What will happen if a DC voltage is applied to the transformer coil?


What will happen if a DC voltage is applied to the transformer coil?
This is a very common question asked by those who are starting up with transformer science. You may come across the same question during interviews too.


Well! Here is the answer:

Before explaining what will happen if a DC voltage is applied to the transformer primary, please keep in mind a few points.

  • The resistance of transformer winding is very less.
  • When we apply DC voltage to the transformer, the reactance of its winding will be zero since reactance is frequency dependent and the frequency of a DC supply is zero.
  • Flux produced by the flow of direct current will be constant.



Now let us enter into the topic.

What will really happen when a DC voltage is applied to the transformer primary?

When a DC voltage is applied to the primary winding of a transformer, due to low resistance, the winding acts as a short circuit across the terminals of the DC source that lead to the flow of heavy current through the winding resulting in overheating of the winding. Eventually, the source or the winding will be damaged. The effect of current on winding depends on the applied DC voltage, rating of the transformer, winding resistance and how powerful the DC source is. “Nothing will happen if 5Vdc is applied to a 50KVA transformer!”.

Can we get voltage at the secondary side of a transformer when DC voltage is applied?

No, there will be no output at the secondary side. But a sudden spike can be recorded due to initial current inrush and nothing else later.

Caution:

Never connect DC supply to a transformer. For testing current controlled source can be connected. 

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