Which Type of Drive Suitable for Paper Mill?

I've worked on printing machines ages ranging from the 60's through to the 2000's all used DC drives with tacho feed back.

You will find at points throughout the process slack take up points. These control the speed of the preceding process, too much slack slow down the infeed, too little speed up. The speed control does have a little leeway in it.

Today's AC Motors with VFDs do offer a very fine speed control. But, it depends upon the speed control methodology adopted (viz.) V/F Control or Flux Vector Control or Closed Loop Vector Control.

The selection of the methodology depends upon the requirement of the driven equipment, like response to speed changes, speed control range, speed control accuracy, etc.

At some point, it does take money to do some things. It is always possible to build your own motors or endoders and feedback system, or go with some large stepper motors or servo motors, but I would guess going with the proper feedback controls similar to what has been used in the past is still the least expensive in the long term.

Oh yes, the steppers are not AC motors but are finely controlled DC motors and may have feedback loops with encoders, but not all do. Servo's are AC motors with significant control system associated with them, again, some don't have encoders and some do. The more control you need, the more you need the encoders.

Methinks a Paper Machine is just a tad outside the realm of stepper motors...

I believe that AC Vector drives have the capability to control torque and speed in a closed loop system. However, you need to compare the cost of replacing and redeveloping an existing DC motor control system. No matter what type of system you use, you will need some type of feed back in order to control it.

My experience is limited to the dry end of paper making machine and in four plants I worked in the drive and speed was controlled by DC motors. I assume there was a good reason, two plants were very old and two were very new at the time.

You can have open or closed loop vector control. For your application you'll definately requirer closed loop, using an encoder. This will provide you with the speed match control you require. The encoder isn't a large expense on top of the whole cost of the project. I've used Eurotherm/Parker SSD 690+ inverter (vector) drives controlled by they're fibreoptic LINK system. I'd steer clear of the fibreoptic LINK system, as it's proved very temperamental and problematic. I believe they have a similar system using Firewire, this sounds a much stabiler platform. I'm sure other manufacturers do similar systems, check them out.