Many of our drives run in torque control. Torque control provides very good control of tension. Torque control becomes a safety concern if the web breaks and torque control is permitted to continue. Then the roll or roller will accelerate, perhaps leading to an unsafe speed or vibration. This safety concern is greater with dc motors as ac drives provide one extra layer of additional protection by limiting the frequency.
Normally a web break detector based on photocells or loss of tension once load cells would take the drive out of torque mode. If the break detector fails or is disabled, the drive should be prevented from overspeeding. This is normally performed in the drive by using the minimum of torque reference or speed regulator output.
This function has different names for different drive vendors. It may be called:
- Speed Limited Torque Control
- Torque with a Limiting Controller
- Torque with Speed Window
- Torque with Speed Error Threshold
- Torque Control with Speed Override
Some drives, particularly Servo drives may not offer a torque mode with speed override. With these drives it is possible to use the speed regulator and apply the torque reference to the torque high limit.
The goal is to prevent an overspeed condition while in torque mode should an undetected web break occur. Overspeeding an electrical motor is a serious safety concern. I know people who have survived electric motors turning into shrapnel and flying everywhere in the building. Initiating a motor overspeed turns drive techs into instant legends. Many of these legends are escorted off-site, never permitted to return.
Other events that may lead to overspeed are:
- Web slipping on a core or roller.
- A broken drive train element such as belt, gear, or drive coupling.