- Joined
- Oct 29, 2012
- Messages
- 1,328
I recently commissioned a dual-capstan setup on a cable extrusion line; it was set up like so: unwinder machine (large spool >8ft dia., bare cable) -> capstan #1 -> extruder (melts a jacket onto the cable) -> water cooling trough -> capstan #2 -> winder machine. The reason for two capstans was to establish a desired amount of tension on the cable during the process, I assume something to do with "pre-load/pre-stress" similar to concrete bridge sections.
For that application capstan #2 (after the cooling trough) required no braking resistor at all but capstan #1 operated continuously in braking/regen mode and required a massive amount of braking resistor capacity. I actually made up a bank of braking resistors and installed an external braking chopper because the drive's internal braking transistor wasn't up to the task. If I had just installed a single resistor, even the largest one the drive was rated for, it would have glowed white hot and fused open in probably under a minute. I could have installed a single resistor on capstan #2, no problem. It wouldn't glow white, red, or orange, ever. Current would never flow in it because what little braking that capstan does, would be absorbed by the DC bus capacitors. It would sit there collecting dust, and become a home for spiders.
I mention this to highlight the fact that not all applications are the same. These two capstans were identical but given the way they were used, had very different braking requirements. Not every machine has or needs a braking resistor, and not every machine which has one, ever gets hot enough to cause problems. I have installed braking resistors inside control cabinets (ceramic encapsulated types) and I have installed them on top (ventilated wire-wound types) depending on the application and amount of expected braking energy.
There is a bit of research and math required to determine which scenario is appropriate and I agree with your advice to consult the manufacturer if in doubt.