function of velocity and belt load. Therefor, energy savings can always be realised with speed control by reducing the belt speed and maximizing the belt load. This reasoning is used throughout literature for various applications and theoretically proven by Ristic and Jeftenic [2012a], Jeftenic et al. [2010] and Ristic et al. [2012]. This theoretical prove will be shown in this section. For sake of simplicity, assume that FN= FSt= FS= δ = 0. Based on Equation 2.3 and Equation 2.4, F
can be rewritten as:
F = [L f g mL0] + [L f g m(m0R+ 2m0G)] (2.7)
By substituting m0
L= ρ A (with ρ the density of the material), power P can be expressed as:
P = [L f g ρ A]v + [L f g m(mR0 + 2mG0)]v (2.8)
If the feeding material flow is constant, according to the conservation of mass the product Av is also con-stant (within the physical limits). Therefor, the first product is concon-stant. The second product is not concon-stant and depends on v. Therefor, according DIN 22101 [1982] energy savings can be realised by minimizing belt speed v and thus maximizing the material cross section area A. For sake of simplicity, it was assumed that
FSt= 0. It may be not clear directly why speed control does not affect the power to overcome gradient
resis-tance. This can be seen with the same reasoning as above, but now assume FSt6= 0 and substite m0L= ρ A in
FSt.
2.3.3.
OTHER ENERGY CALCULATION MODELS
Besides the model derived by the DIN 22101 [1982] standard, there are other energy calculation models to design the drive system of a belt conveyor. These models also derive from standards or specifications, such as ISO 5048 [1989], JIS B 8805 [1992], CEMA [1997] (Conveyor Equipment Manufacturers Association), FDA (Fenner Dunlop Australia) and Goodyear Tire & Rubber Company [1975]. The energy calculation models can be divided into two categories [Zhang and Xia, 2009]. The first category is based on resistance calculation methodology and contains ISO 5048 [1989], DIN 22101 [1982] and CEMA [1997]. The second category is based on energy conversion methodology and contains JIS B 8805 [1992], FDA and Goodyear Tire & Rubber Company [1975]. The first category will not be reviewed in this section, because the basic idea is already made clear in subsection 2.3.1. The second catergory will now be reviewed.
The power of belt conveyor under stationary condition can be divided into three components [Goodyear Tire & Rubber Company, 1975]:
2.4.SPEED CONTROL FOR STARTING/STOPPING 9
• Power to run the empty conveyor: Pec
• Power to move the material horizontally over a certain distance: Ph
• Power to lift the material a certain height: Pl
• Power to overcome special resistances: Pac s
These powers are calculated with the following empirical formulae:
Pec= g f (Lh+ L0)Z V (2.9)
Ph= g f (Lh+ L0)T /3.6 (2.10)
Pl= g HT /3.6 (2.11)
With Lhthe horizontal centre-to-centre distance, L0the compensation length constant and Q the mass of
moving parts of the equipment. L0is an empirical compensation length constant. L0is used to compensate some components of the frictional force, since these components are independent of belt length [Zhang and Xia, 2009]. JIS B 8805 [1992], FDA and Goodyear Tire & Rubber Company [1975] are all based on this energy conversion method but the deterministic specifications for L0are different [Zhang and Xia, 2009]. The total power required by the belt conveyor is:
PT = Pec+ Ph+ Pl+ Pac s (2.12)
The resistance calculation models are believed to be more accurate than the energy conversion models [Zhang and Xia, 2009]. But the disadvantage is the resistance calculation models require many exact design and operating parameters. On the other hand, the energy conversion models are more easy to use since less parameters are required by implementing L0in the model. However, since just one or few compensation length constants are used, there must be some energy calculation errors [Zhang and Xia, 2009]. Zhang and Xia [2009] proposes a new energy calculation model by interlinking the resistance calculation models and the energy conversion models. Zhang and Xia [2009] states that the model is easier to use than ISO 5048 [1989] and DIN 22101 [1982] and more accurate than JIS B 8805 [1992] or FDA.
