Speed regulation and control solutions for belt conveyor drive systems - Snelheidsregeling en controle-oplossingen voor aandrijfsystemen van bandtransporteurs

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Delft University of Technology

FACULTY MECHANICAL, MARITIME AND MATERIALS ENGINEERING

Department Maritime and Transport Technology Mekelweg 2 2628 CD Delft the Netherlands Phone +31 (0)15-2782889 Fax +31 (0)15-2781397 www.mtt.tudelft.nl

This report consists of 42 pages and 0 appendices. It may only be reproduced literally and as a whole. For commercial purposes only with written authorization of Delft University of Technology. Requests for consult are only taken into consideration under the condition that the applicant denies all legal rights on liabilities concerning

Specialization: Transport Engineering and Logistics

Report number: 2015.TEL.7934

Title:

Speed regulation and control

solutions for belt conveyor drive

systems

Author:

R.M. Draisma

Title (in Dutch) Snelheidsregeling en controle-oplossingen voor aandrijfsystemen van bandtransporteurs

Assignment: Literature

Confidential: no

Initiator (university): Dr. ir. Y. Pang Supervisor: Dr. ir. Y. Pang Date: March 28th_{, 2015}

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Summary

Transporting large amounts of material using belt conveyors is a process that requires a lot of energy. Within a mining operation, belt conveyors can be responsible for up to 70% of the total energy consumption. This means that reducing the energy consumption of belt conveyors will have a big impact on power consumption in the industries and countries where they are used.

Belt conveyors usually run at the nominal speed of the drive, regardless of the capacity that is demanded. The main goal of this study is to review the information that is available on speed regulation and the control solutions for large scale belt conveyor drive systems. The study is divided in four tasks:

1. Survey state of the art large scale belt conveyors and conveyor drives. 2. Review variable speed drives and how they work for belt conveyors.

3. Define the energy saving solutions of variable speed drives for belt conveyors in start-up procedure and in steady state.

4. Asses if variable speed drives are a suited method to improve the efficient use of belt conveyors and, if so, in what way they can be used.

From the survey of conveyor drives can be concluded that 3-phase AC electric motors are best suited for belt conveyor applications and that the squirrel cage motor is the most used drive type in this category. Also, in chapter 2 is shown that the torque of a 3-phase AC electric motor can be controlled by either varying the rotor current, stator voltage or the frequency of the power supply.

There are three main categories of continuous VSDs, being mechanical, hydraulic and electronic. Mechanical continuous VSDs are limited by the maximum power that can be applied and hydraulic fluid couplings have shown to be very useful for speed and acceleration control of belt conveyors, however this control is achieved using the slip in the coupling, which leads to a loss of energy. This is not an issue with electronic VSDs. Therefore, the type of VSD that is most up and coming is the electronic Variable Frequency Drive. By controlling the frequency of the power that is supplied to the drive, the applied torque can be controlled very precisely. The two main control mechanisms for VFD are Field Oriented Control (FOC) and Direct Torque Control (DTC). Compared to FOC, DTC is relatively simple and has a shorter control cycle. This, in combination with the fact that DTC has better dynamic speed accuracy, is why it is deemed most suited for the conveyor applications as they are discussed in this study.

For the application of electronic VSDs for large scale belt conveyors several theoretical benefits and challenges were found, as well as some results from actual applications of VFDs for large scale belt conveyors. The main challenges are the ability to determine the amount of material that is

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discharge speed of the material due to the variable speed of the belt. This would require a chute that is able to handle the various speeds in the belt speed control range.

The main benefits of the VFD presented in the theory are the energy savings due to optimal use of equipment and a decline on material strain due to controlled soft starting and the prevention of unnecessary high speeds of the belt. The papers mostly rely on DIN22101 calculations, simulations and small scale experiments. There also is a critical note on these alleged benefits, where DIN22101 calculations, simulations and the Limberg thesis are used to show the lack of impact of reduced speed on energy consumption. However the practical results show that the previous mentioned benefits indeed exist. Several manufacturers, such as ABB and Siemens, and researchers present results from the application of VFD systems. The results are mostly positive in terms of the realization of the expected benefits, although numerical substantiation is sometimes lacking. Three studies, in which VFD is applied on a large scale and used in the daily operation, report energy savings of respectively 15-38%, 20% and 7-24%. Reductions in maintenance costs, although being an expected result, were not yet registered, since it would require long term testing to be able to measure these results.

Based on these results, from this literature study can be concluded that Variable Frequency Drives are a very well suited method to improve the efficient use of belt conveyors by using the system for highly controlled soft starts and adjusting belt speed according to the required capacity, which has proved in theory and practice to lead to benefits in terms of reduced energy use and strain of the belt conveyor and its components.

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List of symbols

: Magnetizing flux or air gap flux (Wb) f : Voltage frequency of power supply (Hz)

I : Current (A)

: Rotor current (A) (depends on load)

: Constants dependent on material and machine design n0 : Synchronous speed (rev/min)

n : Actual speed (rev/min)

p : Number of poles

s : Slip

: Torque available on shaft (Nm) : Stator voltage (V)

Subscripts

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List of abbreviations

AC : Alternating Current

CEMA : Conveyor Equipment Manufacturers Association

CSI : Current Source Inverter

DC : Direct Current

EMI : ElectroMagnetic Interferention

FOC : Field Oriented Control

LR : Liquid Rheostats

PWM : Pulse Width Modulated

VFD : Variable Frequency Drive

VSD : Variable Speed Drive

VSI : Voltage Source Inverter

VVVF : Variable Voltage Variable Frequency

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1 Introduction

Transporting large amounts of material using belt conveyors is a process that requires a lot of energy. For instance, in 2006, 20% of the total energy consumption in South-Africa was used for its mining operations across the country (Department of Energy of Republic of South Africa, 2009). Within a mining operation, belt conveyors can be responsible for up to 70% of the total energy consumption. This means that reducing the energy consumption of belt conveyors will have a big impact on power consumption in the industries and countries where they are used.

Belt conveyors usually run at their nominal speed, which is fixed. The nominal speed is applied, regardless of the capacity that is demanded of the belt conveyor. This means that there often is overcapacity, leading to a needless waste of energy and needlessly wearing out the equipment. One way to improve the efficient use of a belt conveyor is to optimally use its capacity. That can be done by altering the belt speed to the amount of material that needs to be transported, so that the occupation on every part of the belt is as close to 100% as possible, which is confirmed by (Lodewijks, Schott, & Pang, 2011) and (Lauhoff, 2005). That way the system will use less energy. More importantly, with this strategy the system will subjected to less wear, giving a longer life time to idler rolls and other components. Both the reduced use of energy and the reduced wear to the system will lead to cost savings.

