Drives for Web Handling

By Clarence Klassen 
 
Today, while commissioning a new winder (which had been tested by the manufacturer in its plant), the winder was started with no tension feedback. Within a second, tension rose to over 6 PLI. Thank heavens for E-Stops.

For heavy, strong webs like ours today, the web will not break, but the machinery may. Lighter webs will break.
Either condition is to be avoided. That is, we should avoid over-tension, over-speed due to the loss of tension feedback. The way to do this is to test the load cells statically prior to running. Then, start with a very low tension setpoint and very low authority to change torque or speed.

A big reminder for me and others.

By Clarence Klassen 
 
I have worked with drives for over 30 years. They are not much to look at. They are put into beige cabinets, and the cabinets are locked away in motor control rooms. Open the cabinet door, and things get even geekier – a bunch of wires and blinking lights.

Many of the people producing drives see them as so many thousand beige cabinets. Get the order, assemble, test and ship them off the loading dock. They know it is an electronic device for controlling a motor somewhere. Boring!

From the beginning, I saw drives as a means of getting a job done. That was the view of most of the drive-control engineers I ever worked with. Drives are used to form, draw, dry, calender, and wind webs. There is some repetition, but always interesting twists. When not working in web-handling, I might configure a drive for marine propulsion, to test cars or engines on a dynomometer, or to lay fiber cable on the ocean floor.

Most important, drives keep the wheels of industry turning – giving us the jobs we need.

By Clarence Klassen 
 
With enough torque, web slipping past a roller is inevitable. I have seen web slipping on paper machine forming sections, coating rolls and many other places.

Slippage occurs when the forces on the web exceed the traction available between the roller and web. Slippage can be aggravated by wet surfaces (water, coating), high differential tension in and out of a roller, between layers of a winding roll, dry powders such as talc or starch. Some web surfaces such as silicon are very prone to slipping.

The forming sections of paper machines are also very prone to slipping because a synthetic screen (or wire) is driven long distances to form a web out of a slurry containing 97% water. Showers are used to clean the wire as it moves. To reduce slippage and provide the required torque, many motors are used to drive rollers around the length of the wire.

Coating sections are also prone to slipping. Coating rollers have a smooth rubber surface and the web is wet while passing the coating roller. It is often impossible to prevent slippage until the coating is right and the blade pressure is correct. Even then some coatings may run at substantially different speeds than other coatings with slippage being the only explanation.

The drive can detect gross slippage in several ways. Where a tension regulator is used, the output limit high and output limit low signals (part of the tension regulator) indicate the web is probably slipping. It may also indicate the draw is incorrectly set, or the wrong roller diameter is entered in the system during the past roll change.

When no tension regulator is used, slippage can be detected by comparing the speed of two or more rollers. This requires a tolerance of two times the speed variation of each drive speed regulator plus a several second time delay.
Another tool available is to monitor speed and torque changes. Jumps (rate of change) in speed and torque do not necessarily indicate slippage, but are a good indication.

To date, drive engineers have not spent much effort on detecting slip. This could be very valuable to operators and process engineers.

Cars have effective traction control today, but only after decades of development. In web handling, we cannot supply the same real time solutions. That is we can’t make web while pulsing a brake for one or more rollers or by driving from a redundant roller that is not slipping. We can plan ahead and improve traction in the same way we can change our 3 season radials for snow tires during the months snow and ice are guaranteed.

By Clarence Klassen 
 
In a couple of earlier posts, I spoke of the effort in upgrading a large dc variable-speed drive with modern control electronics. I will discuss some of the specific commissioning issues.

The first concern was in upgrading the tachometer to be compatible with the new digital drive. The original drive used an analog tachometer with its known issues of zero speed offset and scaling accuracy which changes with temperature. We had the usual problems of getting the new pulse encoder mounted and new tach cables pulled to the drive. Performance improvements are very much noticed with the new tachometers. When the drive is told to stop, it now stops instead of slowly creeping.

The next issue had to do with auto tuning. Auto tuning is much better now than even 10 years ago. However, auto tune did not work with the 40-year-old, 800-HP dc motors. Manual tuning had to be performed. I have seen many cases where the auto tune function cannot cope with an older motor. Years ago, motors had a lot more copper and iron than an equivalent motor today. This affected the  armature inductance and time constant.

We also had trouble with simple sounding signal definitions. Consider the signal DRIVE HEALTHY. This signal acted differently between the old drive control and the new digital drive. Drive master programming had to be changed to adapt to the difference. This affected permissives to reset and start the drive.