Zhang performed energy calculations with seven energy models to calculate the power of the belt con-veyor under certain conditions given in the paper. The results for varying the belt speed between 1.5 and 6
m/s is shown in Figure 2.10 [Zhang and Xia, 2009]. For ISO 5048 [1989] and DIN 22101 [1982] are both an
accurate calculation method as the C coefficient method used. TwoL0 denotes the new energy calculation model proposed in the paper. Taking ISO 5048 [1989] as a baseline, the relative errors of the other models are shown in Figure 2.11 [Zhang and Xia, 2009]. It is stated that the applicability and validity of the new energy energy calculation model are proven by this comparative study of all energy models [Zhang and Xia, 2009]. Many conclusions can be drawn from the two presented figures. However, the main point for this section is that there are quite a few calculation models to estimate the operating power of a belt conveyor and the predicted energy savings will differ from model to model. On the other side, one could say that they are all in line with DIN 22101 [1982] and thus energy savings can always be realised with speed control by reducing the belt speed and maximizing the belt load.
2.4.
S
PEED CONTROL FOR STARTING
/
STOPPING
To accelerate a belt conveyor, a drive force is applied on the system by using an induction motor. During deceleration this is done by some form of brake. This drive force depends on the inertia of all the moving mass, friction and elevation of the load but also on the starting or stopping procedure. Due the the applica-tion of this drive force and the elastic behaviour of the belt, transient forces occur on the belt during starting or stopping until a steady state condition is reached. The transient forces alternate between a maximum and minimum value. A reduction of transient forces will reduce the required belt size and thus the belt weight. However, minimum acceleration may be dictated by properties of the induction motor and minimum decel-eration may be dictated by safety reasons or material flow at transfer points [Cahners, 1966a].
10 2.BELT CONVEYOR SPEED CONTROL
Figure 2.10: Power curves for belt conveyor with L = 1000m and T = 1200t/h [Zhang and Xia, 2009]
Figure 2.11: Relative errors for belt conveyor with L = 1000m and T = 1200t/h [Zhang and Xia, 2009]
In Figure 2.12 [Lodewijks, 1996] the belt speed at the drive pulley and tail pulley is plotted during a sim-ulation of a direct start of an unloaded belt. The start is not smooth and the (alternating) elastic behaviour of the belt is visible. One could think of alternative procedures. For squirrel cage motors, the most common starting methods are a direct on line start (DOL), star-delta start, softstarter and the use of a AC drive [ABB, 2010]. However, according to ABB [2010], if the AC drive is only used for starting and stopping the motor and there is no need for continuous speed regulation, the AC drive is an unnecessarily expensive, large, heavy and complex solution with respect to for instance a softstarter.
2.5.COMPONENTS FOR REALIZATION OF SPEED CONTROL 11
In Figure 2.13 [Lodewijks, 1996] the belt speed at the drive pulley and tail pulley is plotted during a sim-ulation of a velocity controlled start. The speed of the drive pulley increases linearly from an initial speed to a stationary speed in 30 seconds. The start-up is smoother in comparison with Figure 2.12 [Lodewijks, 1996]. According to Lodewijks [1996] the maximum motor current and couple are much lower and the latter results in lower belt tensions. According to DIN 22101 [1982], these lower belt tensions and lower safety factors (due to smoother operation) will result in a lower minimum required rupture force kNof the belt. Of course, other
velocity trajectories can be implemented, like an “S-ramp” (see Jeftenic et al. [2009] for example) and/or in-troducing “rest” periods. According to Lodewijks [1996] this will give very satisfactory results for minimising peak tensions during transients. For the stop procedure, a mirror like characteristic of the start profile can be used.
Figure 2.13: Belt speed at drive pulley (solid line) and tail pulley (dashed line) during a velocity controlled start [Lodewijks, 1996]
Jeftenic et al. [2009] points out other advantages: controlled start and stop of the belt conveyor reduces wear of gears and belt and elimination of belt slippage on drums during the start and in operation. The latter is eliminated by automatic slippage compensation, without stopping the belt conveyor. If belt slippage is detected, the drive quickly reduces speed to the speed of the passive drum. After the speeds become equal, the drive gradually increases the speed to the previous set speed and continues operation. It is mentioned that belt slippage on a conveyor with conventional control and drive system would have led to a stop of the belt conveyor. This continuous operation is of course another advantage.
Lodewijks [2002] states that there is an important difference in starting a fully loaded belt or an empty belt, as far as power and belt tensions is concerned. Besides that, the some holds for a start after an emergency stop or an start after an operational stop, as far as belt tension distribution and sensitivity to the starting procedure is concerned. A solution is the usage of control procedures based on the worst case conditions, however this puts the conveyors system in all normal operation conditions under a relatively high strain [Lodewijks, 2002].
2.5.
C
OMPONENTS FOR REALIZATION OF SPEED CONTROL
2.5.1.
