Why DC current is not used in transformers?

Answer 1

A transformer couples energy from the input winding to the output winding by means of a changing magnetic field. AC current is a changing electric field, which produces changing magnetic fields when put to a wire and so is ideal for a transformer. This fact is why virtually all power distibution is AC. If DC were put to a transformer, once the winding magnetized in a fraction if a second, the current drawn would be so much it would blow the circuit. In other words, you can not use DC on a transformer because the transformer would be blown up by it.

Another Answer

When any current passes through any conductor, invisible Electro-Magnetic fields are produced in the immediate space around the conductor; the strength and distance of these fields are proportional to the amount of current that passed through the conductor. The electric field is in the plane of the conductor while the magnetic field is perpendicular to the conductor. If the current is DC, the fields grow to their maximum and stay there. If the DC source is removed, the fields collapse. If there is enough current passing through the conductor, the fields will expand, break away from the conductor, and continue to propagate through space at the speed of light. This is the principal of a radio and TV station antenna. This propagation is also a direct loss of source energy.

Induction is described as the relative motion of a conductor through a magnetic field. This can either be the magnetic field in motion with a fixed conductor, or a fixed magnetic field with the conductor in motion. Either of these conditions will induce an electric current in the conductor.

Transformers work under the principal of induction. There is a primary coil and a secondary coil in transformers. The core of the transformer can be air, iron powder, or solid iron; among other material configurations. The ratio of the windings from the primary to the secondary determines if the voltage will be stepped up or down in the secondary. If there are twice as many windings in the secondary as are in the primary, the output of the secondary will be twice the voltage and half the current of the primary. If there are twice as many windings in the primary as are in the secondary, the output of the secondary will be half the voltage and twice the current of the primary. This rule does not take into consideration any loss in the field generation due to: counter EMF, impedance, propagation, etc.

Since the transformer relies on the principal of induction, there has to be a relative motion between the magnetic field and the coil. If you apply a DC source to the primary of the transformer while watching the output of the secondary, what you'll see is a spike on the secondary where the magnetic field in the primary expanded, cutting through the secondary coil, but did not contract. This does not constitute the fundamental requirements of sustained induction. The above contributor is correct when saying the primary would draw as much current as the DC source could provide, usually ending in some catastrophic failure. The reason the DC current could run away is there is minimal resistance in the primary coil wire to DC current.

There is, in fact, a form of resistance to an AC source applied to a coil. This is known as impedance and is caused by some of the factors mentioned above, mainly "Counter EMF". Counter EMF is a phenomenon produced because one loop of the winding is lying next to the next loop in the winding; when current is sent through the coil. When the magnetic field in the first loop starts to expand, it does so in a particular direction. The "Right Hand Rule of Thumb" can be applied to this scenario to determine which direction the magnetic field is traveling around the wire. Imagine using your right hand to wrap your fingers around a wire with your thumb pointing in the direction of current flow. Your fingers point in the direction of the magnetic field. When this magnetic field expands through the next loop in the coil, it induces a small amount of current in the opposite direction of the source current, which acts as a dynamic resistance to the main current source. This impedance is why current lags voltage by 90° in an inductive circuit.

This subject is both broad and deep.

Answer 2

Because transformers don't operate on DC. They require a varying magnetic field to induce voltage in the secondary. DC will simply magnetize the core.