Obtaining a higher return on investment is the goal of every mill manager. Often, the most cost effective means to achieve this is to upgrade the paper machine drives.
Upgrading paper machinery with new digital controls and adjustable- speed drives can increase throughput and improve cost performance. In addition, newer drives can reduce labor costs, reduce downtime and maintenance, and improve safety. For example, by performing automatic setup and system tuning, modern drives eliminate manual tweaking, which improves operator efficiency and reduces labor costs. Maintenance costs are also reduced because the newer electronic devices use fewer parts. Newer drives also document problems as they occur. Therefore, maintenance personnel spend less time trying to find the problem. In addition, there is less risk of running out of spare parts and having to use outdated documentation.
Upgrading drive systems also reduces potential safety problems, particularly when it comes to aging machinery and equipment. Older systems, built prior to the enactment of current regulations, may not present a legal liability problem because of grandfather clauses. But they raise a question about ethics if they present a risk. Newer systems are built to more stringent codes and employ safety interlocks and watchdog circuits.
There are two technologies available for paper machine drives:
Ac adjustable-speed drives. Until the late 1980s, ac drives were not used on paper machines. These machines require constant torque over a wide speed range, and ac motors driven by an inverter could not provide the needed torque, making them a poor choice for paper machines. The introduction of the flux-vector ac drive changed this. Vector drives solved the problem by controlling ac motor torque as well as speed. Many paper machines and slitter winders now use this type of drive.
In a typical system, a shaft mounted encoder provides speed feedback to the drive. This signal tells the drive the rotor position relative to the stator flux. From this information, a complex algorithm calculates the torque and flux — producing components of the motor current — helping the drive control the ac motor torque.
The speed-torque characteristics of a vector-controlled ac motor are similar to a dc motor. Like a dc motor, vector control provides full torque at zero speed, but does not need back electromotive force (EMF) to apply the torque. An ac vector controlled motor can supply full torque at absolute zero speed almost indefinitely.
Dc adjustable voltage drives. Because of the dc drive’s ability to provide constant torque over its entire speed range, it has been the electric drive of choice for paper machines and other equipment that require wide constant torque operation.
Although both drive technologies are suited for paper machines, other factors can influence the choice:
Price. An ac vector-drive system may cost 20 to 30% more than a comparable dc system. The cost reflects the additional complexity associated with the control and power electronics.
Some users may want standard ac motors, which are typically less expensive than dc motors of comparable horsepower. But, adding the features required motor design, provisions for encoder mounting, and motor cooling — offsets the cost advantage. Such changes also make the ac motor non-standard.
Another factor affecting price: Upgrades are usually made to machines with existing dc drives. Some, if not all, of the existing dc motors may be reused with the new drive.
One final consideration might be space in the existing drive control room. An ac vector-drive electronics package is generally larger and requires more panel space than a dc drive.
Spare parts. The cost of spare parts for the drive system follows the 20 to 30% differential mentioned above. The difference in the cost of spare motors can be significant, especially for a mill with multiple paper machines using rebuilds. It is possible for existing spare dc motors to replace some or all the motors in a new or rebuilt dc drive system. It is unlikely, however, that the mill will have any existing spare ac motors suitable for paper machine use with vector drives.
Motor cooling. Dc motors used on paper machines normally come in splashproof, separately ventilated enclosures. This requires a separate ventilating system for motor cooling. Ac motors, on the other hand, do not require a separate system because they use individual fixedspeed fans. The costs of the fan motors and control equipment are usually included with the ac drive system cost. The cost of the central ventilating system and its installation should be added to the dc drives cost.
Cable cost. Ac drive system wiring costs will generally be less than for a comparable dc system. These costs vary depending on the motor horsepower. The larger the horsepower, the greater the price difference. This is a result of the lower current requirement for ac motors versus dc motors of the same rating.
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Power factor. The graph, Figure 1, shows the relative power factors of ac and dc drives as a function of operating speed. The ac drive power factor remains at around 98% as a result of the diode rectifier used on PWM inverters. Dc drives have a power factor of around 90% at 100% rated speed and decrease proportionally with drive speed. Generally, since paper machines run at near design speed, this does not present a great difference to the mill. If the mill needs to improve its power factor, engineers can install powerfactor correction capacitors.
Harmonics. The harmonics generated by the dc drive and the ac drive are about the same at design speed. However, due to the higher power factor, the line currents are lower for the ac system. If the mill has a problem with harmonic currents, then engineers should use deltadelta and delta-wye connected transformers to feed the rectifiers for both ac and dc drives.
Efficiency. There is no significant difference in efficiency between the two systems.
Maintenance. The dc drive has fewer components than a comparable ac drive, thus, it usually needs less maintenance. However, the reliability of modern electronics makes this negligible. The ac squirrel cage induction motor is rugged and easy to maintain. In addition, the ac motor has no brushes or commutator. The reduced maintenance, shorter motor related outages, and reduced motor rewind cost add up to lower maintenance cost for the ac system.
Regeneration. Paper machines typically do not require regenerative drives on the wet end, but do need regeneration for the dry end. Dc drives can provide regeneration by adding a second SCR bridge configured in an anti-parallel mode.
The ac drive does not provide regeneration as easily. A common dc bus for the regenerative sections is a partial solution. In this configuration, drives can regenerate back to the dc bus as long as the total load from the ac line remains positive.
Dynamic braking can be provided for the dc bus for machines that require negative power from the group of ac drives tied to the common dc bus. This adds complexity and heat-producing elements to the ac drive solution.
Another option for the ac drive that requires regeneration is to supply the dc bus from a rectifier with reverse capabilities. This would be like putting a regenerative dc drive on as the supply for the dc bus on the ac vector drive system.
Go with what works
Either ac or dc drives can and will provide a very acceptable solution to the problem of driving a paper machine. If users have been plagued by high maintenance on dc motors, they may opt for the ac solution despite its higher purchase price. If, however, they have many dc drives installed and motor maintenance is not a major problem, they should probably continue to use that system.
Rebuilding, two examples
An engineer has an old cylinder board machine rated for maximum line speed of 350 fpm. The machine is 140-in. wide. By replacing the wet end drives of the machine with newer drives, the engineer can increase the machine speed to 600 fpm. The existing line shaft drive provides adequate power for the dry end of the machine.
Power for the wet end sections was calculated using the Tappi constants for cylinder machines and the machine builder’s power constants.
The rebuild included new digital sectional electric drives including digital control, motors, isolation transformer, and operator’s console. Adding all the horsepower requirements for the primary and main press areas resulted in total horsepower needs of 265.4. Total price to achieve a 71% increase in machine speed was around $215,000 for a dc drive solution, Figure 2, and around $235,000 for an ac vector drive solution.
Multiformer. A multiformer machine is geared in at 400 fpm. The machine width is 170 in. An engineer wishes to rebuild this machine to achieve a new speed of 800 fpm. The existing line shaft drive has adequate capacity to drive the dry end of the machine at the new speed. The wet end will require all new electric drives, Figure 3.
Adding all the horsepower requirements for the primary and main press areas of the machine, the total horsepower requirement was 429.8 hp. The total price for a new digital sectional electric drive, including digital control, motors, isolation transformer, and operator console to achieve the 100% increase in machine speed is around $295,000 for a dc solution, and around $325,000 for an ac vector drive solution.
Ronald Meihofer is senior application engineer, Eurotherm Drives Inc., Marietta, Ga.