INDUCTION MOTOR
Globally, there are two main types of AC motors: asynchronuous motors (also known al induction motors) and synchronous motors. Examples of induction motors are squirrel-cage, wound-rotor and linear induction motors. Induction motors are the motors most frequently encountered in industry; they are simple, low-priced and easy to maintain [Wildi, 2002c]. Belt conveyors are also generally driven by squirrel cage induction motors [Hiltermann et al., 2011], [Lodewijks et al., 2011], [Rocha et al., 2012]. Jeftenic et al. [2009] states that squirrel cage induction motors have high robustness and low price and therefor are very suitable for application in open pit mines. Rocha et al. [2012] states that squirrel cage induction motors meet most often the operation requirements with respect to dust, high temperatures and vibrations.
Figure 2.14 [Wildi, 2002d] shows the torque-speed curve of a conventional 3-phase induction motor. This curve depends on the motor size (and as we will see in subsection 2.5.2 on the supplied frequency and
volt-12 2.BELT CONVEYOR SPEED CONTROL
age). The breakdown torque is the maximum torque developed, at full load the maximum speed is developed. The pull-up torque is the minimum torque developed by the motor while it is accelerating from rest to break-down torque. For low-speed drives it is preferable to use a high-speed motor and a gearbox instead, since for a given output power the size and cost are less and the efficiency is higher for a high-speed motor in comparison with a low-speed motor [Wildi, 2002e].
Figure 2.14: Typical torque-speed curve of a 3-phase squirrel-cage induction motor [Wildi, 2002d]
2.5.2.
AC
DRIVEAn AC drive is a device used to control the speed of an AC motor (either an asynchronous/induction motor or a synchronous motor). In literature, AC drives are known by various other names such as variable speed drives (VSD), variable frequency drives (VFD), adjustable speed drives (ASD), adjustable frequency drives (AFD) and frequency converters (FC) [Vacon, ].
The basic components of an AC drive are a rectifier, inverter and the core of the AC drive, see Figure 2.15. The rectifier converts AC to DC and the inverter does vice versa. The core of the AC drive contains a system that processes inputs from the rest of the displayed circuit and components. The core also contains inputs like a speed or torque reference value. The speed of the motor is controlled via the output(s) of the core by adjusting the supply to the motor. The details of the core depend on the type of AC drive.
Figure 2.15: Basic components of an AC drive
The basic working principle of an AC drive is changing the frequency and voltage of the supply (3-phase voltage) to the motor. Via coils on the stator the supply creates a rotating magnetic field. The rotor will follow this rotating magnetic field. In practice, the changing is done according to the “Volts-per-Hertz” relationship [Wildi, 2002d]. This means that if for example the frequency is doubled, the voltage must also be doubled. By doing this, a constant flux in the air gap between the stator and rotor is maintained plus we ensure that the
2.5.COMPONENTS FOR REALIZATION OF SPEED CONTROL 13
flux in the motor is always close to it’s rated value [Wildi, 2002d]. If frequency and voltage are reduced the torque-speed curve (see Figure 2.14 [Wildi, 2002d]) moves horizontally to the left and vice versa.
Two physical quantities determine the state of the shaft of the motor: speed and torque. The function of the AC drive is controlling these quantities according to the speed or torque reference value. In practice, either one of them is controlled. The AC drive operates either in torque control mode (speed is determined by the load) or in speed control mode (torque is determined by the load) [ABB, 2011]. The velocity trajectories during starting in section 2.4 are velocity controlled. There are different types of AC drives, which have developed trough time. These different types differ in many aspects, like usage of a feedback device, usage of modulator (varies one or more properties of a periodic waveform), control variables, speed and accuracy. An AC drive can operate either closed loop or open loop. In closed loop operation a feedback device (e.g. a tachometer, an ammeter) is used that provides a feedback signal to the AC drive that is used to control the operation of the AC motor. In open loop operation no use is made of a feedback device. According to ABB [2011] the three milestones of AC drives are:
• Frequency control using pulse width modulation (PWM) • Flux vector control using pulse width modulation (PWM) • Direct torque control (DTC)
A closer look will be taken at DTC, since this is the most advanced AC drive technology developed [ABB, 2011] and DTC strategies have extensively been implemented in induction machine drives [Masmoudi et al., 2014]. DTC has gained its popularity since the beginning (in the 80’s) for its simplicity, good performance, independent machine rotor parameters and robustness [Bouhoune et al., 2014]. The simplicity is due to its structure, since no coordinate transformations, current controllers and modulations are needed [Ortega et al., 2005].
