Center vs. Surface Winding Drives
By Clarence Klassen
I will discuss some of the considerations for drives for center and surface winders.
A drive for a surface winder is much simpler than a center winder. The reason is the diameter of a driven winding drum remains constant. The diameter of a driven roll increases steadily.
For speed control, the surface winder requires a constant RPM for a constant line speed. For a center winder, the RPM starts high at the core and decreases inversely with roll diameter. Thus at design line speed, and core diameter, the drive motor must run 5 o 6 times faster than at the end diameter. This impacts the selection of the motor and gearing to get the correct RPM.
For tension control, a surface winder needs a constant torque to provide constant tension. For a center winder, the torque must increase proportionally to diameter to hold tension constant. The motor must be selected to provide the required torque at the lowest RPM at the maximum diameter. The adjustable speed drive must be capable of providing this torque.
The power required for winding at a constant line speed and tension is equal for center and surface winding. Please note that power is proportional to RPM times Torque. The power for the surface winder is simple and efficient. The power for the center winder is produced with the highest RPM and lowest torque at the core. The same power is required with the lowest RPM and highest torque at the largest diameter. The motor must be sized or designed to provide these characteristics. Wide speed range motors (custom and expensive) are available with this torque vs. RPM characteristic. Due to cost, the wide speed range motors are not often used. Instead standard motors are used, but the power rating is 5 or 6 times the maximum power ever used to wind the web. The drive must match the motor power. For example, if process tension and speed establish that 5 HP is required, a 30 HP motor and drive will be purchased.
The surface winder must have a constant drum diameter entered as a parameter. The center winder drive must constantly calculate or measure roll diameter to determine the correct RPM and torque to hold speed and tension.
During speed changes, the drive must compensate for the moment of inertia seen at the motor shaft. The inertia is the sum of the machinery and the time-varying product roll. In each case the inertia of the product roll increases as the 4th power of the diameter. The inertia of the machinery does not change.
For a surface winder, the acceleration rate (RPM/sec) is nearly constant for large or small rolls. The fixed inertia dominates the total inertia. The inertia of the roll reflected to the motor shaft is divided by the roll radius which acts as a gear to reduce its inertia. A constant number entered in a parameter provides an acceptable value for inertia compensation.
For a center winder, the acceleration rate (RPM/sec) decreases with increasing diameter. The changing rate affects the impact of both the fixed and variable inertia of the winder and roll. For most of our web-based products, the torque for acceleration is greatest at maximum diameter. For very dense materials (steel, copper, aluminum), the torque required for acceleration is greatest at the core or mandrel.
The cost of a center wind drive and electrical engineering for the center wind drive is double that for a surface winder. This cost may be insignificant when the process determines the winder type required.