text: <s>[INST] PowerFlex for web handling drives - dynamic brake resistor? Summary: Will I need one?Hi, My company builds web handling equipment that in its simplest form, has an unwind station on one side and a rewind station on the other side. In between the unwind and the rewind, I use two driven rolls to pull the web at a constant speed from the unwind to the rewind. (In between the two driven rolls, the web is subjected to my process). I setup the two driven rolls in a master/follower relationship with one roll following the speed of the other roll. It is very important that the follower matches the speed of the master precisely at all times, otherwise my web will stretch, break, or develop slack that will adversely affect my process. In the past, I have used DC motors with dedicated speed controllers and 4 quadrant regenerative drive boards. I am not 100% sure that the regenerative capability is really necessary for us, but most likely one of our vendors suggested it at some point in time and we just kept doing the same thing. This has always worked for us, but now some of our customers are requesting a wider speed range (on the low end) than what is typically within the realm of DC motors. Anyway, I am planning to make the switch to AC vector drives / motors so that I can get the speed range they want. (I am leaning towards PowerFlex 700S). Anyway, as I have been thinking, reading, asking how to implement my master/follower relationship, I have become aware that I will have to determine whether I will need a dynamic brake resistor, and if so, what size I will need. I started reading document PFLEX-AT001 (PowerFlex Dynamic Braking Resistor Calculator) and got stuck at the point where it says I will need to know my load inertia. I do not know how to calculate this in a web handling application, but my gut feeling tells me that it is probably low because (1) my web speed is very slow - my gearbox/belt reduction is usually such that 1750 motor rpm would equate to no more than 50 feet per minute in web speed (less than 25 rpm for a 8 inch diameter roll) and (2) because my driven rolls are constantly pulling the web against the resistance created by my unwind shaft. My unwind shaft is under tension control (using an electromagnetic brake) so excluding the inertia of the driven rolls itself, the load (my web) is going to lose momentum almost immediately when my driven rolls stop pulling. If my reasoning is correct, I also have the benefit that there are no circumstances in which I would need fast deceleration. All I really need is for when the master drive slows down and/or stops, I need the follower drive to slow down at the same rate. I was just wondering if anyone here has experience with PowerFlex drives in web handling applications similar to mine, and if you have found dynamic braking resistors to be necessary (or useful). Any advice will be greatly appreciated. Thanks in advance, Paul [/INST] You'd be safe with sizing a resistor that is capable of 100% of your motor load, then deciding on a duty cycle. 25% would be a very safe duty cycle; you might be able to go down to 10%. Post Glover is a Rockwell encompass partner; you might give them a call to see if they have some suggestions. \\n </s>