Page 1 of 11 Drive Systems for Vertical Roller Mills Martin Baechler Nahi Khoury Jared Weston Product Manager Sales Manager Product line manager FLSmidth MAAG Gear AG FLSmidth MAAG Gear AG FLSmidth Inc. Lagerhausstrasse 11 2040 Avenue C 2040 Avenue C CH-8401 Winterthur, Switzerland Bethlehem, PA 18017, USA Bethlehem, PA 18017, USA martin.baechler@flsmidth.com nahi.khoury@flsmidth.com jared.weston@flsmidth.com Abstract -- Capacity of cement new production lines has steadily increased over the last years. Reduction of investment costs is one of the most important driving factors for this trend. Because of simple scalability and low energy consumption, the vertical roller mill is a key part for single mill cement lines. The limits of material throughput of VRMs are, unlike other mill types, not defined by the comminution process itself but by the transmissible torque of the drive train. Therefore the size of conventional gear boxes for VRMs have been pushed to their limit and new drive concepts have been developed over the past decade. A conventional drive train contains a horizontal electrical motor and gearbox. In addition to the torque increase, the gearbox also redirects the rotation movement from the horizontal direction into the vertical axis of the mill table utilizing a bevel gear stage. This bevel gear stage represents the limiting factor in torque transmission. New drive concepts either reduce the torque by increasing the number of bevel stages, or simply eliminate the requirement for the bevel gear. This paper discusses different drive solutions for VRMs explaining the technology and discusses limitations and advantages of each. Index Terms Cement industry, Drives, Gears, Motor drives, Permanent magnet machines I. NOMENCLATURE Abbreviation Meaning FEM Finite Element Method kw Kilowatt LV Low Voltage (Voltage level below 1000 V) MV Medium Voltage (Voltage level above 1000 V and below 32,000 V) V Volt VFC Variable Frequency Control VRM Vertical Roller Mill w/d ratio Tooth wide divided by the tooth contact diameter II. INTRODUCTION Since the introduction of vertical roller mills (VRMs) in cement production the importance of this comminution system has steadily increased to the point where the technology makes up more than 66% of raw and cement grinding systems sold since the turn of the century in the United States and Canada. Low energy consumption, compact layout and versatility to quickly adjust operating parameters to produce multiple cement products are only three of the advantages of VRM. The VRM is the only grinding system where grinding, drying and separation take place in the same machine. Grinding processes may be optimized by adjusting parameters such as grinding pressure of the rollers, airflows, temperatures, speed of the separator and additives. Compared to ball mills or roller presses the grinding process in a vertical roller mill is more flexible and adaptions to changing production conditions are faster and easier to realize [1]. The demand for larger mill capacities has increased in recent years as cement production lines have increased as well as a trend towards one raw and one cement mill. The largest vertical mill sold in 2016 has an installed power of 11,600 kw and now more than 18% of vertical mills for raw and cement grinding sold by the four largest Western mill suppliers have an installed power of more than 5,500 kw. The need for increased drive power and a period of increased failures of conventional planetary gearboxes in the last decade has lead drive suppliers to develop new technologies in order to provide these large drives for VRM with a high reliability. The various new drive systems have been presented within several papers and publications by the different drive manufacturers. As the drive is only one part of the mill, to evaluate a drive system it has to be examined in the context of the entire grinding system.
Page 2 of 11 A. Requirements from the cement production process The trend towards single mill cement production lines is driven by reduced required capital expenditure and higher efficiency. The increase in capacity and size is not only limited to the mill and drive system but to all auxiliary units, such as feeders and weighing systems, separators, hot gas generators, ventilators, conveyors, bucket elevators, etc. Fig. 1. Modern VRM with swing-out system for roller replacement The comminution process absorbs approximately 60% of the total electrical energy in the cement production [2]. The efficiency the fast acting grinding process in the vertical roller mill allows the producer to shut down the mill during time periods of peak power costs. The most common causes for unexpected shut downs of vertical roller mills are a result of disruptions caused by material handling equipment upstream of the mill. Periods of material starvation or sudden changes in feed granulometry disrupt the grinding process and result in vibrations [3]. In the event of severe damage on one single roller, a couple modern VRM designs permit the lifting of individual rollers to allow operation with reduced production rate. In order to maintain symmetrical loads and even operating conditions, an opposite intact roller is lifted so that the mill operates with two rollers instead of four, or four rollers instead of six. III. DRIVE SYSTEMS Since the turn of the century, cement production capacity has more than doubled globally. During the peak years of 2006-2006 more than 200 VRM were sold per year. Utilization rates at existing plants increased above 90%, causing plants to push equipment to the limits and minimize maintenance stoppages. As a result, an increase of gear damages with an accumulation especially on bevel gear stages. This led to the conclusion that the weakest part in the torque transmission of conventional gears is the bevel stages and that potential problems are increasing with higher drive power. In the same time the trend toward single mill cement production lines began with higher throughput rates and increasing drive powers. This can be defined as the start of the development of the 4th generation of vertical mill drive. On the other hand as an immediate action many gears were equipped with condition monitoring systems to detect damages in an early stage and counteract against in order to avoid costly unexpected mill stops, repairs and production losses [4].