DTC is an AC drives control principle. With DTC the control of torque or speed is directly based on the electromagnetic state of the motor (and not input frequency and voltage). So as stated before, also this AC drive control principle can operate in speed mode, despite its name. The controlling variables are stator flux and motor torque. The measured input values to the DTC control are motor current and voltage. These values are inputs to a motor model which produces an accurate and actual value of stator flux and torque. The outputs of the motor model are compared to reference values in a controller. Controllers indicate whether the stator flux or torque has to be varied. Depending on the output of the controllers, an electric circuit influences the frequency and voltage of the supply to the motor, which in turn influence the motor torque and flux (and also the motor current and voltage), and the control loop is closed. DTC works with any type of asynchronous/induction motor, plus for most applications no tachometer or encoder is needed to feed back a speed or position signal [ABB, 2011]. A detailed scheme of the core of a DTC AC drive by ABB can be found in [ABB, ].
2.5.3.
FEEDER
In section 2.2 it is shown that the supply of bulk solid materials is not necessarily steady. The loading sce-nario of the belt conveyor also depends on how the material is loaded on the belt conveyor. Intermittent or irregular feeding of the material to the belt is according to Cahners [1966b] not desired, it will result in loss of capacity and probably spillage of material over the edges of the belt. When the supply of material is irregular, it is essential to provide a hopper and some type of feeder to deliver to material to the belt at an uniform rate [Cahners, 1966c]. The hopper, or an other temporary storage device like a silo, decouples the incoming and outgoing material flow. An other purpose of a feeder is placing the material centrally on the belt and with a material velocity in the direction of the belt as nearly as possible to the belt velocity. Since these condi-tions will result in minimum wear on the belt cover, minimum power consumption and minimum material degradation, dusting and spillage [Cahners, 1966d]. Examples of feeders are screw, belt, dragscraper, apron, reciprocating plate, vibratory, rotary vane or drum, rotary disc or table feeders [Cahners, 1966b] . According to Cahners [1966b] the choice of feeder depends upon the characteristics of the material handled, the manner in which the material is stored and the feed rate. A few types of feeders will be highlighted.
A schematic view of an apron feeder is shown in Figure 2.16 [Schulze, 2008a]. An apron feeder is a me-chanical device to discharge bulk solid material from for example a hopper or bin in a controlled way. The
14 2.BELT CONVEYOR SPEED CONTROL
Figure 2.16: Apron feeder discharges wedge-shaped hopper [Schulze, 2008a]
material is fed into the storage medium by for example a dump truck. The apron feeder discharges the bulk solid material by means of a continuous steel belt. The steel belt is made up with flanges (pans or flights) that are projected upwards and connected to and supported by steel chains. An apron feeder is suitable for ex-treme material characteristics, i.e. coarse, very abrasive and sharp-edged bulk solids with particle sizes of up to 1000 mm and temperatures up to 1100◦C or poorly flowing, wet, fine grained bulk solids [Schulze, 2008b].
Apron feeders provide a controlled feed rate and prevents surge loads to other plant equipment [OS-BORN, ], [McLanahan, ]. The speed of an apron feeder can be adjustable via an AC drive [Fördertechnik Gmb, ]. Schulze [2008b] states that the discharge rate should be adjusted by adjusting the apron belt speed (usually on the order of 0.01 to 0.5 m/s) and not by variation of the layer height h. Barfoot et al. [2000] presents a belt conveyor system with a apron feeder. If the production rate of the belt conveyor is adjusted, the speed of the apron feeder is adjusted. The apron feeder is combined with an accelerator conveyor. The role of this conveyor is to increase the speed of the material before it is loaded on the main belt. This accelerator con-veyor is also equipped with an AC drive. However, the proposed belt concon-veyor system is not combined with speed control. Pang and Lodewijks [2011] states that the use a conveyor feeder provides a predictable and controllable material feeding rate as the input for speed control. However, it is mentioned that this requires extra supervisory control systems to regulate the cooperation between the drive of the feeder and drive of the conveyor. No literature is found that combines both drives.
