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Transformer riddle no.3 – Power transformer with cubical core

In mentioned manufacturer country, the transformer core material is an expensive portion of power transformer because it shall be supplied by foreign companies. Therefore they began it by saving material used in core construction.
Generally, the power transformer core plates are cut in rectangular form with different sizes (same length, different width) to shape a cylindrical core. In the mentioned company because of traditional product procedure, this method caused a lot of material loss due to plate cutting.

Therefore they decided to use cubical shape instead of cylindrical core; they thought the rectangular path surface of magnetic flux is important and circular shape of core cross section couldn't cause any technical benefit. Finally they made power transformers with cubical core with less material because they didn't have any waste cut plates. All electrical  tests consist: no load and on load electrical power losses, transformer impedance voltage, percentage of transformer magnetizing current and their harmonic effects were done on new transformers and everything was very well.
However in practice, the new power transformers used in actual power systems couldn't withstand against mechanical stress of sever short circuits.


What is your opinion? How core shape selection influenced the transformer withstand against short circuit current?
Author : Hamid
 
#1
Thu, September 13, 2007 - 11:50

Generally the 'E' and 'I' laminations are extremely efficient in material usage, as the stamping overlap so there is no wastage of materials...

If they are wasting material then why not use a 'C' core where the lamination is a continuous strip so no wastage again?

Or better still a toroidal core using a strip of material as the lamination - the winding maybe slightly more expensive but the core is much easier to make and more efficient.

Your question at the end about how core shape influences short circuit current isn't really related to your body text...

Please explain what it is you are asking... John 
Author : Electroman
 
#2
Thu, September 13, 2007 - 11:51

Presumably you are comparing the alternative square or rectangular built core construction with cruciform construction. These alternatives will not [neglecting labour costs> save on lamination costs at all. However, the cruciform construction will save on active copper cost. All you need to do is to compare the perimeter of one with the circuference of the other for the same CSA. The ability to withstand fault condition has more to do with the design & method of coil manufacture.

 
Author : Greg Lynch
 
#3
Thu, September 13, 2007 - 11:51

From such a detailed description I still don't really understand what you mean. Is this correct?

You're producing a cylindrical core using different sizes of sheets and stacking them to form the cylinder. Then you decided you'd go for uniform sized sheets instead.

Personally, I don't think you have much of a problem. The only requirement I can say is that the area of the core needs to be the same. Plus, of course, the quality of the construction needs to be just as good.

 
Author : Vulcan
 
#4
Thu, September 13, 2007 - 11:52
I think I know what you're talking about. If the changes you are talking about are as shown by the diagram in Vulcan's post #3 you can end up with problems in the windings under heavy loads.

This is going back a long time and I could be wrong, but I seem to remember having dealt with this back in my university days.

The problems are caused by the windings being rectangular rather than circular. As a result they are prone to failure at considerably lower currents than are normally seen if circular windings. The small radius curve in the windings, where they go around the corner in the core, causes considerably greater heating of the winding and as such reduces the tolerance of the transformer to high currents. I would expect that the result would be fairly similar to what you are experiencing with transformers being less tolerant to and failing sooner under high load or short circuit conditions.

I seem to remember that there is a way round this problem. First off you need to design your core so that the cross sectional area of the rectangular core is slightly larger than the circular one. Then the former that you use to wind the core on need to be round rather than rectangular, with an internal diameter the same as the diagonal length of the cross section of the core.

This is going to reduce the overall efficiency of the transformer but the effect will only be marginal and by removing the small radius curves in the windings where it goes round the square corner of the core you will get rid of the problems. 
Author : masu
 
#5
Thu, September 13, 2007 - 11:53
Need more details to be helpful, however, the original windings were likely on a cylindrical bobbin(spool). These windings are sublect to mechanical stress during short circuit conditions. It might be that the cylinder shape was able to handle the stress better than windings in a square bobbin. Just a guess. 
Author : Guest
 
#6
Thu, September 13, 2007 - 11:54
Thank you very much for your good answers. Fortunately you can focus to problem.

I can add to your good answers as following:

In transformers winding it will be seen that any one coil, either primary or secondary, carries current so that the currents in opposite sides flow in opposite directions, and repulsion forces are thus set up between opposite sides so that the coil tends to expand radially outwards in just the same way as dose a revolving ring or other structure due to centrifugal force. The coil thus tends to assume a circular shape under the influence of short circuit stresses, and therefore it is obvious that a coil which is originally circular is fundamentally the best shape, and is one which is least liable to distortion under fault conditions ( similar to cylindrical or spherical pressurize vessels) .

From this point of view the advantages of the circular core type of construction are obvious.

Why? Why we can not have cubical core and cylindrical coil in a transformer? Are there any electrical reasons? 
Author : Hamid
 
#7
Tue, June 23rd, 2009 - 14:35
The probklem can be addressed by adopting a Circular ( Cylinderical ) coil with a rectangular core section .
alternatively you can use Sandwitvch type coil arrangement with Rectangular and rectangular coil .

=Mohapatra-
 
Author : R K Mohapatra - From: India
 
#8
Thu, January 28th, 2010 - 20:07
Why? Why we can not have cubical core and cylindrical coil in a transformer? Are there any electrical reasons?  i visit  this site  because  i  want  the answer  for  above  question 
Author : p.srikanth
 
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