Transformer primary opening

[Cbarn]

Hi there,
I'm new to the forum, so please bear with me.
I have a situation with small circuit power transformers. They have dual 110v primaries, set up for 230v (60Hz)operation, with dual 24v secondaries of 250ma each, in series for 48v powering small motor control circuitry.
The problem is repetitive failure of the transformer primaries, one of them opening, thereby killing the circuit.
My question is this... what kind of action might be expected from a transformer, whose primary has not yet opened, but is beginning to break down, and will finally open? What gyrations might a transformer go through on the way out?
Thanks for thoughts,
Rick

 

[GGOSS]

Hello Cbarn,

I would have thught it be more important for you to determine why the transformer primary is being compromised. If you address that, then you won't need to concern yourself with any possible gyrations the transformer might go through before it ventually fails.

Have you pulled a failed transformer apart to inspect the damage? If you have and can describe what you have seen, there's a strong possibility that someone here wioll be able to identify possible fault causes so that you can address the problem once and for all.

Regards,
GGOSS

 

[buzzp]

It could have something to do with the power being pulled from this transformer being in short bursts. Is it possible this is causing insulation degradation and heating of the transformer? Look at your peak demand on the transformer and use this number to calculate the VA to compare to the transformers rating. Also, look at the in/out of the xformer with a scope and see if there are any transients being generated while the controller is running. I think this will lead you to the ultimate problem.

 

[electricpete]

I believe that one common scenario is that the failure starts as a turn-to-turn fault, generates excessive localized heating, and eventually progresses to a ground fault or open circuit.

I am not sure exactly what would be the early symptoms of shorted turns. A simplistic view suggests that N1 decreases so V2=N2/N1 increases.

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[electricpete]

Thermographic monitoring is another test that might also provide early warning of degradation.

Off-line you could consider TTR, megger.

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[ScottyUK]

Dual 110V primaries... 230V operation?

The primaries in series have a rating of 220V, not 230V. Manufacturers of small transformers are not known for being generous with the size of the iron core, so the flux density is usually on the margin of saturation at nominal voltage. You are applying an overvoltage which will increase the level of saturation. Primary current will increase rapidly with voltage, and will have a highly distorted waveform if saturation is occurring. The high primary current will cause excessive I^2R losses and will burn out the winding.

Can you hang a current probe onto the supply to the primary winding and report your findings?



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If we learn from our mistakes,
I'm getting a great education!

 

[CarlPugh]

Your nose is a pretty good indication of a transformer overheating.
Audible noise that gets worse as the load is increased is an indication that the winding is not properly impregnated.
A sizzling noise that gets worse as the primary voltage is raised is an indication of corona. (Probably not a problem at 230 volt)
A visual inspection can often detect problems. A frequent problem is when the transformer is manufactured, the wire to the terminal inside the transformer is too tight, and with time will open.
As GGOSS suggested, pull the transformer apart.
There are many crummy transformer manufactures out there. Try a different manufacture and see if problem goes away.