Electric motor startup torque feedback for safety 1

[cityjack]

Good morning all,

We have a large metal box/hopper, 20' X 20' X 10' deep that we accumulate a mixture of natural blocks of masticated rubber, resin, calcium carbonate and a few different powdery fillers. This is a feed to more aggressive screws below before we extrude out adhesive.

The bottom screws are powered by a large hp motor, coupled to a very large sprocket and chain drive through a even large phenolic coupling. The 20 x 20 x 10 hopper that holds or feeds the rubber mixture into the bottom screws, it is temperature controlled to keep the adhesive from setting up overnight when the machine isn't running. That way when they starup in the morning, the screws are not trying to grind up huge chunks of setup adhesive. Sort of like trying to re-masticate cinder blocks if you will. They do not have a cover on the hopper so the heat does sort of escape.

The coupling is designed to break if the motor sees too much torque. Sort of like the washing machine motor to transmission coupling we all have at home. Heavy load protection. We are going through 4 couplings every 6 months. It looks like the startup speed of the screws/motor is manually controlled by an operator. If they do not know the state of whats in the hopper or if anything is even in the hopper, they just fire it up at whatever speed they wish.

We can train all day, increase the heat to prevent setup, put a cover on the box to retain the heat, or other things. But I'd like to somehow take the torque of what the motor or coupling is seeing at startup and feed that back to the PLC to prevent the motor from starting. Thereby saving the coupling and 2 hours of downtime with 3 maintenance guys being tied up. It doesn't address the adhesive from being setup or the heat issue that should help, but it does save my coupling, labor, and downtime.

Is there a torque feedback transducer or something I can mine from the motor to tell the PLC not to allow the operator to start up?

Any help would be greatly appreciated.

Thank you and have a nice day.

sid

 

[itsmoked]

Odd. So they pour the juice to the motor and the coupling breaks. Laboriously replace the coupling and.......do what, that apparently starts the system up successfully?

Seems to me that someone is doing something different after coupling replacement to prevent instant re-failure.

Whatever they're doing 'bottle it' and have the operator NOT start the machine but instead request the PLC to start the machine (with a button) and have to PLC do the 'bottled' start.

One caveat: Is the system starting successfully after a coupling replacement because the new coupling is room temperature and the failed one was caught up in the heating and was soft and failure prone?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BrianPetersen]

If the motor is being driven through a VFD, and if the VFD is a decent one (see another recent thread ...), it can be started gently, you can get feedback of motor current (which is a reasonable proxy for its torque), you can limit said motor current, etc.

 

[cityjack]

Good question and now you have me thinking even more about alignment. I was a bystander watching during the whole tear down and coupling replacement.

Once the destroyed one was removed they started to bolt in place the two-piece clam shell coupling. It was then that they noticed the sprocket/chain drive side of the coupling, its big a** pillowblock moved. They loosened the pillowblock, wrapped a few straps around the base of the block and proceeded to tug with a tow motor it into alignment with their eye. Bolted it back together and torqued it with an air impact wrench. I now think, how the heck did it get OUT of alignment if the pillow block was tight initially, or it was tightened out of alignment the last time they replaced the coupling.

I've read that the largest contributor to premature coupling failure is poor alignment and also vibration. To answer your question, I think the operators are given free reign over the HMI and started the motor on full speed. After replacing the coupling, the mechanics started it at 5% speed. We are working on setting the HMI up so that this can no longer be done and there will be a separate HMI startup butto, this will provide a 2 min startup at 5% speed then after timing out, the program will revert back to where the operators can manipulate the speed to where they see fit.

But I still wonder about the alignment. No, the system had sat overnight so it was as cold as the one we pulled off the shelf to replace with.

Thank you so much for the reply and your help.

Sid

 

[cityjack]

Brian, this is a large DC motor. Can DC motors have variable frequency drives? Just by saying frequency makes me think AC and no to my question.

Thank you sir

 

[itsmoked]

Large DC motors can certainly be speed controlled and frequently are. Not cheap but certainly doable. And, correct, not with a VFD. Sounds like you already have speed control (5%) and operator malfeasance.

I'd make the unit always soft-start regardless of what the operator requests. If they request 100% it takes, say, 10 seconds to reach that speed, if they request 50% it takes 5 seconds, etc.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[cityjack]

We are on it just like you say. Thanks a ton sir.

 

[jraef]

Most MODERN digital DC drives will have programmable torque limits in them, but older analog DC drives may not have. DC drives will allow the motor to produce full torque at any speed, so even at 5% speed it will have the same twisting force as at 100% speed. That's what is breaking your coupling and moving your pillow block.

This might make a case for upgrading your DC drive to something that will help prevent this from happening. Yes, I agree that your operators are likely contributing to the next failure in the sloppy way they are replacing the coupling, but even if you fix that issue, it's still an excess torque problem. With the DC drive that has torque limiting capability, you could "dial it in" to the maximum amount of torque the coupling can handle and never let it exceed that, regardless of what the operators try to tell it to do.

Bottom line, your mechanical design leaves a lot to be desired here if you are breaking couplings every 6 weeks or so. If you CANNOT convince the owners to upgrade the DC drive, you should at the very least install a "shear pin" coupling, which is a purposely weak component in the drive train that is easy to replace quickly without needing to realign things. An upgrade to that would be a mechanical device inserted in the drive train such as a "torque tamer", a magnetic particle clutch or friction clutch, that allows the motor to "slip" under excessive torque conditions without breaking anything.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[cityjack]

Jraef, thank you so much. A lot of information you gave me. I had no idea I could get teh max torque at 5% speed. I need to speak with our contreols guy about his current drive setup.
After speaking to our 30 year man here about alignment, he said the coupling is AFTER the gear reduction so the coupling turns really slow. What really slow is, I do not know as I new to this plant. He told me because of the speed at which it turns, there is no way alignment can be a cause of breaking the coupling. Hes been here a lot longer than I and has been around it at least that long. But I will continue to observe after we change the ramp up and torque limiting of the motor profile. We do have to at least do that.

I love your quote about why we are on earth. So true sir, so true.

Have a good weekend.

Sid