secondary current
secondary current
(OP)
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
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






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