kolyur
Lifetime Supporting Member + Moderator
We are building a new piece of equipment and a significant safety issue has come to light. I'm hoping I can get some suggestions here.
We are using a number of rotary trimmers and wire brushes which are individually belt-driven by AC motors (VFD-controlled). All motors are fractional HP. Drives are Delta M-series. There are a lot of exposed belts in the machine, which was not seen as an issue initially since the entire system is guarded.
The problem is that some of the brushes are fairly heavy and take awhile to stop (up to 10 seconds). It became apparent that if an operator was in a hurry and yanked open a guard door and jumped inside, he/she could easily be injured by the equipment even though all power was off at that point.
Part of the issue is the way I have the VFDs wired. The 3-phase lines going from the VFD to the motor pass through a contactor, which is controlled by the safety circuit. So, if a guard is opened or an e-stop activated, this provides a mechanical interlock. For safety's sake, I really wasn't comfortable with just dropping the enable line on the VFD. The problem with this is that when the contactor drops out, any kind of slow-down ramp in the drive won't work; the motor just coasts to a stop.
Does anyone have any suggestions? I am concerned more about the exposed belts than anything else. Unfortunately it would be difficult to guard them individually at this point. I considered adding a braking resistor to the VFD but that would have no effect because of the contactor. I also thought of using brake motors but we don't have room for the additional length. I would like to keep the contactors between the VFDs and motors for safety, but I'm open to other wiring ideas if they still provide a safe system.
Thanks for any help!
-John
We are using a number of rotary trimmers and wire brushes which are individually belt-driven by AC motors (VFD-controlled). All motors are fractional HP. Drives are Delta M-series. There are a lot of exposed belts in the machine, which was not seen as an issue initially since the entire system is guarded.
The problem is that some of the brushes are fairly heavy and take awhile to stop (up to 10 seconds). It became apparent that if an operator was in a hurry and yanked open a guard door and jumped inside, he/she could easily be injured by the equipment even though all power was off at that point.
Part of the issue is the way I have the VFDs wired. The 3-phase lines going from the VFD to the motor pass through a contactor, which is controlled by the safety circuit. So, if a guard is opened or an e-stop activated, this provides a mechanical interlock. For safety's sake, I really wasn't comfortable with just dropping the enable line on the VFD. The problem with this is that when the contactor drops out, any kind of slow-down ramp in the drive won't work; the motor just coasts to a stop.
Does anyone have any suggestions? I am concerned more about the exposed belts than anything else. Unfortunately it would be difficult to guard them individually at this point. I considered adding a braking resistor to the VFD but that would have no effect because of the contactor. I also thought of using brake motors but we don't have room for the additional length. I would like to keep the contactors between the VFDs and motors for safety, but I'm open to other wiring ideas if they still provide a safe system.
Thanks for any help!
-John
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