A Process Driven By Technology

Although the fundamental drive requirements of paper making have changed little, the nature of the drives themselves has undergone a radical evolution. The increasingly software controlled process is now more reliable and much easier to set-up and maintain. Peter Worland reports Over the years, the shunt wound dc motor has been the most common drive in use on paper machines (PMs), but this is changing with the introduction of the high precision 'vector' type ac drive, which offers as good and often superior performance to dc drives. Early PMs were fitted with a line shaft driven by a large dc shunt motor. Cone pulleys were provided to adjust intersection speeds and clutches allowed sections to be stopped and started. Most modern PM drive systems are sectional and a motor and controller is provided for each section (Figure 1). Each section motor is fitted with a digital encoder to provide accurate speed which the motors must operate - very wet at the wire section and hot and humid at the dryers - means that the dc motors have to be ventilated. In contrast, ac motors can be used with no special provisions, additional costs or worries as they are sealed. The maximum power that can be transmitted by a particular roll to drive the felt or wire is limited by two factors: the arc of wrap around the roll and the coefficient of friction between roll and driven medium. Where this power is less than the total required to drive the section, additional power is supplied by supplementary or 'helper' motors, which drive other rolls in the section. Typical arrangements include couch rolls driven by a helper motor to assist the wire forward drive roll motor (Figure 2). Typically, the stock is fed to the wire and recycled through 'broke' pits, situated below the wire section, until the correct consistency is reached. A thin tail of paper is then picked off the wire and threaded through the machine, section by section. When the tail reaches the reel section and is running between the reel drum and the empty reel shell, its width is gradually increased at the wire until a full width is passing right through the machine. At this stage, the paper is wrapped around the reel shell and reeling commences. This means that the drives have to be spot-on, since during the threading process the section motor speeds must be held constant in spite of rapid load changes as the paper passes through the machine and its width increases. Any slight variation in speed at this time will cause the fragile web of paper to break. A typical surface driven paper reel system is shown in Figure 3. The reel section drive normally operates in speed control during threading and reel changes, but tension control is normally used while reeling. As the web travels along the machine, the paper passes through various pressing and drying sections and undergoes changes in length, so the section speeds must be adjusted accordingly. The inter-section speed differentials (draws) may vary from zero to +/-5% of the maximum speed. All the drive sections receive a digital speed reference signal from a master speed reference unit and this is transmitted to the drives over a high speed local area network. Individual section speeds are trimmed with respect to the master reference to obtain the required draws. Normally, one section is tied solidly to the master reference with no draw adjustment. Draws may be individually adjusted, but a cascade adjustment can also be provided to avoid having to reset all draws following an adjustment at one section. The master section is normally the wire (or first) section - a particularly useful arrangement in the case of a cascaded draw system, as all draws can then be considered positive. Some sections have large inertia-to-torque ratios. Dryers, for example, might consist of a dozen or more 2m diameter cylinders to each section. Accelerating these up to speed from a standing start requires substantial overload capacity and designs normally cater for at least 2 to 2.5 times the full torque. Older machines may also have plain bearings that require breakaway torque of up to four or five times the normal running load. These factors should always be given careful consideration when planning an update to an old drive system, particularly if the original drive was a Ward-Leonard system with no limit on the current available. With the advent of microprocessor controlled drive systems, paper machine drives have become more sophisticated, with features such as speed and draw menu storage and immediate set-up by product code. When used in conjunction with a modern PM process control computer, such programs allow very efficient product grade changes and improved start-up times. Once the paper has been wound into reels on the PM, the next stage is to slit it into suitable widths for printing and converting, and then rewind it on disposable cores in known lengths. This process is carried out on a slitter re-winder, which normally comprises an unwind stand, a set of driven slitter knives and a rewind system. The rewind has two drums, mounted side by side, on which the slit and rewound roll of paper sits. The distance between the two drum centres is only slightly greater than their individual diameters, ensuring that the rewound roll is supported by both drums. A rider roll sitting on top of the rewound roll is arranged to rise in a vertical slide as the rewound roll diameter increases. The pay-off stand is controlled by a braking generator and is centre driven, whereas the rewind drums and rider roll provide a surface drive to the rewound roll. The pay-off normally runs in tension control and the two drums in master/slave configuration in speed control, the second drum load-sharing with the first. The rider roll also runs under torque control. The front rewind drum (the one nearest the unwind stand) turns under speed control and sets the speed of the rewinding process. The rear drum runs under torque control, its load being determined as an adjustable percentage of the front drum load. The torque differential set between the two drums is used to control the tension wound into the roll and determines its 'hardness'. Hardness is controlled by automatic load-sharing adjustment in relation the rewound roll diameter, and by adjusting the rider roll torque. The unwind brake generator at the pay-off operates as a constant power braking system to maintain constant tension in the paper fed from the unwind. Constant tension in this part of the machine is essential to ensure that the slitter knives operate correctly. The brake generator has to control tension over a diameter range of about four or five to one and has to deal with a range of sheet tensions. Depending on grade, this could involve a torque regulating range of 25:1. The drive control system for an unwind brake generator is quite complex. The system must be capable of operating under speed control during the winder's threading phase and under tension control once the machine has been threaded with paper and put into the normal operating condition. While threading, the unwind will be set to follow the same speed reference as the rewind drums. But as the rewind is surface driven and the unwind is centre driven, this speed reference must be adjusted to allow for the diameter of the roll of paper on the unwind to ensure a match of peripheral speeds between the two parts of the machine. Once the machine is operating satisfactorily under open loop tension control at low speed, the tension loop is closed by taking a feedback signal from a tension measuring load cell and using this signal via an auxiliary PI control loop to trim the predicted torque level applied to the motor. Normally, the drive designer will endeavour to achieve as accurate a torque prediction system as is possible to reduce the amount of control necessary from the load cell loop. Peter Worland is general/technical manager at Control Techniques