Page 3 of 11 Fig. 2. Average mill size development A. The conventional drive system The conventional drive system for large vertical roller mills consists of a horizontal electrical motor, typically slip ring asynchronous motor and a gearbox. The gear contains the bevel gear allowing a first speed reduction and the redirection of the horizontal input axes into the vertical output axis followed by a single or double planetary gear stage with torque split [10]. To be able to build conventional gear boxes with rated drive power up to 9,000 kw special attention must be given not only to the bevel gear stage but to the entire gear unit. An optimal division of the gear ratio between the planetary stage and the bevel gear stage is critical to keep the gear compact while providing required design service factors. The concept of a double planetary gear stage with torque split reduces the transmitted power in the second planetary state by approximately 30% [10]. In combination with a tooth flank modification on sun pinion and planet wheels, the optimal load distribution in the planetary stage during operation is achieved. A very stiff casing with a minimum of openings and a bearing system with fail-safe shoulders limits the tilting of the torque transmitting parts caused by dynamic grinding loads. The gearing of the bevel gear is always designed as cyclo-palloid gearing with a gear ratio in the range of 1:2. Due to its geometry this type of gearing allows smooth operation and is characterized by an optimal roll-over behavior. For a smooth operation it is necessary to ensure a continuous transition of tooth contact from one pair of teeth to the next. This means that at least one pair of teeth must always be in contact. This characteristic is defined as the overlap ratio [8]. The correct sizing of the cyclo-palloid gearing allows an overlap ration above 2.5 meaning that the torque transmission is always done by at least two pair of teeth. With these design criteria bevel wheel up to a diameter of around 2 m can be manufactured [5]. Summarizing one can say that conventional gearboxes for high power applications must have a customized design for each specific installation. The design focus is not only on the bevel gear stage but on the entire gearing, bearing system and the stiffness of the casing. The result is the well-known conventional gearbox with simple installation, good accessibility, high efficiency and predictable operating costs for VRM up to drive powers of 9,000 kw Fig. 3. Conventional gear box with double planetary gear stage with torque split
Page 4 of 11 B. Modular drive systems All drive systems where the total required drive power is divided into two or more drive modules are summarized under the expression "modular drive system". They all have in common a multiple of two to eight electric motors connected via a gearbox to the grinding table of the mill. Today three different designs are available on the market. 1) Multiple modular drive system The first drive system of this kind was presented to the market in 2010. It consist of 3 to 6 identical drive modules engaging into a central girth gear placed underneath the mill table. The central parts also support the mill table and transmit the grinding forces directly into the foundation. Compared to conventional gearboxes the height of the central unit may be designed with less height, reducing the influence of unsymmetrical grinding on alignment. Fig 4. Schematic drawing of a multiple module drive system [6] The drive module itself contains a low voltage asynchronous motor connected by a flexible coupling to a bevel spur gear unit with a self-aligning pinion meshing into the central girth gear. The entire module is placed on one base frame the way that the motor and the bevel spur gear unit is aligned during final assembly in the manufacturer's workshop. The synchronization of all drive modules can be achieved by a variable frequency converter (VFC) for each motor. With the VFC it is possible to adjust the mill speed to optimize the grinding process [6]. With the division of the total required drive power into 3 to 6 drive modules the design torque of each bevel gear stage is reduced to so the bevel wheel diameter is no longer a limitation. The drive modules require a large foundation around the mill and the alignment to the girth gear is critical. Power supply and auxiliary systems are more complex compared to conventional drives as each module required electrical power, lubrication, monitoring systems and control logic to achieve the necessary speed synchronization. The number of wear parts and components which could fail increase proportionally with the number of drive modules. A few of these units are in operation achieving a power of 11,500 kw. 2) Modular drive system with torque split The second modular drive system was presented in 2013. As the system described above it contains of a central girth gear underneath the mill table to transmit the power from two drive modules to the mill table with an available total power of greater than 15,000 kw. The drive modules are equipped with a vertical electrical asynchronous motor eliminating the bevel gear stage completely.
