fuse blowing in syncronous 1000 kw motor 1

[mottt]

i got a syncronous motor 1000 kw, with a excitation of 400 V dc, with brushes, and a 1000 V ac stator feed, and I feed the rotor with a Motocon system to give excitation on dc, and the fuses burn when I desexcite the motor, 15 minutes after i disconect the stator volts... Motocon is based on thyristors, and I`ve cheecked that the rotor isolation is going down each time i measure it, but still is 1 Meggaohm.... does anybody know why this fuses burn? this machine was working for 8 years....

 

[jraef]

"desexcite"??

Was this a typo? Were you trying to say de-excite? And by that, did you mean that you have shut down the synch. motor and 15 minutes later the fuses blow? Because as far as I know, you should not be turning off the excitation if the motor is still running.

 

[mottt]

the fuses burn when the motor is stopped, and i de-excite it...15 minutes after i´d stopped.... for me it´s very strange... sorry for my english

 

[jraef]

No problem on the English, just wanted to be clear, especially about the sequence of events.

So after you stop the motor, your Motocon rectifier is still powered on for 15 minutes, then you disconnect it from the supply and the fuses blow? If so, I would check the SCRs in your Motocon unit, you may have a shorted one or a bad snubber circuit.

Or do you turn off the motor and disconnect the rectifier from the supply at the same time and then 15 minutes later the fuses blow? if that's the case, that would be very bizarre.

 

[Skogsgurra]

Hi,

Where exactly are these fuses? Main circuit? (probably not). Excitation circuit? If so; in AC or DC path?

I would be VERY concerned having just 1 Mohm insulation resistance in such a system. I think you have to face it: A complete overhaul needed. As quickly as you can get it done.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ssn596]

Sounds similar to a situation we encountered in the Navy. When we shut down or tripped the turbine on a turbo generator AND forgot to turn off the regualator we blew the fuses feeding the regulator. Seems to me you should open the AC breaker feeding the unit and then turn off exictation.

I would have to agree with Gunnar about being concerned with only 1 meg ohm on a synchronous motor. I think I would look and ensure I have no water in it ESPECIALLY if water cooled. I would also look at carbon dust if you are using brush type slip rings. If lots of black dust inside the windings you need to clean this thing out. Have fun - I still have memories of cleaning the DC to AC motor generators.

DO NOT megger the regulator -- disconnect it prior to meggering the rotor and brush rigging This may be as simple as removing the brushes if so equipped.

Dan Bentler

 

[HCBFlash]

I must complement you on your nickname! I'm envious!

 

[Skogsgurra]

Yes,

One can well understand that there can be problems with water. But - if it has gone that far - do you not have other, more serious problems? ;-)

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[mottt]

the sequence is:
- I stop the inversor for the stator.
- I let the excitation during 15 seconds, and after this time i disconect, and in this moment the fuses blow...

about a bad insulation I agree, that is no good, but when the motor is working, during days, i have any problem, and when I start i have no problem, the only problem is when I disconnect the excitation.

I´ll like to know how the snubber circuit or the SCR could do this fault, cause as long I know if the motor is stopped... I´m only feeding a winding... during this time..
Could you explain Jraef.....
and the fuses are on the dc side

thank all of you for your interest.....

 

[Skogsgurra]

There is no commutation between the diodes (or thyristors) when the excitation is switched off. That probably means that current stays on for an extended period (time constant of field winding) so, instead of carrying current 1/3 of the time, it carries current 1/1, which may be too much. The i2t gets longer.

Try to reduce excitation current to a low value before removing the AC. That will make the i2t a lot lower.


And, yes, I do understand that you can run the machine with 1 Mohm. But that doesn't mean that you should do that. You will be quite sorry if you do not do anything about that.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[mottt]

ok, you says that the field remains with power, when you disconnect it, and this current damage the fuses..?? but I got this problem since 6 months ago, and then the isolation was 15 megas, i change Motocon... and 50 fuses or more, and the only thing i see is a degradation on the isolation of the field, and the fuses that burn, i look at the inductive resistence and is good, not short in the windings, and the resistence ohm, is ok too.
And the times of the excitation must be the same...for all the life of the motor field... or maybe something breaks inside the motocon system???
thanks skogsgurra for your kind attention...

 

[Skogsgurra]

The time constant has nothing to do with the insulation. They are not related.

I was only suggesting one possible failure mechanism that I have seen in other high L/R situations. Is the procedure - and the result of the procedure - exactly the same now as it was before this failure started to appear?

Anyhow. You need qualified help. Get a technician there as soon as possible. Your machinery is at risk.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[oftenlost]

I am not familiar with these drives but there is usually a crowbar circuit in these applications where a SCR, thyrite or other device shunts destructive voltages, particularly during switching. I would look carefully at whichever device performs this function.

 

[mottt]

of course, I know i need some qualificated help, but i haven´t it at this moment... for this I try to solve or see some light at the end of the tunnel... please could me explain something about this problems you relate in the L/R circuits...
and in other way, yes, the procesure is exactly the same at the beginning and today....

thank for your kind attention....

 

[ScottyUK]

I don't do much with synchronous motors but they are essentially the same as synchronous generators which I do spend a bit of time working with. The excitation should come off after the stator is disconnected from the AC supply.

As the fuses which blow are on the DC excitation system, does the excitation system have a field discharge resistor to quickly suppress the field current? On big machines this is usually placed in circuit using a make-before-break changeover contactor arrangement which inserts the field discharge resistor prior to disconnecting the field rectifier. If the timing is out so that the contactors leave the field open circuit from a moment then the high inductance of the field will cause very high voltage to develop across the open point. You may wish to verify this. Be careful!


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[oftenlost]

Scotty,
This was the exact arrangement on most ring fed machines. In many of the designs over the past 30 years, the shunting of the field discharge were handled a little differently. In the 1970s, GE utilized thyristers on a rotating ring to bypass discharge, a little later, they used zener triggered SCRs to bypass the discharge currents. They again went to discharge resistors activated by SCRs in the 1980s. In the 1990s to this day, the Canadian GE machines utilize a crowbar circuit with SCRs and the Campinas machines reverted back to the zener controlled units. A failure of the crowbar circuit or other bypass mechanism can blow fuses but they ususally blow them on startup too. I do not remember any machines blowing on shutdown only.

 

[mottt]

thank all of you, i really appreciate your ideas and knowements...

 

[ScottyUK]

Hi oftenlost,

I hadn't gone too far into the brushless systems with shaft-mounted rectifiers or thyristors because the OP said it was a slipring machine: this thread is confused enough!

There are so many variations of excitation systems that without some detailed vendor documentation it is difficult to fault-find. I don't know this specific type at all. The vendor is probably the best option for support because of all the subtle variations in how these systems operate. Most of my experience has been with Westinghouse and Brush designs, some on older Parsons and GEC machines, a little on relatively recent Alstom / CEGELEC types. Not yet had the pleasure of a GE type. Of the types I am familiar with, the one common fact is that they are all different!

motorexplosion,

Can you get any support from the manufacturer's field service engineers? They tend to be expensive to bring in but could well get you a a solution within hours or days. After the length of time you seem to have been struggling I would seriously consider this option.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!