As web handlers, we would like to establish tension at the winder or unwind, forget it and get down to the business of producing web. Sometimes this takes more than 60 seconds. It is very frustrating to have the drive shut down the line because stall tension was held too long (50 or 60 seconds).
The problem is that this single fault is triggered in several ways and only one of these conditions bothers us. This fault results if the drive is running and the tachometer signal fails (the tach itself is not really lost). This is a true tach loss fault and we want to stop or switch operating modes to prevent a motor overspeed.
The second means is checking for pulses from the tachometer. If the delay between tach pulses is too long, we have a problem and the drive should respond.
A third means may compare the tachometer signal with a model of motor speed.
The fourth means bothers us a lot. The winder is told to pull forward against a stopped web. If the speed setpoint is not zero and torque is detected for a time delay, a drive trip results. The drive assumes if it is told to pull forward, it jolly well better be rotating forward. All drives should have a means of ignoring or masking this condition. The drive is stopped and tach loss has no effect.
Note: Since this blog was written, the drive vendor has corrected this fault in newer model drives.
Friday, February 25, 2011 4:46 AM
Sometimes you hear recommendations for stall tension to be 50% of running setpoint. Where did this come from? I can't think of a web handling reason for this. - DR Roisum
Friday, February 25, 2011 6:15 AM
One historical reason for stall tension comes from the paper industry. The winder/slitter crew's first priority was to assist in threading the paper machine whenever the web broke. That meant leaving the winder threaded and stopped under tension for an unknown time.
The dc motors at the time had a problem with local heating on the commutator if tension was applied with no rotation. The solution for saving the motor was stall tension - it reduced commutator heating.
This does not apply to ac motors.
Other web related reasons for stall tension are to allow tension to be applied early during threading. A low (stall) tension setting may work well when only one third of the web width is in the winder and nips are not in place.
Stall tension is also very useful in an unwind when threading a very long machine such as off-machine coater.