1.1 Goal of the study

The main goal of this study is to review the information that is available on speed regulation and the control solutions for belt conveyor drive systems. The study is divided in four tasks:

1.2 Scope

The review will be limited to medium (1-2 km) and large (2-20 km) troughed belt conveyors that are powered by electric drives.

1.3 Report setup

To get an idea of the working of the systems this report will investigate, first a survey of large scale belt conveyors and conveyor drives is done. Then follows an overview of the possible methods to realize a variable speed drive for electric motors, which are divided into three categories: mechanical, hydraulic and electrical. The technical explanation is followed by a review of the energy saving solutions of variable speed drives for belt conveyors in theory and practice.

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2 Large scale belt conveyor and components

2.1 Introduction

The goal of this chapter is to elaborate on the system that is being reviewed and give an understanding on the components that will be investigated in more detail.

2.2 Application

The function of a belt conveyor is to continuously transport bulk materials over a certain distance. Large troughed belt conveyors are mostly used in the mining, chemical, steel and food industry, as well as for coal fired power plants and bulk terminals. Examples of such large scale systems are shown in Figure 2.1.

Figure 2.1: Large scale belt conveyor systems

Belt conveyors have proven to be a cost-effective and efficient system for the continuous transport of dry bulk materials, like coal or iron ore (Lauhoff, 2005). According to the Rulmeca Group transport by truck is the most competitive alternative for belt conveyors, but belt conveyors costs are 40%-50% lower and have the following advantages over trucks (Rulmeca Group, 2003):

1. Reduction in number of personnel 2. Reduction in energy consumption 3. Long periods between maintenance

4. Independence of the system to its surroundings 5. Reduced business cost

2.3 Components of interest

The components that are of most importance for this review are the belt and the electric drive, from which the energy is transmitted to the drive pulley. The components are shown in Figure 2.2.

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Figure 2.2: One end of a troughed belt conveyor

Figure 2.3: Basic configuration of a belt conveyor (Rulmeca Group, 2003)

2.4 Maximizing use of belt capacity

The belt plays an important role in the dynamics of the conveyor system and therefore is one of the components of interest for this study. The effects of accelerating and decelerating the belt will be reviewed further on in this report.

The belt can be loaded with material to a certain maximum cross surface (profile), determined by the angle of surcharge, which in turn depends on material properties (see Table 2.1), and the standard edge margin.

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The angle of surcharge can be defined as the dynamic angle of repose and is taken tangent to the pile of material at the edge of the conveyor (Fayed & Skocir, 1997), see Figure 2.4. In order to make optimal use of the capacity of the conveyor system, the real life cross section of the material on the belt should be as close to the theoretical maximum as possible. From the formula of volumetric flow, also shown in Figure 2.4, we can see that, since the volumetric flow has to remain unaltered, what needs to be done to maximize cross surface S (within material boundaries) is to alter the belt speed.

Figure 2.4: Cross section ‘S’ of loaded belt with surcharge angle ‘A’, trough angle ‘B’ and standard edge margin ‘s’ (Sandvik, 2008).

2.5 Electric motors for large scale belt conveyors

Controlling the electric drive is the method to reach the goal of this review: speed regulation of a conveyor belt. In order to get a good understanding of the motors and their operating characteristics, the basics of electric motors are discussed in the next paragraph. A simplified overview of the

different types of electrical motors is found in Figure 2.5. In order to stay within the scope of this review, the review will be limited to the electric motors that are commonly used for large scale belt conveyors. These are the 3-phase AC induction motors: the squirrel cage and wound rotor motors (Hampton), (Barnes, 2003). Of these two, the most popular type is the squirrel cage motor, which accounts for almost 90% of the induction motors (Noordin, 2007).

Figure 2.5: Simplified overview of types of electric motors (Electrical Know-How)

The reason for the popularity of the AC squirrel cage motors is the high reliability compared to DC motors. The only components that can wear are the bearings (Barnes, 2003).

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2.6 3-Phase AC induction motors

The electric motor has a long history, with the first examples being created in 1834. The 3-phase induction motor was a big improvement compared to these early examples. However, with the first squirrel cage motor being created in 1889, the design basics go back a long way. (Doppelbauer)

2.6.1 Main components

The most important components of an AC induction motor are two electromagnetic parts. The stator is the stationary part of the motor. The rotor is a rotating part that is supported by bearings on each end. Both consist of an electric circuit, which is made up of copper or aluminum to carry current, and a magnetic circuit consisting of laminated steel to carry magnetic flux. An air gap is situated between the rotor and the stator. (Barnes, 2003) These components are visualized in Figure 2.6.

Figure 2.6: (left) Basic configuration of AC induction motor and (right) details of the stator and rotor (Barnes, 2003)

The difference between a squirrel cage and a wound rotor motor is down to the rotor of the electrical motor.

2.6.2 Squirrel cage

For the squirrel cage motor the rotor consists of a couple of copper or aluminum bars that are

connected to each end of the rotor. All these bars, which are the rotor windings, resemble the bars of a cage, hence the name „squirrel cage‟. The aluminum bars are die-cast in the rotor slots, making for a very rugged construction. Although the bars are in direct contact with the laminated steel, almost all current goes through the aluminum bars. (Barnes, 2003)

2.6.3 Wound rotor

The wound rotor, or „slipring motor‟, has three sets of insulated windings, which have connections to the three sliprings that are mounted on the shaft. Brushes provide the external connection with the sliprings. (Barnes, 2003)

2.6.4 Principle of operation

The power supplied to the stator creates a magnetic field that rotates due to the alternating current. The speed of the rotation is determined by the frequency of the AC power that is supplied. According to Faraday‟s law the rotating magnetic field that is created by the stator induces a voltage in the rotor. Due to the short circuiting of the rotor bars by the end rings, an opposing current in the windings of

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the rotor is created. The magnetic field that is created in the rotor reacts to the rotating magnetic field that is created by the stator. As is stated in Lenz‟s law, the direction of the force is that which tends to reduce the flux in magnetic field. That is why the rotor will accelerate to follow the direction of the rotating flux over the air gap and that‟s the way the rotating magnetic field of the stator makes the rotor rotate. (Barnes, 2003)

2.6.5 Torque

The torque that is developed by an induction motor and the magnetizing flux are defined by (WEG) as:

With:

= Torque available on shaft (Nm) = Magnetizing flux or air gap flux (Wb) I = Rotor current (A) (depends on load) V = Stator voltage (V)

= constants dependent on material and machine design f = Voltage frequency of power supply (Hz)

From these equations we can derive the variables that can be altered to adjust the torque of an electric motor, which are the rotor current and the air gap flux. The air gap flux is variable by adjusting either the stator voltage or the frequency of the power supply. The relevance of the dependency of the torque on these variables will become clear later in this report.