Figure 2.17: Illustration of screw feeder [Schulze, 2008c]
An example of a screw feeder (also called screw conveyor) is shown in Figure 2.17 [Schulze, 2008c]. The rotating screw consists of a helical screw blade around a central shaft. The bulk solid material is conveyed trough a circular or U-shaped cross section. Screw feeders are often used for fine-grained and powdery bulk solids (e.g. sand, coal dust) and are less suitable for very abrasive bulk solid materials or materials that are easily crumbled [Schulze, 2008c]. Different types of screw feeders exist by varying the pitch of the screw or varying the shaft diameter over the length. Parallel screws with some overlap could also be used. If necessary, the screw feeder can be cooled from inside of the hollow shaft. Schulze [2008c] states that the speed of rota-tion of the screw feeder should not be too high, due to high centrifugal forces it is harder for the bulk solid to flow into the screw, and that the driving power of screw feeders is relatively high. Screw feeders can also be equipped with an AC drive [Yaskawa Electric, ].
2.5.4.
BELT WEIGHING AND VOLUME MEASURING EQUIPMENT
Belt weighing is a process of determining the mass rate (k g /s) of bulk material that is being transported via a belt conveyor. Belt weighing is performed with a belt weigher. Also known as a belt scale, dynamic scale, conveyor scale or in-motion weigher. The basic principle of belt weighing is measuring the mass m0Lof the
2.5.COMPONENTS FOR REALIZATION OF SPEED CONTROL 15
material on the belt (k g /m) and the (linear) speed v of the belt (m/s). By multiplying these two values, the instantaneous mass rate is determined. Analogue to this volume measuring equipment can be used to measure material cross section (m2) on the belt to determine the instantaneous flow rate (m3/s). If a constant or average density is assumed, the mass rate and flow rate can be exchanged.
According to Ooms and Roberts [], belt conveyor weighing methods generally fall into two broad groups: • Continuous belt weighing
• Batch weighing
Continuous belt weighing refers to a method of weighing whereby a "weigh length" is embedded into a belt conveyor. The weigh length is the section of the conveyor belt where mass of the material on the belt is determined, see Figure 2.18 [Ooms and Roberts, ]. The weigh length is small relative to the total length of the belt conveyor. Batch weighing refers to a method of weighing whereby a separate short conveyor is installed. Sometimes this could be for continuous weighing, but usually this is used for batches of lower mass-flow rates [Ooms and Roberts, ]. In the view of the scope of this literature review, the remaining focus will be on continuous belt weighing. Firstly, contact based weighing techniques will be considered, followed by non-contact based alternatives.
The idlers in the section of the conveyor belt where mass of the material on the belt is determined are called "weigh idlers". The weighing system could be either single idler weighing; usage of one idler set, see Figure 2.18 [Ooms and Roberts, ]. Or multi idler weighing; usage of more than one idler set, see Figure 2.19 [Ooms and Roberts, ]. The weighing is based on the use of load cells or strain gauges in combination with mechanical components. The belt speed could be measured with for example a belt speed tachometer. Single idler systems generally are a cost effective way of determining the mass rate with an accuracy of ± 1% to ± 2%, which is usually sufficient for process flow control purposes and inventory management but not for totalising [Ooms and Roberts, ]. Multi idler systems are generally more accurate (usually accuracy’s of ± 0.5% or better if installed correctly) and is used where totalising belt scales are required [Ooms and Roberts, ].
Figure 2.18: Continuous belt single idler weighing [Ooms and Roberts, ]
Figure 2.19: Continuous belt multi idler weighing [Ooms and Roberts, ]
There are (non-contact) alternatives in the form of volume measuring equipment. Examples are nucle-onic continuous belt scales, optical belt scales and ultrasound belt scales. Nuclenucle-onic continuous belt scales use the principles of gamma radiation absorption. A radiation source (radioactive isotope) is placed above the conveyor belt and a detector below. As the weight of the material on the belt conveyor increases, more radiation will be absorbed by the load. Hence the conveyor load (kg /m) can be calculated [VEGA, a]. So the greater the conveyor load, the lower the radiation at the detector. And vice versa. An example from industry is the W4800 by VEGA, see Figure 2.20 [VEGA, b]. A sealed radioactive source (Cesium-137) is mounted above the conveyor and a scintillation detector below. A fan-shaped collimated beam of radiation is transmitted
16 2.BELT CONVEYOR SPEED CONTROL
from the source through the bulk material and the and the belt to a scintillation detector [VEGA, b]. A scintil-lation detector is a device in which scintilscintil-lations are detected and amplified and an electrical output signal is generated. Scintillations are small flash of visible or ultraviolet light emitted by a phosphor when struck by a charged particle or high-energy photon [Oxford Thesaurus, 2013].
