Siemens-Allis thyristor drive

[Skogsgurra]

Hello,

I was asked to have a look at a DC drive that behaves erratically when switching from motoring to brake bridge and also when it switches back to motoring. The drive had been running off a 480/460 V supply for many years but, since that is not a standard voltage in Europe, was connected to the existing 400 V grid. The transformer was scrapped by ambitious bean counters that got some good money for the copper, so it cannot be reinstalled easily.

It is obvious that the firing angle pre-control is the culprit. It simply doesn't calculate the new retard angle correctly when the mains voltage is off by about 15 %. The potentiometer for adjusting it (R137 in this unit) is at its end setting and trying to adjust it makes things worse.

There is probably a simple way (changing a resistor around that potentiometer, perhaps?) to correct this. Anyone that knows about these drives? And has information about the pre-control and possible action to make it work off 400 V?

The drive is a 506 A 460 V 4 quadrant drive type A1-103-102-505,ISS.05 made around 1986.

Gunnar Englund

http://www.gke.org

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

 

[ozmosis]

Gunnar
I had a chat with a DC expert (Simoreg man):
"When this feeding transformer was removed, it is feasible there was a phase-shift imposed on the stack supply which may have to be in-phase with the electronics control supply in order that the firing pulses are synchronized. This should be checked as it could cause quite an instability. It may be as simple as removing a control transformer which supplies the control electronics with a phase-shift to match that of the main transformer (which is now gone), as the control supplies would usually be 380V rather than 460V.

Otherwise I don’t see that reducing the voltage of the supply a bit will make any significant difference at bridge switch-over.
Please let me know if you can get hold of a schematic drawing showing the system as it was before the transformer was removed as this may shed some more light on the issue.
Chances are it is a Modulpac C drive system, but from the numbers given, this is impossible to tell. Hope this all makes sense."

 

[Skogsgurra]

Thanks Patrick!

I just got an answer from the guy on site. He says the transformer was a D/Y. But, they also put in a transformer for the control voltage. Probably to correct the 30 degrees. But he couldn't tell right now.

When I think about it, it doesn't seem possible to run a drive with a firing angle that is offset + or - 30 degrees. It would mean either a limited speed range or blowing fuses when braking. None of these things seem to happen. Just the exaggerated current peak when changing bridges. I have had this on old Strömberg drives. And it was definitely the start position of the firing angle after swith-over that caused it.

I'll be back when I know what the control voltage transformer looks like, connection group wise.


About ModulPac. No, it isnt. It is built like the old Simoreg, I think it was 6RA22, with those hinged circuit board. But it seems that they replaced the processor board and put an analogue board there instead.

Gunnar Englund

http://www.gke.org

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

 

[1x1x1x]

Gunnar,

1. Using an insulated oscilloscope or high voltage differential probes check first that that the control (synchronizing) voltage is in phase with the bridge voltage.

2. Check the alpha and beta limit (the rectifier and inverter limit) of the control. The easiest way to do it not energize the main bridge just the control (synchronizing) transformer. Please note that it is possible that the main contactor status has to be simulated to release the firing pulses.
However with positive reference the firing pulses shall be at the alpha (rectifier) limit position (it is approx 15-20 electrical degree) and at the beta (inverter) limit approx 125 - 135 degree when a negative reference is given. (The setting of the beta limit depends on the system short circuit power).
Using an insulated oscilloscope and current probes for the firing pulses you can see if the firing pulses are correct at the thyristor gates. The synchronizing voltage is for the #1 thyristor (assuming positive phase sequence is Ur-Ut (R-T phase). The firing pulse order is: 1-2-3-4-5-6 and the phase shift between the pulses shall be 60 electrical degrees.

Please note that the firing pulses will be at the alpha or beta limit compare to the synchronizing voltage, but not to the bridge voltage if the 30 degree phase shift is exist.

 

[Skogsgurra]

Thanks 1x,

Yes, I know rather well about that basic stuff. Did my first thyristor drives at ASEA (ABB now) back in 1963.

Sorry to sound negative, but it will save a lot of unnecessary information transfer if we start from there.

Is this information of yours specific for the Siemens-Allis drive I am asking about? I usually see alpha at 30 and beta at 150 degrees, down to 165 in drives with dual limits and discontinuous operation. That's why I am wondering. Also, the firing limits are alpha(G) and alpha(W) in Siemens parlance, not alpha and beta.

The positive reference for rectifying and negative reference for regenerating also seems odd. Siemens has always used negative for rectifying and positive for regeneraing. So that makes me even more confused. Are you really describing the Siemens-Allis unit?

Do you have any specification for the R137 settings and how it depends on mains voltage? If it is dependent at all.



Gunnar Englund

http://www.gke.org

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

 

[1x1x1x]

Gunnar,

My background is also with ASEA. Sorry but I did not realize that you know DC converters.

 

[Skogsgurra]

No need to be sorry. Viksäng? Finnslätten?

Gunnar Englund

http://www.gke.org

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