Page 5 of 11 Fig. 5. Overview of modular drive system with torque split The basic design concept of this drive unit is derived from the lateral drives for horizontal mills. Directly at the motor shaft the torque is divided into two parallel arranged spur gear trains each meshing with a self-aligning pinion into the central girth gear. The highly flexible coupling on top of the intermediate shaft fulfills three tasks. First it ensures an equal load distribution between the two gear trains, second the timing of the two output pinions relative to each other can be adjusted and third it guarantees a smooth operation thanks to the damping of torque peaks from the grinding process and preventing of torsional natural frequencies in the operation range. The central part with the girth gear supports the grinding table and transmits the grinding forces directly into the foundation [11]. The bearing assembly of the central part is similar to conventional gear boxes minimizing tilting caused by asymmetric grinding forces and vibrations. Fig. 6. Sectional view of drive module with torque split The reduction to only two drive units reduces the complexity of synchronization, control loops and auxiliary system compared to the above described system. The ability to adjust the timing of the total of four output pinions simplifies the alignment of the drive module to the central part. Compared with a conventional drive system additional space around the mill is required. A first drive system of this type was sold in 2016 for a mill application with a drive power of 11,800 kw. Commissioning is planned for 2017. 3) Modular drive system with partly integrated motors The third modular drive system was presented to the marked in 2014. It is based on 8 vertical oriented water cooled asynchronous motors which are partially integrated into the gear casing. The motors are connected via a pinion with a central wheel. The torque is transmitted further through a double cardanic connection into a planetary gear stage equipped with 6
Page 6 of 11 planets. The maximal gear ratio of such a planetary stage is around 1:3.7 therefore the first gear stage (motor pinion to central wheel) needs a very high ratio of 1:14. As the total drive power is divided by the number of motors the torque transmitted by each motor pinion is considerably small and it results in a very small face width of the central wheel compared to the diameter. On the other hand, this large diameter of the wheel is also needed because of the required space when placing 8 motors around the gear casing. According to manufacturer information the gearing of this wheel is hardened and tempered where normally case hardening processes are applied to gears in similar application. The deformation during the hardening and tempering process of a case hardened wheel with these dimensions is not controllable. Torsional flexibility is given in this drive system by using quill shafts between motor output shaft and motor pinion. The torsional stiffness of this quill shaft has to be defined by FEM calculation and must be chosen carefully to avoid natural frequencies in the operating range of the mill. Fig. 7. Sectional view of modular drive system with partly integrated motors [12] The output diameter of the output flange of this drive system corresponds to the dimensions of conventional gears. However the overall dimensions are larger compared to the conventional gears because of the semi-integrated motor and its junction box. Operation is possible with and without frequency converter. For slow maintenance rotation a VFC is required in any case [12]. Based on the above information given by the manufacturer one can state that the modular drive system with partly integrated motors requires minimal foundation underneath the mill and has the largest number of motors of all modular drive systems. The connection of 8 motors to one central wheel increases the complexity of the power supply. The stiffness of the casing is reduced compared to a conventional gear because of the openings for the motors and finally one of the critical parts of this drive system is the central wheel connecting all the motors together. The possibility of partial load operation for this critical item is not given. Since the introduction to the market, this drive system is sold for at least four different projects [8]. The first two units were taken into operation in September 2016. Both are equipped with VFC, one is driving a raw mill the other a cement mill. Both drives are designed for a power of 8,800 kw [16]. 4) General summary In general, all manufacturers of modular drive systems highlight the possible partial load operation of such a system with reduced motors to constitute a higher availability of the mill. With the increased number of drive modules, the complexity and the maintenance requirements of the auxiliary systems such as cooling, lubrication, condition monitoring and electrical power supply increase. Insulation of LV motors has, in general, increased durability compared to HV motors. However, the power cable requirements are much larger, not easy to handle and increase cost with the lower voltage level. In case of failure, partial load emergency operation is only possible when the failure is located in the drive module and not in the central gearing. Modular drive systems requires more space around the mill to enable the positioning of the different modules or simply to allow enough space for the accessibility of electrical power supply as well as cooling and lubrication systems. C. Roller driven mill This drive concept is a new approach to drive a vertical roller mill. Instead of driving the mill table and transmitting the power by friction through the grinding bed into the rollers, each roller is driven by its own drive module. It is also considered a