2.6.6 Speed and Slip

An AC motor will always try to reach the synchronous speed, which is the speed at which the stator flux rotates. However, when the rotor reaches that speed, there is no speed difference between the flux and the rotor. As can be deduced from the previous paragraph, at that moment no current and thus no torque is induced in the rotor. Therefore it cannot maintain that speed (Kumar, Basumatary, & Gogoi). That is why in reality, due to slip, the actual motor speed is lower than synchronous speed. The amount of slip is dependent on stator voltage, rotor current and the load on the shaft. Usually equilibrium is reached at a slip of 3% (Shakweh, 2011). The synchronous speed and slip are defined by (Barnes, 2003) as:

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n0 = synchronous speed (rev/min)

f = voltage frequency of power supply (Hz) p = number of poles

s = slip

n = actual speed (rev/min)

From the definition of the synchronous speed we can deduce that one way to adjust the speed of the electric motor to either change the voltage frequency of the power supply or the number of poles of the motor (WEG). However, there are more (non- electric) ways to make the speed of an electric motor variable. All these will be elaborated on in the next four chapters.

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3 Variable speed drives

3.1 Introduction

AC induction motors essentially are fixed speed motors (Noordin, 2007). In the past years, several systems have been developed in order to be able to vary the motor speed, which are called Variable Speed Drives (VSD). Chapters 3, 3.3, 3.4 and 3.4.4 will provide information about the different types of variable drives and how they work.

In general, variable speed drives are required instead of fixed speed drives when goal is to match the speed of a drive to the process requirements, to match the torque of a drive to the process

requirements and/or to save energy and improve efficiency (Barnes, 2003). Depending on the requirements, two kinds of variable speed drives are available: discrete and continuous. Compared to discrete speed control such as gearboxes or multi-speed motors, VSD provides a continuous range of speed control. This is visualized in Figure 3.1.

Figure 3.1: Speed ranges of discreet and continuous speed control (Ontario Hydro, 1997)

In this review we focus on the continuously variable speed drives, since these will give us a more efficient and thus the desired result.

3.2 Three main types of VSD

There are three ways to control an electrical drive in such a way that its speed is variable, these main types are:

 Mechanical

 Hydraulic

 Electronic

Each type has several variants. The most common types of VSD for industrial applications are briefly explained in the next three paragraphs.

3.3 Mechanical VSD

Obviously the most common and well known mechanical VSD consists of a motor in combination with a conventional gearbox, like the one that can be found in a car. However, this doesn‟t make for a

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3.3.1 Variable pitch drives

The variable pitch drive is the most basic mechanical VSD and is widely used because of their simplicity and efficiency. It is a form of a Continuously Variable Transmission which provides any reduction ratio continuously over a specified range. One example of its application is the use in automatic motor scooters. The basic concept is shown in Figure 3.2. A rubber belt runs between two pulleys of which the diameter is variable. Changing the diameter on a pulley will change the

input/output speed ratio, making it a variable speed drive. There are numerous ways to alter this pitch and which method is used depends on the application of the drive.

Aside from the earlier mentioned benefits of this system, the rubber belt also doubles as overload and jam protection. In those cases, the belt will just slip over the pulley. However, the downside of this variable speed drive is the maximum amount of power it can handle, which is about 100 hp for industrial applications (Machine Design, 2002). This makes the variable pitch drive suitable for a belt conveyor that is driven by multiple smaller power units.

Figure 3.2: Variable pitch drive concept. Sources: left: (Speed Selector, Inc.), right: (Barnes, 2003).

3.4 Hydraulic VSD

A hydraulic variable speed drives consists of a fixed speed engine that is connected to a hydraulic pump that transfers fluid to a hydraulic motor. The torque is transmitted by the hydraulic fluid. The speed is regulated by controlling the fluid pressure or flow. Hydraulic VSD‟s are very much suitable for conveyors and have some great advantages over for instance a standard gearbox. There are two main types of hydraulic drives, each having a different method to control the pressure or flow of the

hydraulic fluid. First the two types of hydraulic VSD‟s are explained, then the main strong points are described.

3.4.1 Hydrodynamic (fluid coupling)

Fluid couplings are commonly used on conveyors (Barnes, 2003). It is a simple hydrodynamic device which connects a prime mover, for instance a squirrel cage motor, to a driven machine. The

mechanical power is transmitted by acceleration and deceleration of the hydraulic fluid. The working part of a fluid coupling consists of two elements, which face each other in a casing and both have vanes. The one connected to the prime mover is called the impeller, the other is connected to the driven machine and is called the runner. Inside the casing is a fluid. When the drive starts rotating, the fluid is picked up in the vanes of the impeller and is thrown out towards the vanes of the runner. The kinetic energy of the fluid makes the runner rotate in the same direction, driving the driven

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machine (McGraw-Hill Dictionary of Scientific & Technical Terms, 6E., 2003). This process is illustrated in Figure 3.3. In this system there is no direct connect between the drive and the driven machine.

Figure 3.3: Basic principle of operation for fluid coupling (Encyclopeadia Britannica, Inc., 2010)

3.4.2 Hydrostatic

When variable output is needed, hydrostatic drives are widely recognized as an excellent means of power transmission. They can outperform mechanical and electrical VSD‟s and traditional gearboxes in terms of response, maintaining a fixed speed under load changes and allowing infinitely variable speed control, from zero to maximum speed (Machine Design, 2002). The basic principle of operation of the hydrostatic system is shown in Figure 3.4.

Figure 3.4: Basic principle of operation for a hydrostatic VSD (Harris, 2005)

(Barnes, 2003) describes this type of hydraulic VSD as following: “This type of hydraulic VSD is most commonly used in mobile equipment such as transportation, earthmoving and mining machinery. A hydraulic pump is driven by the prime mover, usually at a fixed speed, and transfers the hydraulic fluid to a hydraulic motor. The hydraulic pump and motor are usually housed in the same casing that allows closed circuit circulation of the hydraulic fluid from the pump to the motor and back.

