Vertical roller mills are worldwide established in cement plants for the production of raw meal and the cement grinding process. Contrary to other comminution technologies the vertical roller mills have not yet reached their limits of throughput rates. This trend resulted in the development of new drive systems caused by the limitation of power transmission through the conventional gear boxes.
While the majority of the gear suppliers implemented drive systems with multiple motor designs and blowing up the complexity only FLSmidth MAAG Gear with the unique CEM Drive follows the way of reducing rotating parts and providing simplicity. The application range of this drive system is focused on medium to very high drive powers. Keeping this in minded from the very beginning of the design process a smart modularization is realized to cover the power range from 4'000 kW to actually 14'000 kW.
Ingenious motor design
To make the required drive power available FLSmidth MAAG Gear substitutes the bevel gear stage in the conventional MAAG WPV-Gear by a vertical electrical motor, without increasing the dimensions compared to conventional gear units of same drive power.
The ingenious synchronous motor uses a permanent magnet excited rotor. The embedded magnets guarantee an optimal magnetic flux density and the same design protect the magnets against demagnetization by the magnetic saturation of the laminated rotor design in case of a short circuit. The space saving single coils mounted in the stator part allows on one hand to reduce the overall dimensions to an absolute minimum and on the other hand they also reduce the resistive losses by simply reducing the amount of cooper compared to distribute windings. This brings the motor losses down to approximately 1.5%. The factory acceptance test of the first unit proves this amazing value.
Air gab induction and magnetic flux density
But the core part of this motor ensuring the extremely high power density is the cooling system. As the motor is totally enclosed in the gear unit air cooling as used in other electrical motors can't be applied. The risk of bringing dust and dirt into the motor and gear unit is too big and it implement big air cooler near the mill. Conventional water-jacket cooling is not efficient enough and the risk of water leaking inside the gearbox shall be avoided. Therefore FLSmidth MAAG Gear invented a combined air and oil cooling system inside the motor casing itself. It consists of a direct stator cooling with gear lubrication oil and an internal air flow to cool the permanent magnets and the rotor. Cooling channels in the back of the stator allows fresh oil to circulate through the stator and cooling it directly. The ventilator on top of the rotor creates and internal air flow. By cooling down the rotor, the air temperature is increasing. The ventilator is aspirating the hot air and blows it to the back of the stator where it is cooled by the oil cooled stator body. This innovative cooling system allows building a totally enclosed motor fit for operation inside the gear unit without the use of an additional second cooling media and its corresponding auxiliary system.
Innovative motor cooling system
The most flexible drive system for the most flexible mill
Beside the low energy consumption the vertical roller mill has another big advantage compared to other mill systems. With a handful of parameters it is possible to influence the grinding process and adapt it to changing raw material composition or different cement qualities.
The MAAG CEM Drive is equipped with a variable frequency converter supplying the power for the motor. For this component FLSmidth MAAG relies on industrial converters used in many other crucial applications where reliability and space savings are decisive factors, for example ship propulsion. The variable frequency converter used for the CEM Drive gives the cement producer the freedom to adapt the mill table speed according its need to guarantee the best and most economic production. The dynamic torque control together with the direct mechanical connection between motor and gear stage allows a very good electrical damping of excitation frequencies coming from the grinding process. Compared to mechanical spring-damper-elements the electrical damping is not exposed to aging and it can be customized when changing process parameters. The mill can be tuned out of resonances if necessary and therefore unexpected shut downs and peak loads caused by vibration can be reduced. All control and monitoring parts inside the CEM Drive are built with a 100% redundancy to ensure reliable operation.
Fit to use in existing and new vertical roller mills
The overall dimension of the CEM Drive follows strictly the size of conventional gear units where an overlap in drive power exists. The casing is always divided into two parts with a horizontal split line located at the top of the integrated motor. This makes the installation and access to the motor very easy. The gear design and the bearing system to support the grinding table is one by one the same as used in the well-known MAAG WPV gear units and as the motor is also equipped with hydrodynamic slide bearings the only wear part of the entire drive system is the shaft grounding device. This device is easily accessible from outside and can be exchanged within less than one hour. An advanced condition monitoring system allows supervising of operating behaviour of the drive system. Regular maintenance tasks and inspections can be done without removing the drive unit from the mill.
The consequent use of medium voltage for the frequency converter and the motor makes the installation extremely comfortable compared to other new drive systems for large power application. Those are using 3 to 8 motors, where each of them has to be connected to a local motor control centre and a separate circuit breaker. The recommendation of using low voltage motor design increases the power cable cross section compared to medium voltage design. These create high cost for cabling work and limit the accessibility underneath the mill.
The CEM Drive with its variable speed and the compact overall dimensions can replace nearly every conventional gear unit for vertical roller mills. Because of the integrated motor only an alignment to the mill table is necessary. It needs not more than three days of mill shut down to exchange an existing gear unit with the CEM Drive. The first prototype of this drive system is in operation under a raw mill. It operates with a drive torque of around 41'800 Nm in a very smooth and stable way.
First MAAG CEM Drive in operation under a raw mill
The multi-motor drive systems for large vertical roller mills allowing a part load operation in case of unexpected failure of one of the motors or drive units. This may mislead the cement producer to a false sense of reliability of the drive system. Motors of the type in use should normally not fail when they are built in good quality and therefore increasing the number of motor only increase the complexity of the power supply and of the controls, while the promised redundancy is only available when the motor fails. In case of damage of the central part of the drive, for example the axial bearing supporting the mill table, where all motors are connected the entire drive system has to be taken out of operation.
The MAAG CEM Drive on the other hand reduces not only the number of rotating part to an absolute minimum but also the auxiliary systems. The industrial frequency controller with its flexibility in variable speed and electrical damping makes the drive system fit for any type of grinding process, from raw meal to slag or cement. It is designed for the best efficiency. The CEM Drive is the real innovation in the cement industry.
Note: This article was originally published in International Cement Review, April 2017.