While AC drives and motors have become the clear choice for papermakers installing new machines, and often for retrofits, as well, Fort James selected DC drives for their PM20 fine paper machine at Camas, WA. It also chose to minimize outside support, and install the new drives progressively "on the fly." The success of this installation indicates that advances in digital technology allow DC drives to regain equal status with AC.

"We carefully considered the option of replacing both motors and drives, and decided that the greatest value would come from keeping the motors, provided that drives would not be a limitation," explained Somchai Assa, Camas' E & I Engineering/Maintenance leader. "We pushed ahead with the drives retrofit as a measure of preventive maintenance, but we also gained greater control of PM20, and the potential for much higher performance."

Progressive installation
"We did a rolling retrofit over four months, a progression of seven hook ups on scheduled down days, integrating the new drives with our vintage DC motors," said Jim Ludwig, Camas' lead electrical electrician in describing the process for the retrofit.

The progressive installation included 26 individual PROFIBUS® nodes and 26 drives in concert with the existing motors and the Honeywell DCS. Pacific Controls Supply, Inc., a local distributor, provided technology support and training.

The May 2000 launch of the new drives at Fort James Camas started with the reel spool starter, where a dedicated motor spins the roll. The objective to match speed with overdrive potential by a few percent was achieved within the 24-hour window of downtime.

"We worked the bugs out at the reel spool starter, where very quickly our PM20 crews accepted the change, and just took off and ran," said Says Kevin Wikle, project consultant/integrator at Camas. "Not everybody was comfortable with the pit crew approach to installing drives, but it worked so well, even the biggest skeptics were surprised. The success is clearly connected to a good strategy, but also to the simplicity and ease of the drives themselves."

The first order of business was to check out theories about drive functionality for PM20. The original drives, installed in 1984, used a digital master, even though they were the last version of analog drives. They provided a variable frequency pulse train - a signal, proportional to speed, converted to analog to control the drive.

Overshoot not an issue
It is vital to the crews that the machine master speed can be adjusted for grade and process changes while keeping all of the section draws at their set values. The gains are adjusted so the drives will respond quickly to speed and load changes, but not so high as to cause overshoot, as this can cause a paper break. The master acceleration rate from the PLC is then set slower than that of the section with the largest inertia and the whole machine tracks the master accurately.

"We electricians try to keep the drives out of the current limit except for stopping and starting individual sections after a break when the papermakers need them to come up and down quickly," Ludwig said.

One important factor is smooth transitions for grade changes, as Camas runs sheet weight up from 34.6 to 47.2 pounds. Considering that grade changes make the draws change, this is a serious concern. But the digital DC drives can handle this, as well as furnish changes. In this case the vacuum load can vary, which shows up at the press section. But again, the drives have been able to adjust.

What's different?
PM20's management contends that the daily risk they faced of a catastrophic failure is over. But in addition to that operators have a better sense of control. Accuracy with draws is much higher. Even though the old system worked, few people understood it. Information was difficult to get to or interpret.

Troubleshooting is better and easier with the new system. In the past, when a size press paper roll quit working, the whole section had to be shut down before resetting and restarting. Now, checking the LED display on the drive will reveal the status of the drive. In addition, a historical archive is maintained in the drive for the last eight faults. Ludwig predicts that with the all-digital system, speed control-related paper breaks will be eliminated.

Flexibility and ease of use
Simple changes can be made in the PLC MASTER, or by changing connectors or binectors. The Siemens software offers ease of set up with the PLC MASTER, PROFIBUS interface and drives. The learning curve is short.

With the new drives, a technician simply looks at the function data sheets; observe the current operations, and uses simple programming to make adjustments. As for the interface with the HMI (human machine interface), there are only three buttons and an LED read out to program from. Supplemented by laptops, Siemens programs are used to program from one drive and switch it to another, then modify as desired. There is also a handy web site, where upgrades to the drives can be downloaded.

Learning curve and training
The in-house team at Fort James Camas built a trainer, which consisted of a PLC, Siemens DC drive, and operators MMI in a cabinet. This was connected to a test bench with a 7.5 HP motor on the floor. The drive was accurate from the start. Both the liquid crystal display and paper print outs allowed for good recording charts. With a one tenth second acceleration rate, the motor responded well without overshoot.

Ludwig took a short class at Pacific Controls in Portland, then adapted a training program for the mill, focusing on shift technicians to do troubleshooting. They used hot spares and laptops to help everyone learn to configure the drives themselves, then practice uploading and downloading on the trainer.

With only two days of training for drives, SIMOVIS®, PROFIBUS and the PLC, the technicians were ready for action. The electricians who learned the system were largely seasoned veterans with electrical and production experience.

Spare Parts
Spare parts were a problem for what was becoming an obsolete system. Because the Siemens digital drives (from 5 HP to 1000 HP) are universal in design, fewer spare parts are required. When changes do need to be made, they usually can be done easily by the in-house team.

Sharing an identical card rack and BICO (Binector-Connector) software technology, the DC drives allows technicians to install the same option cards as those used in AC drives, including a PROFIBUS CBP2 card, SIMOLINK® SLB card and T400 Technology card for more complex applications such as winder drives. Additional devices such as the OP1S operator control panel are also interchangeable. This concept can greatly reduce inventory and training cost in addition to adding versatility and ease of use.

"The old GE drives required many spare parts, sometimes requiring many months for delivery," said Wikle.

Benefits Beyond PM20
Commenting on overall benefits beyond the drives themselves, Wikle puts it this way: "This is the most important machine in the whole mill. The confidence we have gained from this retrofit has opened the way in the rest of the mill. We know we can buy packaged drives, and install and run well. Siemens drives are not just for wizards."

"A driving need with the project was to make sure that there were no mysteries designed into the system," adds Wikle. "If another team could step right in at the mill electrician level, we knew we had succeeded. Totally maintainable. No consultants. No engineers flew in. Sleeping at night and taking weekends off. That's a good system."

Product Information: Drives Products