Figure 2.20: Illustration of W4800 [VEGA, b]
An other (non-contact) alternative for volume measuring is an optical belt scale. Optical belt scales use a 2D laser scanner. Globally, there are two working principles: pulse time method and phase time method. The first one is the easiest to understand. The time interval between emission of a laser pulse and receiving of the laser pulse is measured (also called time-of-flight). The distance between sensor and object is calcu-lated by multiplication of the time interval with the known velocity of light. It is a continuous measurement of increments of material that is transported, from which the volume of material can be determined [Fojtik, 2014]. An example from industry is the LMS511 bulk scan by SICK. The bulk scan makes use of multi-echo technology (also called multiple pulse time method). In general, one reflection echo is received per emitted laser pulse. In Figure 2.21 [SICK, b] five echoes at different levels are received and evaluated per emitted laser pulse. According to SICK [b] detection of objects has improved significantly and a reliable volume flow mea-surement is realised regardless of the bulk material’s properties or weather conditions. A maximum accuracy around 3% is claimed and the scanner is applicable to a maximum conveyors speed of 30 m/s [SICK, a].
Figure 2.21: Illustration of multi-echo technology [SICK, b]
Popescu [2008] proposes an approximation of material profile cross section at the head of a belt conveyor based on ultrasound transducers (Microsonar UT-212). The working principle is analogue to the optical belt scales. Based on the known velocity of sound, the distance between ultrasound transducer and the material is determined. Multiple ultrasound devices (T1...Tn) are placed in a vertical plane, see Figure 2.22 [Popescu,
2008]. To avoid interference between two or more ultrasound transducers, the transducers should be installed with a minimum distance among them [Popescu, 2008]. The belt velocity is measured with a tachogenerator (TG). Via numerical interpolation among the n measurements, an approximation of the material cross sec-tion is made. The paper presents two interpolasec-tion methods. It is concluded that it is possible to measure the profile height of the transported material at the head of the belt conveyor with an ultrasound transducer.
2.5.COMPONENTS FOR REALIZATION OF SPEED CONTROL 17
Figure 2.22: Illustration of multi-echo technology [Popescu, 2008]
2.5.5.
CONTROLLER
As already stated at the end of section 2.1, a distinction can be made in active/continuous and passive/discrete speed control with respect to the controller. In this section, a part of a control system proposed by Jeftenic et al. [2010] will be discussed. This part contains a continuous speed reference generator. A bench conveyor delivers the bulk material to the belt conveyor. The bench conveyors always run at rated speed. A laser device measures the capacity at the end of the bench conveyor. Based on these measurements, the speed of the bench conveyor and the rated values of the material cross section and speed of the belt conveyor, a reference speed is generated, see Figure 2.23 [Jeftenic et al., 2010]. Via the rest of the control system, the reference speed will be implemented on the belt conveyor. The symbols used in Figure 2.23 [Jeftenic et al., 2010] explained:
Gi stands for gain factors, Ti for time constants, Pifor switches and D for a practical differentiator. The
sub-script n stands for the nt hbelt conveyor, r for rated, r e f for reference, b for beginning and e for ending of a belt conveyor.
Using equation Equation 2.1, a belt speed v(n)c al c is calculated based on maximizing the material cross section on the belt. Subsequently, a higher reference speed v(n)r e f is only set when the calculated belt speed is increasing and larger than the current one. A differentiator is determines whether v(n)c al cis increasing or decreasing. If the first derivative of v(n)c al cis positive then the calculated belt speed is increasing, if the first derivative of v(n)c al c is negative then the calculated belt speed is decreasing. If the calculated belt speed is increasing and is greater than v(n)r e f, then switch P1is in position “1” and v(n)r e f is practically equal (with a small time delay) to the v(n)c al c. So a higher reference speed is set. If the calculated belt speed is decreasing and/or v(n)c al cis less then v(n)r e f, then P1is in position “0”. So no higher reference speed is set but v(n)r e f becomes smaller with a certain deceleration which is predetermined with the negative value of the gain G2. Switch P2only enables/disables speed control. The dynamic characteristics of the belt conveyor system can influenced by adjusting constants in the control structure.
3
A
PPLICATIONS OF BELT CONVEYOR SPEED
CONTROL
This chapter covers different applications (simulations, real-world experiments and case studies) from liter-ature concerning belt conveyor speed control. These applications are discussed in sequence of development in belt conveyor speed control. Three stages are distinguished. The first stages are characterized by further exploiting the theoretical potential for energy savings by means of simulations and real-world experiments. Subsequently, the development is continued by implementing different forms of speed control and investi-gating their efficacy.3.1.
