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July 17, 2006
Torque Control is the Best Tension Control?
A few months back, I made post asking 'Is motor torque control EVER the right?' This was a post based on my experience with several converting controls engineers.
Pete Werner, formerly of Rockwell Automation, posted his comments on this and made a strong case of torque-based motor control should be the first choice. This is quite a contrast from what I've seen over the last 20 years.
Since then, Pete has continued to educate me on this topic. Pete made an interesting presentation at the AWEB conference last month. He makes some good points about how speed regulation and dancer rollers the popular choice, but these are devices invented to solve problem many years ago.
Why does speed-based tension control continue to dominate converting applications?
Probably several reasons. First, controls engineers have learn this and know it works. Speed is easy to understand.
Why use speed reducers (gearboxes)?
Supplier like to boast speed accuracy. Controls engineers like using speed reducers to reduce motor size and increase efficiency. But you shouldn't be thinking about energy or motor costs when your goal is good tension control. Yes, eliminating the speed reduce increase motor size and cost (both torque related), but you save some of that back by not buy a gearbox. The most important motive should be good (or great) tension control.
I always had it drilled into me that you want a motor to run using its full operating speed range. If the motor has 0.01 percent speed accuracy, this is 0.2 rpm variation over 2000 rpms. If you run the motor at 100 rpm with no speed reducer, you're asking for 0.2 percent speed variations. Why would you do that?
This is the wrong question. Motor are not inherently speed devices. They are torque generators. Torque output divided by the mechanical leverage of radius creates tension. Why care about speed accuracy at all?
These are great questions.
I like Pete's question where he says 'You don't usually consider putting a speed reducer on a braked or clutched application, why do it on a motor?'
Today's drives call for new answers. I get the feeling Pete would like to eliminate 90 percent of the speed reducer (gearboxes) and dancer roller in the converting industry.
Let me know your thoughts or experiences regarding speed or torque motor control and when dancer rollers can be eliminated.
tjw
Posted by Tim Walker at July 17, 2006 02:06 PM
