How Power Transformer Works?

From the basic principles of magnetic induction, it is not difficult to see how a rudimentary transformer could be constructed. If a conductor carrying a changing current is brought near a second conductor, then the changing magnetic flux surrounding the first conductor will be linked to the second conductor and will induce a voltage.

Such a rudimentary transformer is depicted in Figure 1.3.
An AC voltage is connected to a primary conductor, shown as the left hand solid bar in Figure 1.3. In response to the voltage, an AC current flows, setting up a time-varying magnetic field surrounding the primary conductor.

A secondary conductor, shown as the right-hand solid bar, is located in proximity to the primary conductor so that the magnetic flux surrounding the primary conductor links the secondary circuit. According to the law of induction, there will be an induced voltage E around the path surrounding the time varying flux.

The configuration shown above is not very efficient in transferring energy because only a small portion of the total magnetic flux surrounding the primary conductor will be linked to the secondary circuit. In order to improve the efficiency of the rudimentary transformer, the magnetic field needs to be channeled in such a way that most of the flux produced by the primary conductor is linked to the secondary circuit.

This is accomplished by surrounding the primary and secondary conductors with a magnetic core material having an affinity for magnetic flux. This modification is shown in Figure 1.4. By adding the magnetic core, essentially all of the magnetic flux produced in the primary conductor is linked to the secondary conductor. Therefore, the efficiency of the rudimentary transformer is greatly increased.
Various types of core materials exist. The important physical property is the permeability constant μ, given in units of N/A2. The relative permeability μr is the permeability constant divided by the vacuum permeability μ0.

Values of μr for some common magnetic core materials are as follows:
   SiFe (unoriented) 400
   SiFe (oriented) 1500
   50–50 NiFe (oriented) 2000
   79 Permaloy 12,000–100,000

A grain-oriented silicon steel conducts magnetic flux 1500 times better than a vacuum.  The ratio of the flux density B and the field intensity H is equal to the permeability of the medium μ:
μ = B/H
H = B/μ

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