D
EVELOPMENT STAGE
I
In a report by Daus et al. [1998] a replacement of a current conveying system in the Nochten (Germany) open-cast mine is considered. A short payback period was aimed for through inter alia energy savings. Therefor, speed control will be implemented. Firstly, the performed tests to explore speed control in relation to power consumption will be discussed. A performance test was conducted on the Nochten belt conveyor system during a period of 14 hours. The power at the supply lines is measured for various combinations of belt load and belt speed. The results of nine (same type of belt conveyor, different lengths) were grouped together, resulting in standardized mean values. The data obtained was analysed by means of linear regression. The tests delivered the relations shown in Figure 3.1 [Daus et al., 1998]. Herein is PNthe nominal power at power
supply lines to the frequency converter and Imis the capacity (m0Lv). The speed varies from v0= vnomto
v0= 0.5vnom. In the report a theoretical derivation similar to the one in subsection 2.3.2 is given. In addition
to this, the following expression is given which is helpful in evaluating the relations in Figure 3.1 [Daus et al., 1998]:
Pe= A + B v where A contains Imand f , B contains f (3.1)
Firstly, it can be seen that there is a linear dependence between the required power and variable load at a constant belt speed. From this it is concluded that in practice f can be assumed to be virtually constant at a fixed temperature and constant belt speed. Secondly it can be seen that the lines for lower speed are below each other. It is concluded that f decreases as belt speed decreases. From Figure 3.1 [Daus et al., 1998] it is derived that the same capacity can be transported with less power by lowering the belt speed. Therefor, the implementation is based on maximizing utilization of the material cross section on the belt.
According to Daus et al. [1998], with a rated capacity Im0 below 50% and an average belt speed of 0.68
vnom, the savings achieved according to Figure 3.1 [Daus et al., 1998] should amount to 16%. In addition, the
potential 25% savings (difference between curves (1) and (2)) have been demonstrated during testing under practice conditions on several occasions by Siemens. However, the author of this literature review is not able to reproduce these potential values from Figure 3.1 [Daus et al., 1998]. For example, the maximum difference between curves (1) and (2) is at maximum 20%, but certainly not 25%. In Jeftenic et al. [2009] the same figure is considered. An example is given: a capacity of 60% can be conveyed with 70% power at rated speed, but also with 55% power if the speed equal to 60% of the full speed is selected. This does agree with Figure 3.1 [Daus et al., 1998].
20 3.APPLICATIONS OF BELT CONVEYOR SPEED CONTROL
Figure 3.1: Relations derived from performed measurements [Daus et al., 1998]
It is stated that belt speed adjustment is implemented as follows. The actual material flow is determined 100 m before the transfer point from a belt conveyor to a permanent head belt by a ultrasound Vegasonde (by VEGA). The control system does not compensate for every surge in material flow and short overload peaks can be absorbed by the conveyor (peripheral loading and storage in transfer chutes) system [Daus et al., 1998]. The load dependent control is implemented by sending the data from the Vegasonde to a shift register. A calculated set point for belt velocity is set. The set point is only adjusted upwards by a later occurring larger value. It is not mentioned if some kind of averaging over a sample interval is used, like shown in Figure 2.5 [Daus et al., 1998]. The speed is adjusted downward by a gradual deceleration ramp. The minimum belt velocity is set to 50% of the nominal belt velocity. An example of the recorded data of the material flow and the belt velocity is shown in Figure 3.2 [Daus et al., 1998]. The average loading rate for this figure is 45.5% with a mean speed of 60% of vnom. However, during long-term testing mean belt speeds of 68% of vnomwere
achieved.
Figure 3.2: Example recorded data and the belt velocity [Daus et al., 1998]
Lauhoff [2005] takes a critical look at the recommendation of taking 100% filling level as the basis of speed control in order to realize energy savings. Resistance relations described in literature and simulations of a fictitious belt conveyor are used for his expert opinion. In the report it is pointed out that f is not constant [Alles, 1979] and therefor should not and cannot be used for fluctuating operation conditions. Therefor, an other primary resistance description than the one in DIN 22101 [1982] is proposed, incorporating the four different components by physical laws. These physical laws were first set up by Lachmann [1954] and Vierling [1956] and describe each of the components individually. By using this description, f is not constant.
3.1.DEVELOPMENT STAGEI 21
In the second part of the report, a simulation is performed with the proposed primary resistance descrip-tion. It is stated that the quantities and parameters from the literary sources are independent from belt speed or filling level. For the simulation a fictitious belt conveyor is used with the properties listed in Table 3.1.