The speed of the hydraulic motor is directly proportional to the rate of flow of the fluid and the displacement of the hydraulic motor. Consequently, variable speed control is based on the control of both fluid flow and adjustment of the pump and/or motor displacement. Practical drives of this type

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• High torque available at low speed • High power-to-weight ratio

• The drive unit is not damaged even if it stalls at full load • Hydrostatics VSDs are normally bi-directional

Output speed can be varied smoothly from about 40 rev/min to 1450 rev/min up to a power rating of about 25 kW. Speed adjustment can be done manually from a hand-wheel or remotely using a servo-motor. The main disadvantage is the poor speed holding capability. Speed may drop by up to 35 rev/min between 0% and 100% load.

Hydrostatic VSDs fall into four categories, depending on the types of pumps and motors.

• Fixed displacement pump – fixed displacement motor

The displacement volume of both the pump and the motor is not adjustable. The output speed and power are controlled by adjusting a flow control valve located between the hydraulic pump and motor. This is the cheapest solution,but efficiency is low, particularly at low speeds. So these are applied only where small speed variations are required.

• Variable displacement pump – fixed displacement motor

The output speed is adjusted by controlling the pump displacement. Output torque is roughly constant relative to speed if pressure is constant. Thus power is proportional to speed. Typical applications include winches, hoists, printing machinery, machine tools and process machinery.

• Fixed displacement pump – variable displacement motor

The output speed is adjusted by controlling the motor displacement. Output torque is inversely proportional to speed, giving a relatively constant power characteristic. This type of characteristic is suitable for machinery such as rewinders.

• Variable displacement pump – variable displacement motor

The output speed is adjusted by controlling the displacement of the pump, motor or both. Output torque and power are both controllable across the entire speed range in both directions.”

3.4.3 Strong points of hydraulic VSD

A couple of advantages the hydraulic VSD are quoted below from (Fluid Coupling Engineering).

1. “Provides a soft start: As the motor is allowed to run up light to full speed, the inherent overload capacity of the motor can be used to accelerate the driven machine. This allows the motor to be sized for the running duty, and not for starting the drive. This is at odds with electronic drives which tend to be 'oversized' to cope with the starting duty, which has a negative effect on the initial cost, and the power factor of the drive.

2. Reduced motor current:

For motors fixed rigidly to the driven machine, they will tend to

accelerate slowly up to speed. During this acceleration time, the electrical current being drawn is typically very high. With a fluid coupling offering the motor a soft 'no load' start, the

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much reduced. The coupling will only ever absorb from the motor the power needed to run the machine and itself when in a running condition.

3. Controlled acceleration: As there is no direct mechanical connection between the two halves of the coupling, the torque is transmitted by the fluid due to the relative motion of the coupling impeller. The vortex created in the fluid then impinges on the runner therefore transferring the torque to the driven machine. This allows the output side of the coupling to gradually and smoothly come up to speed in a controlled and repeatable manner.

4. Overload protection:

As there is no mechanical connection between the two coupling halves,

in an overload or stall condition, the fluid will 'sheer' and the coupling slip. This capability provides protection to the driver and driven machine, without causing any damage to coupling.

5. Isolation from shock loading:

As the two coupling halves are not mechanically connected, any

inherent vibration or shock loading will be damped out of the system by the fluid. This helps to reduce wear on the drive components and increases machine operational life.30+ year old couplings are very common items.

6. Simple and reliable: Fluid couplings are very simple machines with few moving parts. They are very robust, making them extremely reliable in operation. There are no meshing components and meshing parts to go wrong. They are also very low maintenance devices.

7. Tolerant to poor electrical supply: Fluctuations in electrical supply will not have any direct consequence to the coupling performance. A fluid coupling can also help a motor to start a machine in areas of poor electrical supply.”

3.4.4 Weak point of hydraulic VSD

The main weak point of an hydraulic VSD is the fact that the drive speed is controlled using slip. A fluid coupling always suffers slip and the amount of slip determines the amount of power that is transferred. Slip means heat creation and loss of energy efficiency. For electronic variable speed drives, slip in the coupling is not an issue.

3.5 Electronic VSD

Since the application for large scale belt conveyors are investigated, only the electrical VSD type that is suited best, and thus commonly used, will be reviewed. (Hampton) states that this is the variable frequency drive (VFD) for squirrel cage motors. That is also confirmed by (Meakin & Saxby, 2009), saying the trend is to use squirrel cage induction motors in combination with variable frequency control. For an electrical variable speed drive to work, the speed and the torque of the electric motor has to be controlled. In what ways this can be done through the various types of VFD‟s is explained in this chapter.

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According to (Barnes, 2003) frequency control, as a method of changing the speed of AC motors, has been a well-known technique for decades, but it has only recently become a technically viable and economical method of variable speed drive control. In the past, DC motors were used in most variable speed drive applications in spite of the complexity, high cost and high maintenance requirements of the DC motors.

Variable frequency drive is a common electrical drive solution for large scale overland belt conveyors and is mostly used in combination with a squirrel cage motor. It is most cost effective for smaller horsepower or low voltage systems and best suitable when continuous speed variation at a relatively constant load is required. On the other hand it is less suited for applications where variable speed is only required for starting and stopping. VFD‟s for larger horsepower, high voltage conveyors are costly and require special installation considerations (Hampton).

As already stated in paragraph 2.6.6, the speed of an AC motor can be adjusted by either changing the supplied frequency or the number of poles. Changing the number of poles in order to change the speed of the motor is highly impractical and doesn‟t lead to a continuously variable speed. The logical alternative is that modern day AC VSD‟s vary the supplied frequency to control the motor speed.

In order to control the torque, as stated in paragraph 2.6.5, either the rotor current or the air gap flux. The rotor current depends on the load, so that cannot be actively controlled. The air gap flux depends on the voltage and frequency of the power supply. Since the frequency adjustment is already used for speed control, the torque can be controlled by adjusting the voltage of the power supply. As stated by (Barnes, 2003): The AC motor is able to develop its full torque over the normal speed range, provided that the flux is held constant, (V/f ratio kept constant). A standard AC motor reaches its rated speed, when the frequency has been increased to rated frequency (50 Hz) and stator voltage V has reached its rated magnitude. If the flux rises too high, high excitation currents will arise, leading to losses and heating. If the flux is too low, the torque provided by the motor will drop.

More detail on how these systems work, their components and the way in which these systems can be controlled can be found in the following paragraphs in this chapter.

