A Little OT, perhaps

PLC Pie Guy

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I'm sure some of you have seen the video of the ski lift that malfunctioned in Georgia. Its on the CNN site.
Its likely that the investigation will be tight with details but I'm curious if anybody knows what happened there??? Improper wiring or a programming mistake. Why did it go in reverse and at full speed. I would think the drive to have a maximum frequency parameter set and the reverse to be disabled as well.
Surely there was a lack of emergency stopping mechanisms. I don't ski but I can imagine that there are a lot of people examining their ski lifts now for potential bugs like this one.
I hope all were ok, it looked pretty bad in the video!
 
PLC Pie Guy said:
Surely there was a lack of emergency stopping mechanisms.
Click to expand...

I could not believe that they dont have them everywhere... at every gas station we have them but not there? I think the drive had to fault out because two things happened, it ran in reverse and full speed reverse

It was horrific to watch, I could not emagine being there and seeing it happen
 
Do those have position controllers?
I don't ski, so have no clue how those things work, but a position controller with no sanity check and no safeguards in place could potentially do something like that.
 
It looks to me like the braking gave out and it was simply the mass of the bodies forcing the lift to run in reverse at such a high speed. Much like a crane would do if the braking failed with a load in the air.
 
Lynx777 said:
It looks to me like the braking gave out and it was simply the mass of the bodies forcing the lift to run in reverse at such a high speed. Much like a crane would do if the braking failed with a load in the air.
Click to expand...

That seems like a more logical explanation than "it ran in reverse at full speed" So perhaps it was a braking system that failed. We will likely never know.
 
Do those have position controllers?
Click to expand...

I think they are just big inverters.... and they just run at a slow speed in forward direction, I would think they would have reverse wired in so they could if they had to

I worked for one place where it was common practice to use a screwdriver in a contact and run a motor manually, I hope it was nothing like that here

braking gave out
Click to expand...

I never saw the beginning... and that would be possible but it should start out going backward very slow then speeding up, that would truly suck because than you have no chance of stopping it
 
Lynx777 said:
It looks to me like the braking gave out and it was simply the mass of the bodies forcing the lift to run in reverse at such a high speed. Much like a crane would do if the braking failed with a load in the air.
Click to expand...
I agree. The initial reports I saw said the lift stalled, then the repairman "hooked up a transformer backward". I find that implausible. Given the fact that Georgia has been economically depressed for decades under Soviet rule, then left on their own since independence but constantly fighting to stay that way, it's likely that this is a very old system, no VFDs or modern safety systems. In the old days, ski lifts were run by Wound Rotor Induction Motors (Slip Ring induction motors) with dynamic braking on the rotors and a mechanical brake that was mainly for holding, but sized for emergency stop. If the rotor braking failed, i.e. anything in the rotor circuit failed, the mechanical brake should be able to stop it. But I suspect lack of maintenance, oversight, periodic testing etc. lead to that brake failing and then there would be no way to stop what happened there.
 
Can a ski lift be considered to be an elevator ?
A regular elevator has an inherently safe design. If the lift should somehow start to fall, a self-tightening clamp will engage and stop the lift. (see: https://en.wikipedia.org/wiki/Elisha_Otis)
We use the same principle for bucket elevators. If the elevator is filled with material when power is cut, without a backstop the mass in the buckets would make it run reverse at a catastrophic speed. Because of this we fit a selftightening backstop on the drive shaft that makes it impossible for the elevator to run reverse.

edit: Such a mechanical backstop is a bit more safe than an electromechanical brake. The backstop will work always, irrespective of an eventual VFDs programming or failing. An electromechanical brake would possibly not have engaged if an eventual VFD was running in reverse.
 
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You are all thinking controls. I would venture that a chain broke or a coupling separated. Comments above about an impoverished nation are true and probably many of those parts should have been replaced or just lubed.
 
