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

 

[PWR]

From the information you have supplied, the mechanical rollback prevention devices failed for some reason. Since squirrel cage induction motors have no way to reverse except by reversing the phase rotation, it would seem that the designer has decided to place the blame on something he clearly does not understand. Your analysis of "this is impossible" sounds correct to me. Unfortunately, I can offer no experience on the mechanical side of things. I don't know if there is a forum that might address those issues better than this one can.

 

[rbulsara]

Tell the mfr. : "Good Try." Now find the real reason.

 

[Skogsgurra]

No, never heard of anything like that. But heard and read lots of other "interesting" theories with regard to lightning. Ask the manufacturer for an in-depth explanation. Has he any references to similar occurrencies?

It is possible that the manufacturer confuses voltage loss and phase reversal (unlikely, but possible). If that is so, an explanation to what MIGHT have happen is given below:

If there are brakes that are activated when the motor voltage is cut off and if these brakes are connected directly to the motor terminals AND if there are PFC capacitors also connected directly to same terminals; then an abnormal power cut (not via control system) MIGHT lead to self-excitation of the motor so that motor voltage keeps brakes open and allows belt to accelerate in wrong direction.

Only if all these rather improbable conditions co-existed AND your mechanical safety device failed, would your belt run backwards. But not in any other case.



Gunnar Englund

http://www.gke.org

 

[jraef]

Wow Gunnar, and I thought I was the king of improbable possibilities!

Seriously though, skogsgurra has made a valid point about the remote possibility of the lightning causing some other sort of failure in the system and backup systems (if any), but there is no way on this planet that lightning can make an AC motor run in reverse.

First off, lightning is DC not AC, so even if the lightning directly hit the motor leads (and miraculously somehow did not vaporize them), the worst it could do is lock up the motor, similar to applying a DC injection brake. People see a lot of strange things result from lightning strikes, but in all cases they follow the laws of physics. We think of them as strange only because they are outside the realm of our experience. Changing lightning's nature from DC to 3 phase AC power is as close to impossible as you can find in science!

I know that it is technically debatable that lightning is DC, but it is definitely not 3 phase AC. Even if you stretch the definition of AC to include a variable pulse in one direction (see link below), it still could NEVER be 3 phase because you would need to have 6 precisely times pulses in opposite directions with each pair being 120 deg's apart, and do it precisely that way repeatedly for long enough to create a field in the motor that would cause it to reverse. And all this without damaging the motor or power supply components!

Second, if a nearby lightning hit were to cause some sort of control system malfunction, you would STILL need to have a reversing motor starter for this to cause the motor to change direction. As PWR said, the only thing that can make an AC motor revers direction is to change the phase rotation.

3rd, if it did something at the power generation end of your power supply to cause the phase rotation to reverse, ALL of your AC motors would have reversed.

That guy should go into law or politics. Obfuscation and denial are typical tricks of those professions.



Lightning AC or DC debate link (for the termially curious like me):

http://www.physicsforums.com/archive/t-70874_Lightning_AC_or_DC?.html

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

 

[Skogsgurra]

Thanks jraef, I probably have studied too much Rube Goldberg machinery...

Gunnar Englund

http://www.gke.org

 

[machmech]

Hello gentlemen I have a great deal of respect for your expert advise it is some of the best available. I do not have electrical expertise that you folks have but have been a mechanic for many years. My thoughts are excluding any electrical surge or powers of the almighty. If the weight of the loaded conveyor would cause a reversal then even the most basic ratchet style anti reverse devise should have prevented this problem!

 

[jraef]

Hello machmech,
Yes, you are right, although with the size of conveyor raisinbran is refering to (5000HP), a ratchet would be a noisy device! There are however supposed to be back-spin preventors of some sort, and I think raisinbran was trying to prove the point that whomever supplied this system was deficient in that aspect. The system supplier on the other hand has tried to foist this "devine intervention" excuse instead.

Raisinbran, you sure have some interesting large conveyor applications thread237-124926. What are you mining?

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

 

[raisinbran]

Jraef and others,
I work for a coal mining company, and the sizes of conveyors that I have written about are "typical" for the industry.

Skogsgurra,
There are no electrical brakes on this installation, but most conveyors that I have seen do not rely on electrical brakes.
The only rollback protection provided are mechanical rollbacks (sometimes referred to as backstops or roll back preventers). They are on the high speed side of the gear reducers, which is not typical. Most installations have these protective devices on the low speed side - just incase of a gearing failure.

