Lightning - Motor reversal?? 2

[raisinbran]

I already have an opinion on this issue, but there are a lot of smart people on this forum with a lot more experience than I will ever have, so I thought that I would ask.

We have an inclined conveyor that failed to hold under load. The conveyor was stopped due to a normal sequence stop. The conveyor ranaway in the reverse direction and did some damage, albeit small in comparison to what could have happened. Fortunately, no one was injured. Yes, as usual, there was a lightning storm in the area.

The conveyor is driven by two 2500 HP, 4160 volt motors and uses across the line, non-reversing starters.

The designer of the conveyor system is claiming that the lightning caused the conveyor motors to reverse and defeat the mechanical rollback devices, allowing the conveyor to accelerate in the reverse direction.

No other motors in the plant (literally hundreds) were affected, and no electrical components of the conveyor system (transformers, cables, surge arrestors, PLC, etc.) were changed after this incident, but he stands by his claim.

My position is that this is impossible, and that there is a deeper mechanical/systems issue to blame for the incident. Bottom line, we do not want a re-occurence, and we must push to identify the actual cause of the problem, regardless who pays. Has anyone ever heard of anything like this happening before?

Thanks,
Raisinbran

 

[jraef]

Hmmm...

Machmech, what was it that I typed that made you engender me as a Mrs.? Let me know so I can avoid that in the future!

 

[rmw]

I think he/she was commenting on your missus's potential reaction to his/her initial post regarding the worm size comment, not what you posted.

rmw

 

[itsmoked]

hehehehehe

 

[Skogsgurra]

hahaha good luck to all involved

Gunnar Englund

http://www.gke.org

 

[jraef]

Oops, [redface]
Totally misinterpreted that...

 

[raisinbran]

Machmech,
We (I) also have concerns about the design and what set of circumstances occurred to allow this to happen. Short term - we have installed a disc brake on the high speed shafting that engaged at zero speed and at any indication of reverse rotation. This is in addition to the high speed roll backs - that were dismantled and examined by the OEM. Long term, we are planning to install low speed roll backs on the drive drum and an additional high speed disc brake on the the other gear case. At that time the existing high speed roll backs will be removed.

Once again, thanks to all for your insight and responses.
Raisinbran

 

[itsmoked]

Yeah but how do ANY brakes of any kind help when the output shaft (low speed) snaps like a stale pretzel???!?!?!? I have seen five inch output shafts snap off on concrete drum drives and 8inch shafts shear off leather processing drums..



Side note: When the concrete drum drives snapped, I had to climb into the drums with a jackhammer to clean out the drum. I clearly remember looking at,(and cursing) the sheared off axle with the chain gear mounted on it laying on the ground still engauged in it's worthless chain.

 

[mc5w]

I believe that the antirollback devices are similar to those in hoists that prevent the load from dropping if the brake should fail. These work on a similar principle to a seatbelt clutch and require that the motor be turning in the down direction for the hoist to go down. In a hand cranked hoist there is a dynamic braking version of this type of clutch otherwise you could never lower the load in a controlled manner.

These devices are not foolproof which is why you normally use a real brake to keep the load from dropping. However, if lets say that the operating brake is 99.9% reliable and the antirollback clutch is 99.9% reliable then you have more like a 1 in a million chance of failure rather than a 1 in 1 thousand chance of failure. The idea is that redundancy prevents problems particularly if 2 different technologies are extremely unlikely to fail simultaneously.

I still think that this alledged expert is trying to pin the blame on God instead of admitting that he has no idea of what went srong.

In fact, you may never determine what went wrong but you could develop some theories and then investigate as much as you can.

 

[machmech]

Hello Raisinbran

These sound like some very good improvements " Necessity is the Mother of invention " I don't know if this would apply in your situation you may want to consider the possibility of Legal responsibility if the system should fail again.

At our company we developed a way to place a hoist on to a machine for lifting rolls, the hoist worked very well and most likely prevented many sore backs how ever a visit from O.S.H.A and we were advised to have each one of our hoist tested by a person certified to do this, then they had to have weight restrictions labels and of course documentation.

I do believe the original manufacturer needs to be taken to the wood shed!

 

[jraef]

raisinbran,
Mike Cole's analogy of a seat belt catch raises an interesting point, Is it possible that your high speed antirollback device failed because the belt reversed too slowly?

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[mc5w]

Different versions of seatbelt clutches behave a little differently. When a General Motors seatbelt clutch locks up the only way to unlock it is to open the door. Maybe this is safer but it is annoying.

 

[jraef]

mc5w,
With all due respect, let's not make this into a discussion about seat belts.

 

[DanDel]

I have seen motors reverse direction when one phase is lost due to damaged motor starting contacts, but this was a LV two-speed two-winding motor. In this case, the motor started up in the wrong direction about 50% of the time (with a load).