Thanks for the comments so far - as usual my problem is not defined well enough, so here's more information.
My machine usually has a motor/drive for controlling unwind tension with a dancer roll as the feedback device. An air brake is also provided for emergency stop only (the drive is disabled during emergency stop). The air pressure is profiled with diameter to prevent bending rolls when emergency stopping with smaller diameter rolls.
As well as emergency stop (all drives disabled), there are two other stops, normal stop at end of run (say 90 seconds to stop from full speed), and a fast stop due to a fault (say 30 seconds to stop from full speed). When choosing an unwind drive/motor, the worst case scenario is stopping a large roll at full tension during fast stop as the motor must generate the torque for tension as well as the torque to slow down the roll. Of course, the frequency of occurrence of fast stops at full tension with a large roll are not that often, but the motor/drive are sized to cater for that situation. The idea is to reduce the motor/drive size and then use the controlled brake to assist the motor during the odd fast stop with large roll.
elevmike - note that braked unwind tension controlled machines are quite common if the production environment can tolerate the brake dust and the energy thrown away is not that significant. Changing a set of disc pads is no big deal.