3.5.2 VFD Components

(Ontario Hydro, 1997) states that a VFD system controls a motor through four main components:

1. Rectifier 2. DC Link 3. Inverter

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The rectifier is used to convert AC to DC. For this rectifier the diode type, which is also known as the six-pulse uncontrolled rectifier, is the most commonly used one. The inverter generates an alternating current and is controlled by the regulator.

To illustrate how these components interact with each other and the motor, a simplified VFD system is shown in Figure 3.5.

Figure 3.5: VFD control setup (Ontario Hydro, 1997)

In the VFD system, the inverter is a critical part. In order to get a better understanding of the way the inverter adjusts the motor speed, the working of the most commonly used inverter is explained in the next paragraph (Ontario Hydro, 1997). How the system works as a whole is elaborated on in

paragraph 3.5.4.

3.5.3 The Pulse Width Modulated (PWM) inverter

Inverters work as an interface between the energy source and the induction motor (WEG). There are three main types of inverters, the Current Source Inverter (CSI), the Variable Voltage Inverter (VVI) and the Pulse Width Modulated (PWM) inverter. The last two fall in the category Voltage Source Inverters (VSI) (Brumbach, 2003) and are the only ones suited for conveying applications. The PWM inverter is the most complex and expensive one, but also is the most reliable and precise one (creates the least harmonic noise), making it the most used inverter for industrial motors in applications that involve speed variation (WEG). That means that the PWM inverter can use the motor more efficiently and with less jerking than the other two. PWM inverters are suitable for drives up to 5000

horsepower, which, together with their reliability, makes them very suitable for large scale conveyor applications. (Ontario Hydro, 1997)

The PWM inverter adjusts the width (i.e. duration) of the output voltage pulses. By altering the pulse width, the average value of the voltage and current that are fed to the load are controlled. This is illustrated in Figure 3.6.

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The PWM uses this technique to achieve an almost sinusoidal output with the desired voltage and current. It does that by switching the power supply, which has a fixed frequency, on and off at a certain rate, creating a resultant wave form that must be perceived by the load as a smooth sine wave. Therefore, the PWM switching capacity is much higher than what would affect the motor. An example of such output and the difference with the VVI is illustrated in Figure 3.7.

Figure 3.7: Wave forms – (left) PWM and (right) VVI (Ontario Hydro, 1997)

3.5.4 How does VFD work?

In the rectifier the supplied AC voltage is converted to DC current and voltage. After that, they are transported via the DV link, where the DC current and voltage are filtered to smooth out any peaks. Then that DC current and voltage is supplied to the inverter. There they are converted into variable AC voltage and frequency which can be supplied to the motor. The inverter simulates a variable AC sine wave, of which the characteristics are determined by the control system (or rectifier). In order to prevent over fluxing the motor, the output voltage is controlled, so that the voltage-frequency ratio remains constant (Barnes, 2003). The regulator, or control circuit, determines what action has to be taken to get the actual speed as close to the reference speed as possible and modifies the inverter switch characteristics accordingly so that the output frequency can be controlled. The effect of varying the frequency on the motor speed was already explained in paragraph 2.6.

3.5.5 Control system

All the components of a VFD cannot operate by themselves. The control system is needed to manage the operation of the VFD system. It makes sure that the motor speed will match the reference speed, which is input from the operator. According to (Barnes, 2003) the overall control system can be divided into four main sections:

1. Inverter control system 2. Speed control system 3. Current control system 4. External interface

(Barnes, 2003) also lists the following objectives that have to be achieved in the design of the VFD control system:

1. High level of reliability

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3. Minimizing inverter losses

4. Possibility to integrate control system in overall process control system

5. High tolerance to power supply fluctuations and ElectroMagnetic Interferention (EMI)

In VFD‟s, several control types can be distinguished, these are elaborated on in the next paragraphs.

3.5.6 Control types

Within the subject of control types for VSD‟s, the different literature sources are inconsistent in their designation of the control types. That does not benefit the clarity of this subject. For the sake of clarity, this paragraph seeks to give an overview of the different names which are used for the different control types. A schematic overview is found in

Figure 3.8. The detailed functioning of the systems is explained in the next paragraphs.

Figure 3.8: Overview of types of Variable Frequency Drives (VFD)

With the variable frequency drives for AC motors, two main control types with three sub types can be distinguished (ABB, 2011), (Barnes, 2003):

1. Scalar control

a. V/f control, also called open loop drive or scalar control 2. Vector control

a. Field Oriented control, also called (flux) vector control or closed loop drive b. Direct Torque Control

Each type of system has their characteristics and variations. They all control the speed of an AC induction motor by varying the voltage and frequency of the power supply, but they all use a different design to do so (Turkel, 1999).

VFD controls the torque and speed applied to the conveyor by varying the input voltage and input electrical frequency to the motor (ABB, 2011), which are related in a ratio (V/Hz) that has to be kept relatively constant for the AC motor to produce adequate torque (Ontario Hydro, 1997).

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3.5.7 Scalar control

The simplest type of VFD is an open-loop control system. It uses the technique that is explained in the previous paragraph without any feedback device, which makes it very simple and low cost. It is suitable for applications that do not require high levels of precision (ABB, 2011), (Barsoum, 2012), such as fans. A simple block diagram of the aforementioned components in an open-loop drive configuration is shown in Figure 3.9.

Figure 3.9: Block diagram of open-loop drive, or V/f control.

The inverter sends out a PWN pattern that is designed to maintain a constant V/Hz ratio, which is why this system is also referred to as the V/f control system. This ratio is put out by the inverter under the assumption of ideal conditions. How the motor actually reacts to that PWN pattern is dependent on the load conditions, which are not measured in the open loop drive. So, even though it is relatively simple and low cost, this system also comes with a big downside. The lack of feedback control means that the field orientation, which is the alignment between the stator and the rotor, is not controlled. This can result in torque and current oscillations, making it so that the torque of the motor cannot be controlled accurately, but that the motor torque depends on the load conditions. (Texas Instruments, 2006)

(ABB, 2011) lists the following advantages and drawbacks of an open-loop drive, V/f control system:

Advantages:

 Low cost

 Simple – no feedback device needed

Drawbacks:

 Field orientation not used

 Motor status ignored

 Torque is not controlled

3.5.8 Vector control

According to (Barnes, 2003), developments in power electronics over the last 10 to 15 years has made it possible to control not only the speed of AC induction motors but also the torque. Modern AC variable speed drives, with vector control, can now meet all the performance requirements of even the most demanding applications.

