Contactor getting welded when shifting from Star to Delta 1

[askoppal]

Hi,

PROBLEM

When the 10 HP 420V 50Hz motor of my lathe machine gets shifted from Star to Delta all three contactors (Main, Star and Delta) gets welded and it trips my workshop supply altogether.


OUR INITIAL ANALYSIS
We thought we had a faulty contactor since the machine is too old, So the star contactor could have stuck during the transition to delta causing a short circuit and it welded the links of all the three contactor that was in use during star delta transition.

After changing all the 3 contractor and also the 4th contractor for reversing, the motor was okay in a few trials and we run the motor for a while and we thought the problem is solved and we had a tea break and to our surprise when we started the motor after the break the exact problem repeats. OMG!!!!

AT PRESENT ANALYSIS
What we suspect now is we have a faulty timer. The fraction of time taken to shift from star to delta is negligibly small causing a short circuit. (I am not referring to the long time taken to shift from star to delta while the motor gains its inertia.)

I would like to get more comments and somebody please help me to set these relay timers because I have no idea. I have attached the photographs of the timers.

But before concluding let me know whether I am right about the timers or is there anything else I should be looking in to ? I do not want to make a mistake again.

 

[Skogsgurra]

Your problem is not a faulty timer. It is the well-known fact that the rotor has a frozen magnetic field which induces an EMF in the stator winding and that the EMF hasn't decayed when the delta contactor switches. If the voltages are in the wrong phase, then the contactor welds.

What phase they are in is a result of phase sequence and the tiny time difference between wye contactor out and delta contactor in.

There are two ways out. One very simple, but works sometimes. The other one involves an extra timer.

The simple one is to change phase sequence to the starter and then, to maintain motor rotation, change phase to the motor. The phase 'drift' then will change and, if you are lucky, to the better.

The other one is to introduce a delay between wye out and delta in. Make the time long enough to allow the rotor field to decay. One or two seconds is usually OK. That will surely help. But try the simple one first.

Good luck!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

I mostly agree with Skogs.

We thought we had a faulty contactor since the machine is too old,
What we suspect now is we have a faulty timer.

If nothing external has changed recently then:

The other one is to introduce a delay between wye out and delta in. Make the time long enough to allow the rotor field to decay

In the absence of other information I suspect that the second timer suggested by Skogs is already installed and has failed.

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

 

[crshears]

This may be a dumb question, but is there any possibility that no interlocking limit contact has been provided to ensure the wye contactor is completely open before the delta contactor can close? Including this provision is just one more means of preventing what is essentially a dead-short condition from occurring if/when the timers develop problems.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[askoppal]

What phase they are in is a result of phase sequence and the tiny time difference between wye contactor out and delta contactor in.

The tiny time difference between the wye out and delta in is actually controlled by timer. I do not how how to configure it.I have attached a photo of the timer in the first port.

Your problem is not a faulty timer. It is the well-known fact that the rotor has a frozen magnetic field which induces an EMF in the stator winding and that the EMF hasn't decayed when the delta contactor switches. If the voltages are in the wrong phase, then the contactor welds.

What you mean by "voltage in wrong phase" ? If what you said is right, why it is not happening in the initial trials? We did atleast 8 to 10 start and stop of the motor.

This may be a dumb question, but is there any possibility that no interlocking limit contact has been provided to ensure the wye contactor is completely open before the delta contactor can close? Including this provision is just one more means of preventing what is essentially a dead-short condition from occurring if/when the timers develop problems.

I had asked the same question to myself "What happened to the safety interlock and the OLR that is being installed to protect in the event of a short circuit" ?

Interlock Failure : There was no mechanical interlock only electrical interlock so it could fail (I hope so!).
OLR Failure: There was not much time, it happens so quick, the MCCB trips even before the OLR could act.

 

[Skogsgurra]

OP:

The answer to your question can be found here:

http://www.mastercontrols.com/Articles/Wooddall/TA_RWood.htm

Figure 2 is very illustrative.

Please note that this is a very old reference (45 years). The problem is known to the EE community for a lot longer time than that.

But I can agree with other views. Missing or faulty interlock is a very probable cause for the problem. In cases like this, I connect a single-shot scope or a fast recorder to see what is happening. Guessing games are entertaining, but not very effective.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

This is the main reason that I NEVER recommend Star-Delta starting methods. The ONLY benefit it offers is that it is slightly less expensive than a solid state soft starter. That's it, but then it comes with there inherent risks. And by the way, what you have now witnessed as a catastrophic failure in the contractors is just the tip of the iceberg, the "weakest link" so to speak. It has been pundinf your motor and drive train components with torque spikes too.

Why now after years of being OK? The effect is partially the result of residual magnetism in the motor, plus the phase angles of the sine waves at the exact moment of transition and howuch the rotor slows down as the open transition starter waits. The pounding that your drive chain has been taking all these years has taken a toll on everything and the juxtaposition of all of these elements is no longer allowing for any tolerance.

