Winder Tension

[rswolff 9]

ok.....hopefully there are a couple of EE or Physics Gurus out there. Here's the question. I have a 20hp motor connected to a spindle. Gearin is 1:1. Motor is being driven by a Closed Loop Flux Vector drive so I can (or should) get full motor torque at zero rpm. A 20hp motor generates roughly 70 ft-lbs of torque. I have 24" of material on the spindle. I connected a spring scale to the material and measured the torque available and I did have roughly 70 lbs. (drive was set to rotate motor at 5rpm. Current limit was gradually increased to 100%) This appears correct, unless I'm thinking about this incorrectly. If my material width on the spindle is 60", then presumably I could have a little over 1.0 pli (lb per linear inch) of tension. And thats what I seem to actually have. The machine builder is saying I should have more with the 20hp system. Additionally, given the motor rating of 70 ft-lbs, I expect I'd have 480 lbs of torque at 3" (basic physics). But that would also mean I'd have 40 lbs available at 36". Again, this seems to be very low from what the builder is saying he's had with other systems in the past. What am I doing wrong (or am I simply mad as a hatter?) Are my assumptions correct or am I missing something? any insight greatly, greatly appreciated. Edited 24 Mar 2011 by rswolff

[Steve Bailey 118]

What is the nameplate speed of your motor? That would be the case with a 6 pole (1200 RPM) motor (87.5 Foot pounds) or an 8 pole (900 RPM) motor (116.7 Foot pounds).

[rswolff 9]

nameplate speed is 1800 rpm. The specification on the motor (its a 20hp Marathon Black Max) is 60 ft-lbs. I'm actually getting slightly more at the spindle as the actual gearing is a bit more than 1:1. Builder is saying same motors, same gearing, same web width, on other machines but believes they have possibly 2-4 times as much tension (?). Standard calculation for most of the industry is based on calculating maximum work being done by motor (basically speed x tension x width x diameter) and converting the work to hp. In this case the calculation indicates the motor is undersized, however the builder says that in past they've simply lowered the linespeed (not change the gearing) and the tension will increase. So they should be able to run at lets say 300fpm even at a 24" diameter and get significantly more tension. My calculations indicate for a 60" wide web at 24" diameters they can't get more then 1 pli regardless of the linespeed. I'm just trying to determine if i was missing something obvious or just possibly insane.

[paulengr 44]

If you are below base (nameplate) speed on the motor, then you are in the constant torque (HP) region. Lowering the speed simply decreases speed. It does not increase torque. As I was more or less hinting at, you can certainly use a drive to make a motor exceed nameplate torque. There is SOME (not a lot) room to push it a little. But you're not going to get 2-4 times as much without either a different motor or a gearbox. As has already been recommended, increasing the number of poles allows you to keep a fixed horsepower but decrease speed. And since torque is proportional to the inverse of speed, it increases torque, if you are changing motors. With the same motor, torque is constant (decreasing speed does not affect torque output for a given motor in the constant torque region).

[rswolff 9]

thanks....you're all just reinforcing what I've been saying to my customer. In actuality it appears that the calculation the builder is using is correct. The calculation however is based on the motor at full speed (full hp). So what they did was gear the machine to a high speed, but use a much lower fpm in the calculation. So they need 50hp but thought they could use a 20hp motor if they simply lowered the running speed of the machine. It would work if the machine was actually geared to the lower speed. I told them it was beyond the laws of physics as I knew them, but they guaranteed me it worked in past. This forum was only one of the places I touched base with, but it appears the laws of the universe still hold sway....or at least they seem too. I had already recommended that they change the gearing. They are looking into it but still were skeptical as to why it was necessary. Many thanks to everyone!!!!