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There are two types of vector control: Field Oriented Control (FOC) and Direct Torque Control (DTC). The review of (Garcia, 2006) concludes that FOC is more suited for steady state behavior and DTC has a quicker response in its torque dynamics. Which type is more favorable depends on the application.

3.5.9 Field Oriented Control (FOC)

Field Oriented Control (FOC), also called flux vector control, deals with the aforementioned downsides of V/f control. It is a control system that can be used for applications which require up to 5000 horsepower (Ontario Hydro, 1997).

FOC drives are available in direct and indirect configurations, which are also referred to as open and closed loop configurations (Toshiba International Corporation). Since the open loop system does use sensors and feedback, it is actually named incorrectly, but since this is the accepted naming in the field, this system will be referred to as the open loop or direct vector control. The difference is that the direct system uses current sensors inside of the drive to monitor the system and compares it to a mathematical model. The closed loop or indirect system also uses feedback measurements from rotor speed and angular position relative to the stator field for the motor control, making it more precise (ABB, 2011).

In order to understand FOC, first the principle of vector control must be explained. As we know, the electric motor operates through the interaction between the magnetic fields in the stator and (consequently) the rotor and their interaction over the air gap that is separating them. As shown in paragraph 2.6, the torque of the motor depends on the air gap flux.

With the FOC control method the three stator currents are used as space vectors in an (a,b,c) coordinate system and converted via the Clarke (a,b,c  α,β) and Park (α,β  d,q) transformations to a two vector coordinate system, the two vectors representing the torque and the magnetic flux of the AC motor. The PI controllers provide renewed (d,q) components, which are translated back and eventually fed in the PWM, in order to keep the motor speed and torque at the level of the reference values.

(Texas Instruments Europe, 1998) summarizes Field Oriented Control as follows:

“ The Field Oriented Control (FOC) consists of controlling the stator currents represented by a vector. This control is based on projections which transform a three-phase time and speed dependent system into a two coordinate (d and q coordinates) time invariant system. These projections lead to a

structure similar to that of a DC machine control. Field orientated controlled machines need two constants as input references: the torque component (aligned with the q coordinate) and the flux component (aligned with d coordinate). As Field Orientated Control is simply based on projections, the

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A block diagram of a typical Field Oriented Control system is shown in Figure 3.10.

Figure 3.10: Schematic representation of a direct FOC system (Kazmierkowski, 2011)

(ABB, 2011) lists the following advantages and drawbacks of a Field Oriented Control:

Advantages:

 Good torque response

 Accurate speed control

 Full torque at zero speed

Drawbacks:

 Feedback needed

 Costly

 Torque controlled indirectly

3.5.10 Direct torque control (DTC)

DTC uses the voltage and current measurements from the motor in order to calculate an estimated magnetic flux and torque. These are compared to reference values from a look-up table and based on the deviation action is taken.

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Figure 3.11: Schematic representation of DTC (Abb)

Advantages over FOC (ABB, 2011):

 Field orientation without feedback, less complex

 Control cycle time 10 times smaller than FOC

 Better dynamic speed accuracy

 Torque controlled directly

Considering these advantages over FOC, DTC is deemed more suitable for the belt conveyor applications as they are discussed in this literature study.

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4 VSD for large scale belt conveyors

In the previous chapters the different potential methods for using continuous variable speed drives were discussed and their working explained. In this chapter review on the benefits, challenges, other considerations and the use of such a system on large scale belt conveyors is performed.

4.1 Benefits of VSD for belt conveyors

Logical reasoning leads to the conclusion that being able to adjust the speed of a belt conveyor to exactly speed that is required can only bring benefits. (Saidur, Mekhilef, Ali, Safari, & Mohammed, 2011) states an appropriate summary of the possible benefits of VSDs:

“VSD installation increases energy efficiency, saves energy consumption, improves power factor and process precision, soft start up and over speed capability. They also eliminate throttling mechanisms and frictional losses affiliated with mechanical or electromechanical adjustable speed technologies and expensive energy-wasting. Other benefits of VSDs include the prolonging of the life of the equipment, by adjusting the motor speed to meet load requirements. Generally, energy savings translate into cost savings and reduction in GHC emissions for a given level of production. “

(ABB, 2012) refers to the ability to recover energy when braking downhill conveyors as another benefit of a VSD system. Downhill conveyors require continuous braking to minimize wear. The speed and torque control of a VSD can be used for slowing down the conveyor and feed the recovered energy back into the grid.

Additionally, (Antoniak, 2006) lists these advantages of VSD compared to conventional drive systems:

 A soft start of the belt conveyor which does not cause voltage drops in the mains and also reduces belt tension during the start-up, which enables to use belts of a lower strength

 A flexible control of the torque and of the rotational speed of the motors, thus also of the belt speed in a function of the run-of-mine, a possibility of men-riding at a safe speed

 A resistance to disturbances and changes of the mains parameters

 A possibility of a flexible change of motion direction

 A possibility of full automation of the transportation system

 Nearly ideal equalization of loads among the motors driving one conveyor, enabling the use of multi-drum drives and intermediate drives

 Smaller release of heat

 An operation at full cross-section of the trough, enabling a temporary storage of the run-of-mine on the belt and thus having a conveyor able to act as storage facility.

Furthermore, (Hiltermann, 2008) lists these benefits of VSDs for belt conveyors:

 Lower noise production due to lower idler rotational speeds

 Lower dust emissions due to lower average belt speed and reduced surface area per transported unit of material

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 Better belt tracking behavior as belt is heavily loaded

 Less belt wear in loading areas (material has to be accelerated less and is deposited in a thicker layer)

 Less load cycles for all moving components

4.2 Challenges of VSDs for belt conveyors

4.2.1 General

(Hiltermann, 2008) lists these drawbacks of VSDs for belt conveyors:

 Possible detrimental vibration modes at certain speed and load combinations

 Higher constant load on all components (which may increase wear)

 Increased motor temperatures (which may reduce motor life)

4.2.2 Chute

As also confirmed by (Lodewijks, Schott, & Pang, 2011), a hazard for belt conveyors with VSD is that the discharge speed of the material is variable. That means that the chute that catches the material must be designed to either be able to handle the material regardless of the belt speed, or to be adjustable depending on the belt speed.

4.2.3 Harmonics

According to (Saidur, Mekhilef, Ali, Safari, & Mohammed, 2011), VSD creates current and voltage harmonics in the electric system. The harmonics lead to overheating of transformers, cables, motors, generators and capacitors, providing useless power and increasing operating cost. Applying a harmonic filter to prevent these problems is advised in this study.

