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12 MW refiner motor - Excitation issues

12 MW refiner motor - Excitation issues

(OP)
I did my first measurements on machines like these 15-20 years ago.

The exciter shorts out the excitation winding during start (high induced voltage due to high slip makes the control unit gate the thyristors on) and when near synchronous speed, the induced voltage is reduced so the thyristors (SCR:s) stay off and the rectifier just rectifies. All fine and dandy.

Now, there was a spare rotor that a winder shop wanted to test before returning it to the plant. They have done this test many times before, but this time - the control unit let out some smoke. It still worked as intended. But the smoke was not pleasant and I think that life of the unit may be reduced by this. Pictures below:





The waveforms show how the ctrl unit trigs the SCR:s at around 400 V - which is OK and works well in this application. This picture is from around 2004.

The "Smokey" had been set to a higher trigger voltage. Instead of 400 V peaks, we had 800 - 820 V. The original Smokey had burned a hole where you could see the smoke exit. The second unit didn't smoke. But got quite hot in the same place. We switched off before there was any severe damage to it.

The funny thing is that both units behaved well and showed the same characteristics.

The units that we have seen before (from 2000 - 2010, cirka) could all be tested without any problem. The new ones can't tolerate AC voltage for more than ten or twenty seconds.

We do not need speculations. But if someone has had this problem and can say if there are any differences between the older units and the newer ones (from 2014 and onwards) we would be very grateful.

For political and business reasons, I do not disclose supplier or type. But anyone who has had this kind of problem will know what it is about.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: 12 MW refiner motor - Excitation issues

SCR not triggering could be causing the gating circuit to carry too much current.

RE: 12 MW refiner motor - Excitation issues

(OP)
Thank you, Lionel.

They do trig. Flawlessly. That is what we were monitoring continuously. Scope and incandescent lamps were used and even if we missed a non-trig on the scope. the lamps are quick to indicate such an occurence. There wasnt the slightest flicker.

Our thinking right now is that the trigger unit, which seems to be a "hand-crafted" replica (or rather functionally similar) device that was not supposed to run much longer than a normal start would take - that is 20 - 30 seconds.

We were running it for around five minutes, which was OK with the original units, when this unit started to let out smoke. There are no papers or labels warning for this. And, as said, the original units could be run for hours without any problem.

I am actually asking for similar experiences. If there are any.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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