Minus sequence and heat in synchonous motor rotor?
Minus sequence and heat in synchonous motor rotor?
(OP)
I know about quite a few ways that a rotor can run too hot. Like repeated resynchs, too high excitation current, poor ventilation and such things. I can also understand that excessive minus sequence grid voltage can cause eddy currents in the rotor (saient poles). But, is it possible that a .6 percent minus component can do that?
That is what I heard as an explanation to accelerated aging of rotor coils. I have seen .6 percent and more in other places. But no known problems with rotor heating. So, why on this particular 10 kV machine? Or is it just not so?
That is what I heard as an explanation to accelerated aging of rotor coils. I have seen .6 percent and more in other places. But no known problems with rotor heating. So, why on this particular 10 kV machine? Or is it just not so?
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.






RE: Minus sequence and heat in synchonous motor rotor?
Is there anything on the nameplate which suggests this machine is being pushed to the limits? For example, what thermal class is the insulation system, and what is the expected temperature rise? Big machines traditionally use a class F insulation but are designed to run at a class B rise, which is part of the reason why they last a long time and part of the reason why they can tolerate moderate NPS heating without damage. By contrast a class H machine running at a class H rise won't be as tolerant of NPS heating in the rotor.
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If we learn from our mistakes I'm getting a great education!
RE: Minus sequence and heat in synchonous motor rotor?
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(2B)+(2B)' ?
RE: Minus sequence and heat in synchonous motor rotor?
There is another section (33) which covers smaller "definite purpose" generators and which has an identical paragraph.
So I think, this would apply to all generators purchased to US standards, below 75MVA.
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(2B)+(2B)' ?
RE: Minus sequence and heat in synchonous motor rotor?
Pete, can I assume that a negative current sequence roughly corresponds to the negative voltage sequence? Or is there an impedance involved as a conversion factor?
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Minus sequence and heat in synchonous motor rotor?
I'm not sure about synchronous generators. I'll poke around my references tonight if no-one else responds by then.
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(2B)+(2B)' ?
RE: Minus sequence and heat in synchonous motor rotor?
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(2B)+(2B)' ?
RE: Minus sequence and heat in synchonous motor rotor?
Is it a coincidence that the "amplification factor" is approximately the same as the starting current multiplier?
In my mental picture of the negative sequence voltage and current, it is a little like plug breaking a motor. The current then is also five to ten times higher than rated.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Minus sequence and heat in synchonous motor rotor?
Both situations correspond to the impedance seen when slip is very high: s=1 for locked rotor and s~2 for negative sequence. As a rough approximation, the impedance is Z=j*w*L1+j*w*L2 for both cases. (neglects the term R2/s which is very small at these large slips, neglects small variations of L2 with frequency due to deep bar effect, magnetizing branch neglected for simplicity but doesn't change between the two cases).
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(2B)+(2B)' ?
RE: Minus sequence and heat in synchonous motor rotor?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Minus sequence and heat in synchonous motor rotor?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Minus sequence and heat in synchonous motor rotor?
I would guess a salient pole machine will have a similar relationship between X2 and X''d, although the values will be considerably higher than for a turbo machine (roughly double those for a turbo machine).
There's some discussion in the NPAG although it's aimed at big utility machines: http
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If we learn from our mistakes I'm getting a great education!
RE: Minus sequence and heat in synchonous motor rotor?
There is a set of thyristors parallel to the rectifier and those thyristors are there to create a parallel path for current opposite the diode current (so the excitation winding acts as an amortisseur winding during start). The triggering voltage for tose thyristors is usually kept at 700-800 V. So it is not very likely that the negative sequence activates the thyristors. But, if it does, that would mean repeated out-of-sync and restarts. Had that once on a Siemens machine. This one is ABB. But same arrangement with diodes/thyristors.
I have not been given all the details. And I haven't seen the machine. The guy that told me about it has signed an NDA. Don't ask me why - but that is what he says.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Minus sequence and heat in synchonous motor rotor?
Not sure I follow your opening paragraph. Are you thinking that the NPS current turns up in the field winding? That's true to some extent, but the main problem is that the 2x line frequency current circulates in the rotor forging itself where it causes eddy current heating of the unlaminated rotor core.
Hadn't seen the problem you describe with thyristor false triggering causing a loss of sync. That's pretty serious! Have you any more details on that event?
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If we learn from our mistakes I'm getting a great education!
RE: Minus sequence and heat in synchonous motor rotor?
I didn't see that thyristor triggering problem myself. But a Siemens guy phoned me during comissioning and said that they had problems with a large heat pump drive. I asked what the triggering point was and it was quite low. He then set it to, I think, 800 V and the problem disappeared. Not sure if it was the induced EMF that caused the triggering, but that was what we agreed on. Then. The need for an explanation isn't always the same thing as the need for absolute truth.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.