construction

construction

1. What Construction Refers To (Single-Phase Transformer)

Construction of single-phase transformer = core, windings, insulation, tank, cooling that realize efficient mutual coupling with low loss.

2. Main Parts (exam diagram — label 6)

Part	Construction	Function
Iron core	Laminated silicon steel (CRGO, 0.35 mm stampings) E-I or shell type; stacked to reduce eddy currents (lamination insulates), mitred joints reduce reluctance	Provides low-reluctance path for mutual flux $φ$ (links both windings), confines flux (low leakage)
Windings (coils)	Copper (enamel insulated) wound as primary and secondary; concentric (one over other) or sandwich (interleaved) to reduce leakage flux	Carry currents, link via mutual flux; concentric gives tight coupling ($k≈0.98$)
Insulation	Paper, oil, varnish between turns, between windings and core	Prevent short between turns / windings / core
Tank and oil (for power transformers > few kVA)	Steel tank filled with transformer oil (mineral oil)	Insulation and cooling (oil circulates by convection, radiators)
Bushings	Porcelain + oil	Bring out terminals insulated from tank
Cooling system	ONAN (Oil Natural Air Natural), ONAF (forced air fan), etc.	Dissipate core ($P_{core}$) + copper ($I²R$) losses
Conservator + breather	Conservator tank above main with oil expansion, breather with silica gel (absorbs moisture)	Accommodate oil expansion, keep oil dry

Types by core:

• Core type: windings surround core limbs (two limbs, one winding per limb, flux single path) — easier to wind, more leakage.
• Shell type: core surrounds windings (three limbs, central limb wound, flux splits) — less leakage, more iron, used for large.

3. Why Laminated Core

Solid iron would have large eddy current loop ($e∝dφ/dt$ in conducting core) ⇒ $I²R$ loss. Laminating into thin insulated sheets breaks loops ⇒ eddy loss ∝t² reduces drastically. Use CRGO (Cold Rolled Grain Oriented) steel for low hysteresis loss (domain aligned).

4. Exam Points

• Draw labelled diagram (core, primary, secondary, tank, oil, bushings) — 7 marks, very frequent (almost certain to ask construction).
• State core is laminated silicon steel to reduce eddy currents — 5 marks, very frequent.
• Distinguish core vs shell type — 5 marks, frequent.
• Mention copper windings and oil cooling — 3-5 marks, frequent.