I also vote for a mechanical failure, maybe a broke or decoupled transmission.
Weighing more the ascending part that the descending the sense reversed without control.
it would lack a parachute system as the elevators have.
 
gas & Ife are spot on. When I used the term "brake" failure in my previous post I was thinking along the same lines. Something failed in such a way that the braking system could not do it's job.

I would have thought that there should be a speed brake or emergency brake on the idler end of the system as a backup for this kind of failure. But, of course, once poor economics and lack of maintenance get involved, all bets are off.
 
I had read this yesterday which adds a little more insight into what apparently is being said happened from one eye witness...

The Guardian said:
Ryan Wilkinson, 24, from Kent, was queuing at the bottom of the lift with a group of friends when he saw the incident happen. He said the ski lift had been closed for repairs all week, but reopened on Friday morning.

“The lift stopped for a minute. There is a language barrier with the Georgians, so we didn’t know what was happening. Then it started reversing backwards slowly, then it got quicker. There was a guy in the office smashing on a machine and shouting who seemed powerless to stop it,” he said.

“All the chairs were full of people. Everyone was shouting in Georgian and English for people to jump off. The only thing that stopped it getting worse was the friction caused by the pile-up of chairs at the bottom. People would have kept flying round otherwise.

“One woman in red was too scared to jump. It looked like she’d been spat out of a washing machine when she hit the bottom. It was like a scene from a Final Destination film.”

An emergency stop led to the chairs sliding back at high speed, the economy minister, Dimitry Kumsishvili, told journalists, adding that the incident was “allegedly caused by an electricity outage”.

“The interior ministry has launched a criminal probe into alleged violation of safety norms,” he said.

The health minister, Davit Sergeenko, said the affected tourists – who were Georgiain, Russian, Swedish and Ukrainian – did not suffer serious injuries.

“Two of them including a pregnant woman from Sweden were airlifted to a hospital in Tbilisi,” he said.

A spokesperson for Doppelmayr, the Austrian manufacturer of the chairlift, said two technicians were in transit to Georgia to respond to the incident. No initial cause was given.

The fixed-grip four-seat chairlift was installed at the Georgian ski resort in 2007. The design includes an emergency brake and backup power generator, the company said.
Click to expand...

It would appear, at the outset, that after a power outage the backup power generator failed to kick in and then the emergency braking system also failed to engage or be signaled to engage. Perhaps it should be as Jesper suggests and be double redundant where if an electromechanical emergency braking system fails then a mechanical backstop should be available. I have plenty of experience with bucket elevators carrying heavy grain loads and the absolute necessity for a mechanical backstop. Even if it has to be manually applied in the event of catastrophic systems failure, it is imperative that one is available. And this is just where dealing with product only. Once people and the general public are involved, lift Safety gets, or should get, very serious indeed.

It will have to be investigated and established if any safety regulations were in breach here, such as circumventing manufacturer designed Safety systems or failure to correctly maintain such systems. You can be sure that the lift manufacturer will be quick to assess and spot any shortcomings or misuse of their equipment, which I'm sure was supplied with fully operational Safety measures.

Wishing all affected a speedy recovery.

Regards,
George.
 
I work at a ski area on weekends. I'm pretty sure our lifts have anti-rollback mechanisms on every tower to prevent a loss of the main brake from causing a situation like this.
 
T Gibbs said:
I work at a ski area on weekends. I'm pretty sure our lifts have anti-rollback mechanisms on every tower to prevent a loss of the main brake from causing a situation like this.
Click to expand...
I doubt there is a single ski lift in the US where this could happen, our safety requirements and inspections are (or at least were) second to none. People whine and complain about Governmental regulations, but here is an example of what can happen with a lack of oversight. Undoubtedly the lift mfr, having built it in 2007, had all the necessary safety systems in place. The issue will undoubtedly come down to someone having tampered with them, bypassed them or simply ignored them for 11 years.

Given that the lift was down for a week prior, I'm going to bet on it being a rush to re-open it because of pressure from the management, leading to safety systems and/or tests being bypassed and there being nobody watching over them to ensure public safety.
 

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