Explain the construction and working principle of a single-phase transformer with a neat labelled diagram.

Construction:

• Core: made of thin (0.35 to 0.5 mm) silicon-steel laminations, insulated from each other by varnish, to reduce hysteresis loss (silicon steel) and eddy current loss (laminations). It provides a low-reluctance path for flux.
• Core type: windings surround the two limbs of a rectangular core (single magnetic circuit). Shell type: both windings are on the central limb, which carries full flux; outer limbs carry half the flux (double magnetic circuit).
• Windings: primary (connected to supply) and secondary (connected to load), made of insulated copper; LV winding placed next to the core, HV winding over it, to reduce insulation requirement and leakage.
• Insulation, tank, transformer oil (cooling and insulation), conservator, breather, bushings in larger transformers.

   Core type                         Shell type
  +--------------+              +-----+-----+-----+
  |##|      |##  |              |     |#####|     |
  |##| core |##  |              |     |#P S#|     |
  |##|limb  |##  |  P on left,  |     |#####|     |
  |##|      |##  |  S on right  |     |     |     |
  +--------------+  limb        +-----+-----+-----+

Principle: mutual induction (Faraday's law). When an alternating voltage is applied to one winding, the alternating flux it produces links the other winding and induces an EMF in it.

            laminated iron core (flux phi)
        +-------------------------------+
        |   ---> phi (alternating)      |
  V1 o--|-- N1 turns            N2 turns|--o  E2 / V2 -> load
   ~    |  (primary)           (secondary)|
     o--|--                           --|--o
        +-------------------------------+

Working: the primary winding (N₁ turns) is connected to an AC supply V₁. It draws a small magnetising current which sets up an alternating flux φ in the low-reluctance laminated core. This flux links both windings. By Faraday's law, a self-induced EMF E₁ is induced in the primary (it nearly balances V₁) and a mutually induced EMF E₂ in the secondary. If a load is connected, secondary current I₂ flows; its MMF tends to reduce the flux, so the primary draws extra current I₁' such that N₁I₁' = N₂I₂, keeping the flux nearly constant. Energy is thus transferred from primary to secondary through the magnetic field at the same frequency, without electrical connection. A transformer works only on AC (DC gives constant flux, no induced EMF).

EMF equation: \( E = 4.44 f\phi_m N \) V; transformation ratio \( K = \dfrac{N_2}{N_1} = \dfrac{E_2}{E_1} = \dfrac{V_2}{V_1} = \dfrac{I_1}{I_2} \) (K > 1 step-up, K < 1 step-down).