4.3 Material profile section determination

The determination of the material profile section is not a part of the actual VSD system, but it is a requirement for being able to use VSD for belt conveyors. Knowing the material profile section and the belt speed at a certain point in the conveying system, leads to the desired belt speed of the next conveyor in order to maximize the this profile section.

(Popescu, 2008) has performed a study concerning the quantity determination of belt conveyor loads. He states that determining the quantity of material on a belt can be done by using ultrasound sensors to measure the height of the material compared to the belt surface, see Figure 4.1.

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After measuring the height of the material, the material profile section can be approximated by interpolation. In his paper, (Popescu, 2008), presents two methods for interpolation, being the Lagrange and the spline functions methods. It is determined that the most accurate method for approximating the material profile load section is the spline function method, as is displayed in Figure 4.2. It is stated that this approximation is efficient because it determines a cubical polynomial function for each pair of points. The research concludes that it is possible measure the profile height of the transported material using ultrasound measurements.

Figure 4.2: Spline versus Lagrange interpolation for two samples (Popescu, 2008)

After the determination of the material profile section, there has to be enough time left in order to accelerate or decelerate the belt to the required speed. The goal is to quickly react to changes, however there are limitations to the acceleration due to the belt dynamics. (Siemens, 1998) reports from its practical application of VFD, that the material profile section is measured 100m before it reaches the belt.

4.4 Soft start & steady state behavior

An important application of a VSD for large scale belt conveyors is the soft start. The torque and current characteristics for a typical squirrel cage motor without using VFD are displayed in Figure 4.3. In this case the motor can only be turned on or off, so the torque cannot be controlled. Not being able to match the motor torque to the load conditions during the start leads to uncontrolled starting behavior of the belt conveyor. In practice this means jerking of the belt, causing wear and possibly damage.

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Figure 4.3: Torque and current characteristics for a typical squirrel cage motor (ABB, 2010)

(Siemens, 2013) states that using a VFD the torque and current characteristics for a belt conveyor can be varied for optimal torque and speed, as shown by (ABB, 2010) in Figure 4.4. The converter

changes the speed and torque according to a defined ramp, preventing peak loads. According to Siemens this will enable smooth acceleration of the belt conveyor, preventing jerking of the belt in natural and resonant frequencies and making the operation less harsh for the drives, bearings, belt pulleys, brakes and rolls, also avoiding belt vibrations and tears. At the same time a controlled acceleration can prevent the belt from violently lifting up under hard acceleration when it is slightly hanging down between two idler rolls, throwing the material it holds in the air in the process. Furthermore, preventing peak loads means that a belt with a lower tensile strength can be used, reducing investment costs.

Figure 4.4: Possible torque and current characteristics for a squirrel cage motor using VFD (ABB, 2010)

According to (Moss, 2014) VFD‟s are expanding their use as the soft starter of choice. The main challenge for a starting system is that it must produce enough torque to get the belt load moving, while doing this in a smooth and controlled way in order to prevent the negative mechanical effects of high speed acceleration. Conventional starting systems lead to a starting current of 700% of the nominal current, while a VFD starting system can achieve the same amount of torque with a fraction

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curve or a combination of those two. See Figure 4.5 for an example of a ramp-up, steady state and deceleration procedure for a belt conveyor with VFD.

Figure 4.5: Example of acceleration, steady state use and deceleration of a belt conveyor using VFD (Moss, 2014)

4.5 Energy saving theory

4.5.1 General

(Saidur, Mekhilef, Ali, Safari, & Mohammed, 2011) states that significant energy savings can be achieved when the rotational speed of the motor is decreased to match with the load requirement, as deducted from Figure 4.6. The research also states that the application of VSD typically leads to energy savings in the range of 20 to 40%, with a payback period is 1 to 3 years.

Figure 4.6: Relation between motor speed and power from (Saidur, Mekhilef, Ali, Safari, & Mohammed, 2011)

4.5.2 Calculation

(Hiltermann, Lodewijks, Schott, Rijsenbrij, Dekkers, & Pang, 2011) presents a methodology to predict power savings of troughed belt conveyors by speed control. In the article is concluded that speed control for belt conveyors promises considerable savings in the annual power consumption. DIN 22101 can be used to predict possible power savings if the calculations are physically validated, with the equation for power required to overcome motion resistances.

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Measurements on a frequency controlled belt conveyor were performed in order to validate the calculation results. Deriving a trend of the friction coefficient instead of using a fixed value was judged as not adding any value. Using the DIN 22101 calculation for power, this has led to an example of energy saving results using VFD which is displayed in Figure 4.7.

Figure 4.7: Example of energy saving results (Hiltermann, Lodewijks, Schott, Rijsenbrij, Dekkers, & Pang, 2011)

The research states that in practice the power savings using speed control have been proven to be substantial enough to recommend to install all new belt conveyors with VFD. It is also expected that VFD will reduce maintenance related costs.

4.5.3 Simulation

(Zhang, 2010) states that using VSD for belt conveyors leads to considerable energy savings of 15.35%. This conclusion is based on simulations that were done for a case study for coal conveying in a coal-fired power plant. The simulation model is based on calculations using the standards of DIN 22101, ISO 5048, JIS B 8805 and the Conveyor Equipment Manufacturers Association (CEMA). Based on these simulation results, the payback period for VSD‟s is roughly 1.15 to 1.26 years. The ratio between feed rate and motor power that results from the simulations is shown in Figure 4.8.

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22101 and their influence on the energy use of the system has been reviewed using literature, simulations and the Limberg thesis. The outcome of the review is that the fictitious resistance coefficient of belt conveyors largely depends on the filling level of the belt and that the actual belt speed only has a small influence on the total resistance. Furthermore, Lauhoff concludes that speed control is non-effective for filling levels of 60% to 100% in terms of energy usage reduction.His final conclusion is that with a constant conveying speed v, that means with a decreasing filling level (ϕ gets smaller), the specific energy requirement reduces, while, by reducing the conveying speed v, that means controlling the conveying speed, the belt load (ϕ = constant) is maintained.

4.6 VSD for belt conveyors in practice

In this paragraph the application and results of VSD for large scale belt conveyors are presented for several locations, which are reported by researchers and, mining- and production companies.

4.6.1 Nochten opencast mine (Germany)

According to (Siemens, 1998), in the case of the belt conveyor system at Nochten, the task of adjusting the belt speed according to the various quantities to be transported so as to exploit the available load cross-section to the greatest possible degree was turned into reality. The extensive report is summarized in this paragraph.

