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Topic Title: 440/440 Isolation Transformer Issue
Topic Summary: Overheating Problems
Created On: 07 December 2009 05:07 PM
Status: Read Only
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 07 December 2009 05:07 PM
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michaelfivey22

Posts: 27
Joined: 21 January 2004

Hi all,

Having a bit of a problem with a transformer on one of our ships and I am trying to see if any transformers experts can maybe offer some advice.

The transformer is a 440/440 delta/delta isolation transformer between two 440V boards. The unit has been generating excessive heat for a while now. We have now isolated it and had the chance to look inside and test it.

using an infra red thermometer we measured the temp of the core which was 150 degrees centegrade - the windings were all about 60-70 degrees. This was with very little load on the unit as the vessel is in dry dock.

All IR readings are fine (GOhms)

Ductor readings 4mOhms primary and 2.6mOhms secondary - differnt sizes of windings primary and secondary could account for this.

When we reinstated power top the transformer we experienced a loud cracking noise coming from it so we decided to isolate it until we could figure out what was causing the problem.

We suspect that the core laminations are breaking down and eddy currents are generating the heat - discoulouration on the centre core section would back up this theory.

The transformer casing is not very well bonded to the ships hull but to me that would not solve the core heating problem - perhaps if the casing was heating up via eddy currents here but in this case it is clear the casing is heating up due to the core temp.

All help appreciated.
 08 December 2009 09:29 AM
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sfchew

Posts: 589
Joined: 10 December 2002

Primary and secondary voltages are equal so it is questionable that winding resistances are different.

Please use a variac and inject voltage slowly and measure the magnetizing current with secondary output open.

Repeat the above on the opposite side of the winding.

Regards
Chris Chew
 08 December 2009 01:06 PM
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Euclid

Posts: 27
Joined: 14 November 2002

It is reasonable to assume different wire sections as one winding will be wound inside the other, therefore the inside winding would operate at a lower Current density than the outside winding. Therefore I can understand the resistance variation. You don't indicate the kVA size of the transformer, are there cooling ducts within the windings and is there free ventillation for them or are they blocked? Is the ships power supply based on 50 or 60Hz? and is the transformer matched to this? Transformers designed for 50Hz may operate satisfactorily on 60Hz but transformers designed for 60Hz systems are unlikely to operate satisfactorily on 50Hz without a corresponding reduction in output voltage and hence VA rating. Can you see the core lamination joints? are they interleaved or just butted together? Modern practice on small VA units (upto approx 200kVA) is to use butt joints with a glued join to reduce noise emissions and labour costs. I hope the above helps in finding solutions. Euclid
 08 December 2009 06:58 PM
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michaelfivey22

Posts: 27
Joined: 21 January 2004


Sorry the unit is 500kVA.

I dont think it is a cooling issue - the vessel is in dry dock and this is for the engine room switchboard so was only running at minimal load - 20kW or so - still producing a core temp of 150 degrees C.

Measuring across some of the plates in the core pack we can measure a dead short over an inch or so suggesting the laminations are breaking down and eddy currents ciruculating in the core are the issue.

 10 December 2009 08:06 PM
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fredman

Posts: 221
Joined: 26 July 2007

its not a 60 HZ unit running on 50 HZ?
 22 December 2009 03:48 PM
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Baz

Posts: 146
Joined: 24 May 2009

I think you have answered your own question. If the winsings are cool and the core is hot then I would suspect excessive iron losses. being a maritime transformer the salt air and water may have corroded the core and broken doen the very thin insulation between the laminations, causing the problem. Alternitively, the bolts holding the core together may be missing insulating washers this allowing a short circuit between the clamping plates of the core. Check the no load current draw, this is all due to iron losses, compare this with manufacturers data.
Hope this helps
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