Contactor failure and how to predict it 5

[panter]

Hello everyone.
I'm involved in an crane incident investigation and contactor failure as a possible cause.
Previous practice for the right time to replace it is based mostly on a visual inspection of electrician and terms when the contactor had the expected number of operations .
we are thinking about termovision, measurement of the voltage drop at contacts and other possibilities to prevent total failure of the contact , contactor and consequences.
What are your experiences and recommendations .
Thanks in advance

 

[BrianPetersen]

Failed in what way?

Failed to energise (motor wouldn't start when commanded to start), or failed to de-energise (motor wouldn't stop when the run command was removed) which is also known as sticking or welded contacts? Or something else?

Usually something that doesn't stop when it should, is a much bigger problem than something that doesn't work at all. Welded/sticking contacts can be addressed through redundancy and automatic monitoring (e.g. by safety relays).

 

[waross]

The overwhelming number of failure of motor contactors are failure to close.
Welding happens but is often associated with fault currents and so is independent of contactor age or wear.
Monitoring the condition of the contacts may be quite expensive and may not be that reliable.
Consider that if a monitoring scheme is implemented regular inspections may be dropped.
In addition to time based inspections I would add a counter to monitor the number of operations of the crane.
The operation monitor would alarm in cases when there were an unusually high number of operations before a scheduled inspection.
Also you may consider reviewing your criteria for replacing contacts at regular inspections.
At the last inspection there was probably some indication that the failed contacts were near the end of their live.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[panter]

Brian , Waross ,
thanks for yours inputs.
Yes , contactor faild to deenergize and In the investigation of the incident, they claimed that the problem is remanent magnetism .
The contactor was replaced three weeks earlier and the manufacturer guarantees over 2 million operations.
With additional tests, we could not confirm that switching off the voltage at contactor spool will not switch off but poor contacts, big power dissapation per pole and sticking contact should be good reason for incident .
We installed a new contactor of the same manufacturer and shortened the term of inspections so that we hope to point out the possible problems and real reasons .

 

[ScottyUK]

Who are 'they' who claim that the cause is residual magnetism? I've not come across that as a cause before.

Overloading, or marginal contactors with every last atom of non-essential metal removed from the design, or bad installation resulting in a slightly warped frame, or badly-mounted auxiliary switches all seem credible.

 

[itsmoked]

If the failed contactor was only weeks old, no amount of inspection is going to prevent a repeat, and probably soon!

Something is wrong or has changed. The controller should be improved. A MUCH larger contactor should be employed. Or, as was suggested by Brian, series contactors should be employed with one set always closed during operation and the other cycled as needed. When the cycled contacts weld shut the backup drops out and is electrically kept dropped out until someone manually resets it. On power up one always needs to set the safety contactor, which tests it frequently. This precludes the need for flaky inspections.

Inspection? Really? The road to Hell is paved with well meaning inspections.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

With a crane repeated, rapid, short closures to position a load may result in the contacts welding in such a way that the load cannot be de-energized.
Forty or so years ago when U-frame motors were common we saw this occasionally, but not often.
With T-frame motors the motor may fail before the contactor welds.
However crane motors are often more robust.
I will second Keith's suggestion. Install a larger contactor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I would also agree that 2 weeks to failure means something is badly wrong with the setup and there is no monitoring system that could help.

You could also switch to a solid state device that won't weld. It could fail in other ways, but properly sized it should be more reliable.

2 million operations is likely with no load and not jogging duty.

 

[ScottyUK]

Any chance the contactor installed was an AC-3 rated type, and you've put it on a crane application which is usually AC-4 duty?

AC-4 duty allows for inching / plugging / jogging of motors and AC-4 rated contactors are usually at least one full frame size larger than AC-3 rated ones. Until this year our plant had a history of repeat failures in the starters for a specific group of drives. Our analysis determined that the starter gear was being used in AC-4 duty but it had been incorrectly specc'd for the much less onerous AC-3 duty. It's a mess which dates from long before my time, but once we identified the underlying cause of the repeat failures we installed massive AC-4 duty LV vacuum contactors in place of the ones which were failing and the problem has ceased to be a problem. Sometimes the big ugly brute force solutions are the best answer.

 

[panter]

Skotty,
According to the project for 100kW lifting motor should be AC3 contactor 250A. People from crane maintenance have installed a 265A contactor, but design is poor and there is no valid quality data from suplier .
The story of remanent magnetism as cause for unreliable operation of equipment and incident was pushed by an investigative team who, due to production problem , made a bad investigation and propose unconvincing reasons .
Realizing the omissions, a new team was formed and now, besides seeking the real reason for the incident, found a lot of spare equipment in a warehouse of suspicious quality.
AC 4 duty contactor is good suggestion .
Thank you all for the comments and your worthy views on our problem .

 

[waross]

All contactors ARE NOT created equal.
We had a customer with chronic flash-over problems in a bridge crane. Our service man was called out about twice a month to replace fuses blown by another flash-over.
We replaced all the contactors with same rated contactors of a different brand and a different design.
Never had another call-out on that crane.
Residual magnetism is so slight in a contactor that it is only an issue when combined with another failure such as the failure of a return spring.
All modern contactors have an air gap incorporated into the magnetic circuit to greatly reduce residual magnetism.
It is difficult to be in a position of answering to someone with more authority less knowledge.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

If you're looking for suitable contactors for AC-4 service I can recommend the Siemens Sirius 3TF6 series vacuum contactors: the range goes up to about 190kW in AC-4 applications (and for comparison those same contactors are rated for about 450kW under AC-3 conditions), or alternatively consider ABB's 'R-Line' bar & shaft contactors for severe service applications.

Edit: The 3RT12 range is similar to the 3TF6 range but covers slightly smaller motor sizes in the low hundreds of kW.

 

[LionelHutz]

AC3 contactors and cranes should never be put together. That's the reason for your issues.

 

[panter]

Thanks for your time .
Good suggestion from LionelHutz.
Interesting thing is that the project has defined AC3 contactor on the stator winding, while for rotor resistor bank specifies 4 steps and AC2 contactor for all , which is by definition for crane wound rotor motor .

 

[LionelHutz]

Chosing AC3 duty was probably based on the theory that the stator current would be limited because of the wound rotor resistance. The resistance likely limits the continuous current to around 150% of rated or less.

But, what is often forgotten is that the stator still has the same initial inrush current as any other squirrel cage induction motor until the magnetic field is established. This inrush could easily be 10X to 20X the rated current and happens every time the contactor closes. Do any rapid switching and the AC3 contactor is in trouble.

 

[jraef]

Ah, wound rotor you say? Add to the possibilities already expressed:

The initial setup may have REQUIRED that any start command would step THROUGH the resistance steps. In that scheme, the AC3 rationalization may have been valid from a sizing standpoint based on the inability to command any single contactor to cycle repeatedly. But somewhere along the way, someone in operations became frustrated/bored with having to do that and altered the controls (by a willing accomplice or by themselves) to eliminate that requirement so that they can start directly at any step they like. In doing that, they invalidated the previous selection criteria for the contactors because starting at a low resistance step without building to it means you end up with VERY high stator current per unit of torque, plus the ability now to “bump” that one contactor repeatedly. I’ve seen this several times on WR cranes. By the way, this is also VERY hard in the motor as well because often the motor is ALSO designed to be started and run this way, so the rotor is not rated for the higher current when starting with low resistance.

Retrofitting the WR controls with a good quality VFD that has Vector Control has been a decent solution for me on those occasions.


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