Implementation and testing of belt speed adjustment and energy saving theory

Electricity consumption represents a major cost factor when operating belt conveyor systems and is influenced by a large number of parameters. Testing was done in order to confirm that maximizing belt occupation through VSD leads to power and thus cost savings. Variable belt speed based on the quantity of material was applied to seven belt conveyors. The quantity is determined 100m before the point of transfer using an ultrasound sensor.

The tests led to the following conclusions:

1. The trail results confirmed the relationship between load, belt speed and energy consumption 2. Attempts to have the maximum cross section occupied were unsuccessful, with an average of

below 50% and a maximum value of 84.36%.

Energy conservation results

On completion of a long-term study electricity savings of 20% have been registered. This figure takes the converter equipment into account, which causes 3.5% in energy losses. The savings can be broken down into seven categories, which are listed below:

1. Load-dependent belt speed adjustment (16%) 2. Removal of slip resistors (3%)

3. Removal of starting resistors on slipring motors (0.5%) 4. Acceleration energy reduction (0.4%)

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5. Reduction in start-up losses due to simultaneous start-up of converter-controlled belt conveyor systems (3%)

6. Recovering braking energy (0.5%)

Effect on equipment wear

Data on equipment wear was not available for this research, and would require more years of operation, monitoring and gaining experience using the VSD system after the report of (Siemens, 1998) appeared. However, based on theory and assumptions, some estimations on wear reduction were made.

Assumptions:

1. Kinetic energy (friction, acceleration) accounts for more than 90% of the energy consumption 2. The energy savings achieved by the reduction in speed stem exclusively from the kinetic

energy.

Using these assumptions and the equation for generated frictional work, it can be concluded that a speed drop to 68% of vnom results in an overall wear reduction of 15% for the moving components in the system.

For the material stripping devices the wear is directly proportional to the belt speed, so a reduction in wear of 30% is expected.

In the study a reduction in wear to the baffle flaps and wear plates due to a reduced kinetic impact is anticipated at 20%.

It has been calculated that a saving factor in the electromechanical maintenance in the upper half of the savings range will be achieved by implementing the new VSD system.

Soft start

Using soft starting a reduction in 28% of starting torque was achieved, reducing unnecessary strain on the belt conveyor and its components.

Conclusion/summary

Following effects of use of vsd for large scale belt conveyors are possible: 1. Power savings between 15% and 38%

2. Savings in maintenance cost: 15%-30% main components, up to 50% for special components 3. Reduction in required drive performance of 15%-30% and up to 30% reduction in dynamic

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Drmno open pit mine. The optimal belt speed is calculated as a function of transported capacity of bulk material and the maximum possible material profile section. The data results of the experiment are shown in Table 4.1. Over a measurement period of 8 weeks an energy saving of 20% compared to the historic data from the constant speed situation was achieved.

Table 4.1: Experimental results of applying load dependent speed variation for belt 3 at Drmno open pit mine (Jeftenic, 2010)

4.6.3 Tamnava West Field mine (Serbia)

An interesting evaluation of the actual application of variable speed drives for five of the eleven belt conveyors in the Serbian Tamnava West Field mine has been done by (Kolonja, Jeftenic, & Ignjatovic, 2003). The study reports a reduction in energy costs of 7 to 24%. These figures are mentioned in the conclusion, but their exact origin is hard to determine. (ABB, 2000) does report this project, see Figure 4.9 for a scheme of the belt conveyors in the mine and the application of VSD. The report states the goals of reduction of energy use through load dependent speed control and a reduction of belt and gear wear through the application of VSD, but no numerical results are reported. ABB summarizes the benefits of controlled drives based on this project as:

 An optimum loading of the belts through controlling the belt speed

 A reduction of gear and belt wear through smooth starting and stopping of conveyors

 The elimination of belt slippage at driving drums

 More even torque on driving drums through load equilibration

 Control of speed differences between motors on one driving drum

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4.6.4 Siemens

(Siemens, 2013) describes the application of variable speed drives for belt conveyors that are used for the coal supply of Jänschwalde and Schwarze Pumpe power plants in eastern Germany, as well as in the Lusatian lignite coal field. The benefits of the implementation of VFD drives that are mentioned match the ones listed earlier in this chapter, such as energy consumption reduction and a reduction of wear and jerking of the belt during the starting procedure. In the article it is stated that these

expectations are fulfilled, but no numerical values are presented.

4.6.5 ABB

(ABB, 2012) reports two successful implementations of variable speed drives, although numerical substantiation is lacking.

In the Los Colorados Mine in Chile the following results were achieved: 1. Improved process control

2. Elimination of motor problems

3. Longer lifetime of conveyor equipment 4. Minimized downtime

5. Lower impact on electrical network

In the Worsley Alumina mine in Western Australia, the following results were achieved: 1. Increased conveying capacity

2. Improved conveyor control 3. Improved start/stop performance 4. Reduced maintenance

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5 Conclusions

The main goal of this study is to review the information that is available on speed regulation and the control solutions for belt conveyor drive systems. The study is divided in four tasks:

From the survey of conveyor drives can be concluded that 3-phase AC electric motors are best suited for belt conveyor applications and that the squirrel cage motor is the most used drive type in this category. Also, in chapter 2 is shown that the torque of a 3-phase AC electric motor can be controlled by either varying the rotor current, stator voltage or the frequency of the power supply.

From the review of the different techniques for VSD can be concluded that the electronic Variable Frequency Drive (VFD) is most suited and also becoming the drive solution of choice for belt conveyors. By controlling the frequency and current of the power that is supplied to the drive, the applied torque can be controlled very precisely.

The main benefits of the VFD presented in the theory are the energy savings due to optimal use of equipment and a decline on material strain due to controlled soft starting and the prevention of unnecessary high speeds of the belt. The practical results show that the previous mentioned benefits indeed exist. Several manufacturers and researchers present results from the application of VFD systems. The results are mostly positive in terms of the realization of the expected benefits, although numerical substantiation is sometimes lacking. Three studies, in which VFD is applied on a large scale and used in the daily operation, report energy savings of respectively 15-38%, 20% and 7-24%. Reductions in maintenance costs were not yet registered, since it would require long term testing to be able to measure these results.

From this study can be concluded that Variable Frequency Drives are a suited method to improve the efficient use of belt conveyors by using the system for highly controlled soft starts and adjusting belt speed according to the required capacity, which has proved in theory and practice to lead to benefits in terms of reduced energy use and strain of the belt conveyor and its components.

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