In my roll as a drive consultant performing audits on web handling equipment, I always begin by comparing the actual winding roll diameter to the diameter measured or calculated in the drive system. The roll diameter is vital to all 4 key ingredients of winding, TNTs (Tension, Nip Loading, Torque and speed).
Drives regulate motor speed as measured by frequency, or a tach or encoder. The motor speed is in RPM not line speed of FPM or MPM. For a winding roll the motor RPM must steadily decrease as the diameter builds for a given line speed.
RPMroll = Line_Speed
Center wind drives control Torque at the motor shaft. This torque is used to establish Tension on the web. The torque required to hold steady tension increases as the diameter increases. This is because the roll diameter acts as a pulley. The motor provides a torque constant (dependant on HP and rated RPM) relating power to torque. The torque at the motor shaft required to hold tension is given by:
Torqueroll = Tension_SP*Diamroll
If a contact or rider roll is used, its Nip Loading probably is tapered or profiled with diameter. The controls for nip loading may or may not be implemented in the drive system. The diameter sensor for the nip loading may be the same or independent of the winder drive. In any case his diameter must be accurate and scaled correctly.
One common problem is that the diameter used by the drive system deviates from actual during speed changes. On many lines, speed changes including starting, stopping, and slow speed operation affect the diameter signal. The diameter deviation shows up in tension variations. Ensuring that the diameter is accurate is a simple way to improve your tension control. See the chart below showing the diameter error when a center wind unwind starts and stops 4 times. The actual diameter for an unwind never increases.