Why use motors vs. brakes or clutches?

(Caveat: As usual, when I’m writing about controls, it’s best to get some confirmation about what I say.  -tjw)

When I think about motors vs. brakes and clutches, I can think of three differences.

  1. Motors have a faster response time.
  2. Motors (the right motors) operate in all four quadrants of control, in either forward and reverse and in either power or regeneration (braking) mode. This is especially helpful for unwinding large, high inertia rolls. Braked unwinding systems rarely apply and compensate for inertial torque. Without inertia compensation, an unwind’s tension will increase during acceleration and decrease, sometimes causing a slack web, during deceleration. A motor-driven unwind can add more torque to help accelerate a roll and act as a brake during decel.
  3. Motors can run in speed mode. Every web line needs at least one pacer.

Early in my web handling days, I tried to set up a torque-to-torque web line. I was working on a torque-controlled winder (at the end of a film line) and wanted to run some winding experiments off a portable braked unwind. So we put in the unwind, threaded the web to the winder, and turned things on. Nothing happened. The brake torque’s tensioning was too high relative to the winder torque tensioning and the unwind won the ‘tug of war.’ If we turned the brake setting down, the winder would win for a while, then stall as the unwind tension went up (due to unwind tension = torque / decreasing roll diameter). If we turned the unwind brake too low or off, the winder would accelerate. This system had no pacer, so it was a battle of torques and F=ma to determine the line speed.