Center Winders and Unwinders: A VFD and DC Drive Overview

Center Winders or Center Unwinders  are among the most challenging applications for which to set up a VFD or  DC drive. The following is a short over-view.

(NOTE:  IF you’re having a service issue with a winder or unwinder, contact us by either calling 800-848-2504 OR by one of the methods on the right of this  page) When I first got into the drive business,  most drives were analog. Setting up a winder or unwinder using analog drives was very cumbersome. It almost always required a number of external components to do the diameter calculations, match speed, do dancer positioning, load cell control, and to provide a speed and torque reference.

Not only that, but since most winders require tension tapering, stall tension, and other logic, a number of either relays or a small PLC was usually required.

Since the adjustments were often done with potentiometers, which tended to vary both from environmental and user tinkering over time, they required periodic readjustment.   This wasn’t always simple, and would often result in both down time and damaged materials. (we used to hand out “tweakers” and joke with the plant maintenance people that these insured additional service calls for us)

Until the introduction of Vector Controlled AC Drives, almost all center winder applications were driven with DC Drives.  EMA worked on, and continues to occasionally work on, old Reliance MaxPak winders, Louis Allis Winders, GE Drive winders, and others.   Later, as Digital DC drives became common, we did a number of winders using the Emerson Mentor or Quantum DC Drives, the SSD DC Drives, and the Bardac DC Drives.

In general, with no material on the core, the winder is adjusted to be at line speed, plus whatever addition tension the material needs.  Since there is very little weight on the roll at this point, the torque required is low.  So you have the classic constant horsepower application at this point, with high speed and low torque.

As the roll builds, and the diameter increases, speed must slow down proportionally to the material buildup, but since weight is increasing as well, torque must increase.  So at the end, with a full roll, full torque is required, at relatively slow speed.

The figure above is a Louis Allis control system from many years ago, that was designed to regulate an eddy current clutch center winder.  Since an Eddy Current Clutch is essentially a torque machine, this board looked at the line speed vs the actual winder speed to calculate the buildup of material.  It then allowed for more torque reference, which kept the center winder pulling at the desired tension.  The WK2 on the lower left was a differentiated line speed reference, the purpose of which was to adjust torque to maintain tension during acceleration and deceleration.    From looking at this, you can see the inherent adjustment problems.

If the application is a center driven unwinder, then the operation is simply reversed.  You start with a full roll, and at the end, are at core.

The way a center winder or center unwind  is controlled depends both  upon design, and the material being wound.  With heavier materials, many designers simply use a diameter calculator to indirectly control the roll.  These have been around for years, and work well.

Other situations require load cells, dancers, or more elaborate means to control the speed and torque.  Regardless, the overall physics are the same.

With the introduction of microprocessors, drives began to have significantly more internal processing power, and most drive manufacturers began to incorporate the center winder algorithms and processing within the drive. In both DC Drives and Vector Type AC Drives, it’s now common that PID controls, diameter calculators,  summing junctions and expanded logic are all inside the VFD or DC Drive.   Drive manufacturers who cater to system type applications have incorporated specific  CTCW (Constant Tension Center Winders) and SPW (Speed Winder Programs) software into their VFDs and DC Drives. Most system drives have the capability of accepting four to five separate analog inputs all at once.

The Bardac drive and the Parker SSD in particular give the engineer/technician the flexibility to incorporate the PIDs into these analog inputs as they deem necessary.

The benefit to this, from the customer’s standpoint, is that it’s not necessary to purchase a number of ancillary circuit boards and controllers to allow a VFD or DC Drive to control a winder or unwinder.

EMA has experience setting up Winders and Unwinders  with not only the Bardac PLX drives and the SSD 590 Plus drives but also with the Control Techniques Mentor/Quantum III drives. The Mentor/Quantum III utilizes an additional core-processor called MD29 card which is a separate controller that handles processing the CTCW and PID instructions. We’re also very familiar with older products such as the Reliance Maxpak center winder controls, the older GE drives,  Louis Allis winders, and others.

In addition to applying new controls to winders, EMA is adept at tuning existing winder and unwinder applications whether they be indirect (using diameter calculation only) or direct types using load cells or dancers.  We also teach classes on winder and unwinder setup and adjustment