Page 7 of 11 modular drive system where the power of each module is equal to the total required drive power divided by the number of rollers. Each module consists of a roller unit with bearings, hydraulic system to apply the grinding pressure and a bevel planetary gearbox attached to the end of the roller unit. The standalone horizontal motor is connected by a cardan coupling to the gearbox. The gearbox itself is not connected to the foundation [13]. Because of the roller movement it must be free-floating. The circumferential speed of each roller is controlled and adapted by frequency converters. According the supplier of this drive system the change from driven mill table to driven roller influence the grinding process fundamentally. At least two of this roller driven mills are in operation. Fig. 8. Sectional view of modular drive system with partly integrated motors [13] Because of the special roller unit design this drive can only be used with the corresponding mill of the same manufacturer. The entire drive unit, excluding the frequency converter, must be at the same level as the roller units. This requires larger roller foundations around the mill. To support the grinding table and transmit the grinding forces into the foundation a hydraulic axial bearing with a corresponding casing in similar dimensions as a conventional gearbox is required underneath the mill. Similar to other modular drive systems the complexity of power supply and auxiliary systems increases with the number of drive modules, or in this case with the number of rollers. However, the control system is less complex as this system has no mechanical connection between the different drive modules beside the friction between roller and grinding bed and therefore small speed deviation between the rollers can be accepted. Fig. 9. Drive arrangement of a roller driven mill [13] D. Integrated drive systems The integrated drive system is based on the substitution of the bevel stage with a vertical motor and keeping identical outer dimensions as conventional gearboxes. The key to success of this mechatronic system is a special designed, high efficiency vertical oriented electric motor with a compact two stage planetary gear combined in one casing. The motor provides the necessary power at medium speed while the planetary stages reduce the speed and increase the torque according the requirements of the mill. With this arrangement a bevel gear stage is no longer needed. This drive system reduces the number of rotating parts to an absolute minimum and enables a highly efficient torque transmission. The planetary gear stages are similar to the well-known and accepted design of conventional gears.
Page 8 of 11 The motor is the key part of this innovative technology. To minimize the drive system to achieve the same outer dimensions as conventional gears the motor needs to be extremely compact. Only permanent magnet excited synchronous motors with an efficient cooling system are suitable for this application. The permanent magnet excited rotor requires a VFC to be operated and is totally wear-free. Two different integrated drive systems are available on the market. The relevant difference between them are hardly noticeable from the exterior. It is manly the design of the motor stator and the way of cooling it. 1) Integrated system with distributed windings The first integrated drive system was brought to market in 2010. The motor consists of a distributed winding. This type of winding is used in most of the industrial motors. Its disadvantage is a larger winding overhang which needs installation space and creates resistive losses without participating in the torque creation. The stator is capsuled with static seals from the rest of the motor making a separate chamber. Transformer oil is pumped through this chamber to provide cooling. To reduce the motor speed two planetary gear stages are arranged in series on top of the motor where both gear stages transmit 100% of the drive power. The axial thrust bearing supporting the mill table is identical to the conventional gear boxes. This drive is designed to a maximum power of 15,000kW [14] [15]. One prototype of this drive system was in operation for one year [8]. Figure 10: Cross sectional drawing of integrated drive system with distributes stator windings [14] The heat transfer from the stator to the oil in such a system, without turbulent flow, is based on natural convention and therefore manly dependent on the stator surface. The requirement of efficient cooling is contrary to the miniaturization of the motor. 2) Integrated system with single coil windings The second integrated drive system was presented in 2011. In this unit the motor stator contains single coil windings. This type of winding is characterized by very short winding overhangs. It requires approximately 50% less copper compared to the distributed winding. The resistive losses are reduced by the same amount because they depend on the quantity of copper windings. The motor cooling is achieved by using the lubricating oil of the gearbox. It is guided via pipes directly through the stator body where it creates a quasi-turbulent flow even when using high-viscous gear lubrication oil. The motor is currently designed for drive power up to 14,000 kw.