Table 3.1: Properties of belt conveyor for simulations [Lauhoff, 2005]
Conveying length 1000 m
(nominal) Capacity 2794 t /h
(nominal) Belt speed 4.00 m/s
Belt widh 1000 mm
Belt weight 28.4 kg /m
3-part troughing angle 40 ◦
While the density of the material is set to 1600 kg /m3, two simulations are performed:
• Belt speed is kept constant at nominal speed v = 4.00m/s, filling level φ is varied between 0.6 and 1.1 of the nominal filling level
• Filling levelφ is kept constant at nominal filling level, belt speed is varied between 2.40 and 4.40 m/s It is important to note that the capacity is thus not kept constant, but the capacity is the same for the two simulations. A specific energy consumption equation is established by dividing the power by the capacity M and length L of the belt conveyor:
Wspec=
P
M L with unit W s
kg m (3.2)
The simulation results are shown in Figure 3.3 [Lauhoff, 2005]. From this graph it can be seen that the specific energy consumption is lower if belt speed is kept constant for the range of filling levels between 0.6 and 1.0 . Another important notice is that the specific energy consumption shows a minimum at a filling level of 0.7. On the other hand, the curve for constant filling level is essentially in proportion with the belt speed. According to the report, this latter is related to the characteristics of the indentation resistance of the belt. This resistance has a major share in motion resistances for a horizontal belt conveyor (see Figure 2.9 [DIN 22101, 1982]) and rises progressively in dependence on the load on the idlers. For completeness, the data presented in Lauhoff [2005] is analysed in Table 3.2. Taking the resistance description from DIN 22101 as baseline, significant energy savings are presented by maximizing utilization of the material cross section on the belt. However, taking the proposed single resistance method as baseline, relative small increases in energy consumption are presented.
22 3.APPLICATIONS OF BELT CONVEYOR SPEED CONTROL
Table 3.2: Specific energy consumption W s/(k g m) for a 1000 m belt conveyor [Lauhoff, 2005]
Conveying capacity [t /h] 1676 1956 2235 2515 2794 3073 DIN 22101 v = vnom 0.348 0.328 0.314 0.303 0.294 0.287
DIN 22101φ = 1 0.294 0.294 0.294 0.294 0.294 0.294
Energy saving [%] 16 10 6 3 0 -2
Single resistance method v = vnom 0.285 0.283 0.284 0.289 0.294 0.298
Single resistance methodφ = 1 0.291 0.292 0.293 0.293 0.294 0.295
Energy saving [%] -2 -3 -3 -1 0 1
Lauhoff [2005] also incorporates a thesis of Limberg [1988] in his report. Limberg performed measure-ments (of the power up take, amongst others) at belt conveyors in stationary operating conditions under on-site conditions. The measurements of two belt conveyors were translated to specific energy consump-tion by Lauhoff, see Figure 3.4 [Lauhoff, 2005] and Figure 3.5 [Lauhoff, 2005]. The properties of these belt conveyors are listed in Table 3.3 [Lauhoff, 2005].
Table 3.3: Properties of belt conveyor no. 4 and 6 [Limberg, 1988]
Belt conveyor no. 4 no. 6
Conveying length 530 1026 m
(nominal) Capacity 5000 4600 t /h
(nominal) Belt speed 4.0 2.62 m/s
Belt widh 1400 1400 mm
Belt weight 45.6 44 kg /m
3-part troughing angle 30 30 ◦
Motor 2 x 250 3 x 160 kW
From the graphs it can be seen that also here the specific energy consumption for constant filling level is not always lower than for constant speed. Plus, the specific energy consumption shows minima again. It is important to note that the specific energy consumption of belt conveyor no. 4 is considerably higher than the specific energy consumption of belt no. 6. The report states that this result is explicable by findings of Hintz [1993]: different rubber materials for the cover plates can be the sole reason for this difference. On this subject will be elaborated in more detail in subsection 4.1.3. In the report by Lauhoff it is concluded that all the analysis he performed have one thing in common: for a constant conveying speed and a decreasing filling level, the specific energy consumption reduces. And therefor, speed control does not save energy at traditional filling levels between 0.6 and 1.0. The author of this literature review would like to comment that this is not true for belt conveyor no. 4. In Figure 3.4 [Lauhoff, 2005] it can be clearly seen that for a constant conveying speed and a decreasing filling level, the specific energy consumption does not only reduce and energy savings are realised for filling levels between 0.6 and 0.8.