As Gunnar said, the phase angle change (called "rolling") MIGHT help, and it's free other than your time. If it doesn't, you might try tossing that dinosaur and getting a modern soft starter. That's what I would do.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

If it worked for years, it probably doesn't need rolling. I suspect a failed second timer.
Now, if someone has been working on the machine and there is a possibility that the wiring has been changed, consider that the connections may need to be rolled.

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

 

[KuanYau]

In Singapore, there are hundreds of thousands of LV SD starters currently in use. Autotransformer and closed-transition SD starters are not widely in used here. There is no report of motor shaft been broken due to star-delta starting.

There were numerous cases of contact welding identical to what you had encountered.

Assuming that there was no changes to the original control circuitry or motor rewinding or rewiring, the problem is the time-delay timer.
Replace the timer with a identical timer as the original, which was working fine.

If the original timer is not available, replace the timer with a "special" timer specifically designed for use in start-delta starters.
This product is available from ABB and OMRON. Attention, a normal timer is not acceptable.

 

[LionelHutz]

Ya, if it's been working for years then your mechanical interlock or wye to delta delay timer is the issue.

The overload relay is only for motor overload protection. Don't expect the overload to trip on a short circuit.

 

[KuanYau]

To Mr LionelHutz,
In Singapore, with hundreds of thousands of LV SD starters currently in use; none of them is installed with mechanical interlock, interlocking all three; the Main, Star and Delta contactors. Usually the Main and the Delta contactors are of the same size (current rating); whereas the Star contactor is usually selected one size lower.

It is desirable to have mechanical interlock between the Star and the Delta contactors. These two contactors must not be in the closed condition simultaneously, even momentarily. If the Star and the Delta contactors are of different sizes, standard mechanical interlocking device from the contactor manufacturer may not be readily available.

To Mr askoppal,
The time-delay timer is the only culprit.
Please share your experience if the problem is resolved. How?
Or, are you still looking for other brands of "special" SD starter timer in your location.

 

[itsmoked]

Welcome to your active phase at EngTips KuanYau. Glad to have you aboard.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[KuanYau]

To Mr itsmoked,

Thank you for your kind welcome.
Singapore is a very very small state ; a small "red-dot" in South-East-Asia.
IEC is prevailing here.
My opinion is based on the practices prevailing in Singapore.

 

[LionelHutz]

KuanYau - attempting to interlock all 3 contactors would be rather dumb. I'm not sure why you're making such a big point of telling me this because I never posted anything about interlocking all 3 contactors. I just posted to agree with the other responses that have already pointed out the interlock as a possible problem.

 

[crshears]

Now now, Lionel, don't be too hard on KuanYau; by the look of things he's only just joined the forum, and we should give it a little time before engaging in no-holds-barred riPOSTing with him...

That being said, Lionel, I do take your point; you did not in fact post anything about interlocking all three contactors - and who would want to? The motor would never run...

As to jraef's comment about NEVER recommending wye-delta starting, would not the availability of qualified service personnel also be a factor at the design stage? True enough, the capital savings aren't huge, but if it will be tough to find competent, knowledgeable staff to attend to any problems, I would think surely that should be considered as well.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[askoppal]

It is solved. We didn't find the exact problem but will mention what we did.

We replaced three contractors (Main, Delta and Star) and an OLR (Old OLR was not resetting and it looks like the links are completely melted during short circuit)

We checked the old timer it was working perfectly but since we didn't know the reliability we replaced it with a new one. It is not a star delta timer. (I should wait 15 days to get it). Its a normal ON delay timer. But this time we checked the current drawn by the motor and set the time delay such that star - delta shift happens when the least current is drawn, in its highest load, our time was 4.5 seconds. So this will lower the sparks / inrush current.

The timer manufacturer (Snider) is claiming the internal switching from star to delta is 20 to 40 micro second which is actually too fast switching.

It is working so far we tortured the machine like kids. It was fun, we were all expecting it will blow and it didn't happen. But like Jraef we are not completely happy with it because we are not quite sure what we fixed here.

We made the electrical interlock stronger these times, we insured that the control voltage to coils are maintained even at full motor load.

Thanks to Skogs, Jraef. Those eng tips and information was really worth knowing. I added it to our library for future reference.

To Jraef,
The reason why we still stick on to the old method of delta and star switching is because the availability of parts. We sell machinery to oil industry workshops and it is always located in hostile/remote environment. We just came back from Basra, Iraq. Supplying parts to these regions is really hard.

To Skogs,
I didn't get the time to study those article entirely but I did tried your methods, I will let you know the practical side of the article when I get time to play with motors again. This time we where too eager to solve the issue and wrap it up.

Thanks all, I am new to this forum community and I am really happy. I have a feeling that I am sharing all these with the right people.

Askoppal