Page 9 of 11 Figure 11: Drive arrangement of a roller driven mill The double planetary gearing together and the axial thrust bearings are designed as the conventional gearbox described at the beginning of this paper. The efficient double planetary gear stage with torque split optimizes loss and reduces number of rotating parts of this drive system [10]. The first industrial trial of this drive was in 2014 where it was installed and commissioned under a raw mill in Ecuador in only 3.5 days. The drive was in operation for three month and showed a stable and smooth operation. With this experience the design of the motor was improved. In December 2016 the new and optimized drive system was reinstalled and commissioned. Since then the drive is in operation showing the same stable and smooth behavior in combination with improved motor temperatures. Figure 12: Prototype of an integrated drive system installed in a raw mill Having only one motor and one cooling system to operate the VRM reduces the complexity of the auxiliary systems to a minimum, similarly to a conventional drive system. With identical outer dimensions to conventional drives, these integrated drive systems are suitable for exchanges and replacements of existing drives..
Page 10 of 11 Compared to the other drive systems described above, the integrated motor is not accessible without dismantling of the gear casing. However, it is a wear-free motor should not require accessibility for normal maintenance works. Additionally, horizontal split divides the gear casing into a gear and a motor part simplifies the disassembly if needed. IV. SUMMARY Increased drive power requirements for large VRMs has led to a variety of new drive technology developments in recent years. While only a few installations with drive capacities above 10,000 kw exist, the market is growing. VRMs requiring drives in the range of 5,500 kw to 9,000 kw have become much more common. Drive systems for large power application can be divided into 4 categories: 1. Conventional drive systems (up to 9,000 kw) 2. Modular drive systems up to the highest power ranges 3. Driven roller systems up to the highest power ranges 4. Integrated drive systems up to the highest power ranges The conventional drive unit in combination with an efficient condition monitoring system and regular preventive maintenance is a valuable alternative to the new drive systems. Cement producer can rely on proven 3 stage conventional drives up to a maximum power of 9,000 kw. Modular drive systems contain of a multiple of two to eight drive modules, depending the total required power and the specific manufacturers design. The drive modules are engaged by gearing into a central part underneath the mill table. Therefore the design power of a single drive module is equal to the total required power divided by the number of modules. These systems allow partial loaded operation of the mill in case of damage of an individual drive module. The possibility of partial load operation may provide the operator a false sense of security as damage to the central gear will still lead to complete shutdown of the mill. The complexity of the auxiliary systems, maintenance requirements and control logics increase with the number of drive modules. Synchronization of drive modules become critical to avoid risk of dynamic overloads leading to uncontrolled operation and damage. The roller driven mill is unique compared to the other described systems. Each roller unit is driven by its own motor and gearbox which fundamentally changes the grinding process. An increased roller foundation is required on the level of the roller to support the roller drive system with motors. Contrary to the modular drive systems no mechanical connection exists between the single drive units, therefore synchronization of the different drives is less complex. Nevertheless, increased installation and maintenance efforts are needed for the multiple roller units. The integrated drive system contains a vertical oriented motor in combination with a proven planetary gear stage which minimizes the number of rotating parts. The motor is key to this development and its performance is depends on and efficient cooling system in combination with minimal losses. The motor as well as the gearing are wear-free and the entire drive is built in the same dimensions as conventional gearboxes. It is suitable for replacements of existing vertical mill drives. V. CONCLUSION With the trend towards large single VRMs for raw material and cement grinding the reliability of the drive systems are only becoming more critical. Many competing drive technologies have been developed with the focus of increasing reliability. The reliability of conventional drive units in combination with condition monitoring systems remain a cost efficient option for drive systems up to around 9,000 kw. Modular drive systems do provide redundancy by permitting the mill to operate at reduced capacities under a partial load when an individual drive has failed. However, the added complexity with increased auxiliary equipment, control systems, electrical infrastructure and number of rotating parts with modular drive systems need to be taken into consideration. Integrated drive systems providing an efficient means to transmit power with the minimal number of rotating parts which could fail and reduced maintenance requirements.
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