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

This is a quite common question asked by many. In fact, it is necessary to know the answer for why we don’t apply DC voltage to a transformer coil. You may come across the same question during interviews too. This article can help you clear your query.

DC voltage applied to transformer winding.

 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 the 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. 

Similar questions

  1. Why does a transformer not work on dc supply?
  2. If the primary of a transformer were connected to a dc power source
  3. Why doesn’t a transformer work with direct current?

This Post Has 12 Comments

  1. Gary Wayne McSpadden

    You will also get a voltage spike on the secondary when power is disconnected from inductive kick.

    1. admin

      That is a very good point to be noted. You will also get a voltage spike on the secondary when power is disconnected from the inductive kick. Thank you for your contribution.

    2. Harvey Raygrove

      Electromagnetic induction is defined as a moving magnetic field , OR a change in a stationary magnetic field. While purely applying a dc source will wreak havoc, when dc is applied a large field builds around both windings. When the dc source is interupted, then instead of having continus induction, one large induction takes place on the secondary from the field collapse. If you disagree please remove a spark plug wire while the engine is running and place your finger inside:)

  2. Rahul

    What if we short the output of transformer and give it the d.c supply at input. Having a load connected in the input terminal.

    1. admin

      It does not matter whether you connect a load at the secondary of the transformer or not, due to low resistance, the winding acts as a short circuit and lead to the flow of heavy current and damage the winding.

  3. Jar Jar

    How come it is possible to use DC to power an electromagnet though, as in, for example, an EI shape microwave transformer core with the original primary coil, and the work piece as a (single turn) secondary coil?

    I know it is not effective or smart, but this is the arrangement most people seem to go for.

  4. Rui Figueiredo

    Everything ok generally speaking, however before the “power electronics” some electrical application used controlled DC to a separated wind as the way to saturate the magnetic field to change output voltage or inductance in another wind
    It was used by Philips decades ago in analytical X-ray

  5. Faran

    If during performing insulation resistance test of transformer by mistake the start point is connected to ground what can happen.

  6. Flash Man

    So, then why not paults DC on the primary. Other then it will be on the noisy side maybe but, the heat is dissipated to a lesser value and the secondary has what it wants, the on to off state to make the magnetic field that excite the core and winding of the secondary. In short you are introducing a square wave to the primary at a given frequancy. And the frequance can be changed to match the transformers iron core so it would not be noisy as if even 60 hz was used. I wouldn’t believe it would have to pass the zero side into the negative side of DC to work. This should produce an output on the secondary in theory. Just as long as it did go to zero volts then back up again.

    1. Electrical Classroom

      Here we are talking about applying a constant DC voltage to the transformer primary. If it was a square ware, as you said the scenario will be different.

  7. Bret

    “But a sudden spike can be recorded due to initial current inrush and nothing else later. ” Could this phenomenon have any practical application within a circuit? ie. spot weld, initial ignition etc.

    1. Electrical Classroom

      The current inrush can damage the winding and the insulation.

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