secondary current

[angryman]

We are using a 250VA xxx/120 transformer in a system to supply the 120vac for various components. The various components, although not continuously, draw approximately 3a during normal use, maybe around 8a total for short periods (under 3 min).
I'm figuring that the 250va transformer at 120 is rated for about 2.1a give or take. How is it determined at what point over the rated current that the transformer will fail...

Also, would a large current draw on a secondary sorta work in reverse and cause the primary to draw more current from teh supply?
This is a pretty basic step down and we use various voltage inputs, but I am having trouble finding a good description of secondary current draw and it's effect on the primary and the transformer overall....any info would help....thanks,
Mark

 

[ScottyUK]

Mark,

A transformer is one of the most efficient electrical machines ever conceived. Power out = power in, if you neglect the small losses. So yes, there is a definite relationship between output current and input current: they are directly proportional.

Transformer ratings are determined by temperature rise of the windings. The problem isn't normally the copper but the insulation on the winding. The highest temperature which any normal insulation is rated for is 180 deg. C (insulation Class H) but not many small transformer windings will be built with Class H insulation. The highest temperature will be deep in the winding so it will not be easy for you to measure anyway. Small transformers are a commodity item and as such they are designed with virtually no margin for overload simply because extra margin pushes the cost up. The power rating will assume a certain ambient temperature. If your application places it in a cooler environment then you should be able to squeeze a little more out of it. Conversely if you place it in an environment hotter than it was rated to work in then you will have to derate it.

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Sometimes I only open my mouth to swap feet...

 

[rbulsara]

Not an expert on votlage regulation of control transformers, but have you looked into if the transformer will maintain acceptable output voltage at 8A, about 4 times the rated current for 3 min?

 

[CarlPugh]

3 amp continous from a 2 amp transformer. Not advisable.

8 amp for short periods. Some small transformers will handle this, and some transformers output voltage will drop drastically.

 

[angryman]

we have had several units running under these parameters for some time...I believe the last report was closer to 9 amps.

 

[itsmoked]

Well you are running on borrowed time..

If those transformers are running nonlinear loads you are even pushing your luck with; load current = transformer current rating.

You have to ask the question "what happens when this transformer fails?"(as it will) Will it trap people in an elevator? Or will the water fountain stop? Depending on this answer you should either plan for failure at the most inopportune time or correct the design. IMHO.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[angryman]

right....I'm working on correcting it....any idea how far above the rating is catastrophic? i know that's a general question, but anyone got any generalizations from experience?

 

[itsmoked]

I would say running 260VA out of a 250VA transformer is asking for it. Seriously! These little buggers are quite marginal in my experience. I am surprised to hear you are getting away with things as far as you are. If you have dramatic air flow past your xformers then you might be able to run some percentage over ~ 5-10 maybe.

Remember that the core of the transformer is commodity sized to be just large enough to support the flux required at the rated currents. If you over amp them they will go into saturation! Then the primary current is no longer restrained by what the secondary is drawing it is only constrained by the primary winding resistance. (a baaaad thing!)

I also wonder about the safety of your unit if you are not fusing things to prevent overloading/heating/fire of your transformers. HmmmmMM?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com