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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 In addition, there is less risk of running out of spare parts and having to use outdated documentation.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.

Drive choices

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.

Other considerations

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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