DC applied to primary of transformer

This was test question for ems comapny,they don't describe Volatge/Current Source?here is original question: DC is safely applied to the primary of a transformer:

*When limiting resistance is in series with primary. *When -------------------------Parallel------------. *When neither of preceding applies

Thanks a lot Sir,I appreciate your detailed response due to that we get some knowledge by these forums.I am civil engineer working for ems cos in ON.

The windings of a transformer act just as any other piece of wire where DC is involved, so think of how you could safely connect wire across a DC supply.

If the supply is a voltage source (voltage essentially constant, no matter how much current is drawn from it), than adding other loads in parallel with the wire would only provide additional paths for more current, without limiting or reducing the current through the wire.

Resistances connected in series (the wire is just a low value resistance) add. So a series resistor will reduce the current that passes through the transformer winding.

If, however, the DC supply is a current source (which means that the current is nearly constant, regardless of how much resistance is connected across it because it varies its voltage to whatever is necessary to drive that current), then paralleling the wire with other resistances would detour some of that total current to other paths, lowering the total voltage the supply needed to produce to maintain the fixed, total current. This lowers the fraction of that total passing through the transformer.

Since your mention of "applying DC" does not specify whether the source is more like a voltage source or a current source, or something in between, the question cannot be answered, except to discard the third choice.

Tell your teacher that you learned this on the Internet, not in their class.

Does my answer make sense to you?

Since voltage sources are so much more common than current sources, I suspect they are expecting the first answer.

True or false? Transformer cores are sized to be the smallest/lightest/cheapest they can be and still not saturate at rated drive levels.

If true, then any significant amount of DC current is a bad thing. Matters not whether it was placed there by Mr. Thevenin or Mr. Norton. mike

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IOW, (D) None of the above. There is _no_ safe DC value that can be applied to the primary of a transformer. Unless, of course, it's a transformer that was designed for single-ended class A output. In that case, it'd be series resistance, or more strictly speaking, series transconductance.

If this is a test question, then Thaqalain needs to reread the textbook before re-taking the test. Or ask the teacher what answer they're trying to extract from the students.

One of the ways test questions are recognizable is by the date. It's FINALS WEEK! ;-D

Cheers! Rich

Generalization...A.

Tom