I thank all of you for your responses.

Regards,
Raisinbran.

 

[Skogsgurra]

Well then, there you are: Mechanical device to prevent roll back. Device failed. What is the problem? As I see it, there was no electricity involved in the failure at all.

Gunnar Englund

http://www.gke.org

 

[raisinbran]

Skogsgurra,
I wish that it were that simple. When the rollbacks were disassembled, they were in almost brand new condition - no heat, no broken parts, etc. They worked when tested, but they rely on the speed going from + 100 rpm to - 100 rpm (on the high speed side)in a time frame that allows them to react. If you can somehow (and I do not know how) get the system to change direction fast enough, centrifugal force takes over, and you have defeated the devices. Yes, there were two, and we fooled both of them somehow. If we ever get to the bottom of this, I will let you know what we find.

Raisinbran

 

[buzzp]

Something would have broke if the motor was expected to provide momentum immediately back to the system, in the opposite direction, in a very short amount of time. Furthermore, the motor would have been providing reverse power to the rest of the system, without blowing any fuses, breakers, or tripping any protective relays (if there are any). If it was providing a appreciable amount of power, to accelerate faster than the gravity pulling on the load, something would have tripped or something. We certainly know the phase reversal did not magically happen in a matter of seconds.

This question is giving me deja vu. I believe this was posted sometime in the past or maybe it is a similar problem?

 

[OperaHouse]

A motor under a high load condition would reverse very quickly if you lost a phase. Once the motor started spinning in the reverse direction it would single phase and quickly go up to speed. Since it is going in the reverse direction, it would draw less cirrent and not trip any current device. This is a real problem with screw compressors since they screw out through the case. They require a phase loss relay that is very fast, not the couple seconds that many are. This could explain what happened if the power lost a phase temporarily.

 

[jraef]

OperaHouse,
Interesting info, thanks. I'll have to remember that when dealing with phase loss protection of screw compressors, but I doubt it is applicable to this specific problem. I can see how a loss of 1 phase would result in a severe loss of torque and allow the motor to be reversed by the load if it was hevy enough. However it cannot reverse any faster than the load can physically reverse it, and in this case he had a mechanical device designed specifically to prevent that. The claim is that somehow the motor reversed FASTER than the load could have done it, thereby defeating the mecahnical device.

I still think the mechanical guy is fishing with the wrong bait.

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

 

[buzzp]

It is a bunch of crap. Not going to happen.

 

[Barry1961]

If this is the same conveyor as in the previous thread the motor would not have to quickly reverse to quickly reverse the rollbacks. The fluid coupling between the motor and gearbox would not be able to transmit much torque at low speed.

I would think it would be possible for the gearbox to reverse while the motor was still turning forward at a slow speed. It should still take a jam or overload to make the conveyor spring back quick enough to fool the rollbacks.

I don't know how fast you can reverse a 2500 HP motor but I would think you could reverse the gearbox quicker with the fluid coupling "disconnecting" the motor at low RPM.

Barry1961

 

[buzzp]

I thought I stated the obvious - what about the rest of the equipment in the plant if the phases were actually reversed? How can a motor connected to the line get its phases reversed and not the rest of the plant? If the motor was moving backwards, the motor could be generating voltages in the reverse phase with the line - imagine that! I can't. Bad things would happen.

 

[machmech]

Hello Jraef

I said even the simplest of ratcheting device should have prevented this problem. Meaning correctly designed and mechanically sound this or these devises whatever they are should not have failed if reversal is evident then the issue is safety and we could have dead employees.

Since machinery powered with electricity can fail for many reasons blown fuse, power surge, there are too many possibilities to list, There for the safety must be designed into the mechanics of moving energy.

My point is clear the design of the conveyor mechanically has much to be desired the engineer behind this design must be incompetent or this would not have been an issue.

You sir may be using the right bait, but your worm is too small!

Chuck

 

[jraef]

LOL!

Small worms must be my problem with fishing too!

 

[machmech]

Hello Jraef

Yup! I was hoping to get a star from Mrs. Jraef...., All
kidding aside I am looking foward to what Rasinbran shakes
out of this and will watch for his post.

Chuck