Which has lower leakage reactance?

Shell type, using interleaved sandwich coils

Core type, with concentric coils

Which type resists short-circuit forces better?

Shell type, as core and coils form a rigid block

Core type, as coils are fully braced

Core type, as it has more limbs

How do the two compare in cooling?

Core type coils are exposed so oil reaches them easily, while shell coils are enclosed

Shell coils cool easily and the core cools poorly

Core type cools poorly because the coils are hidden

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Core type versus shell type transformer

Short answer

Core type has windings surrounding the core, concentric coils, higher leakage reactance, easier cooling. Shell type has core surrounding windings, interleaved sandwich coils, lower leakage reactance, better short‑circuit strength but poorer coil cooling.

Differentiate between core type and shell type transformer on the basis of construction, mechanical design, leakage reactance and cooling.

In a core type transformer the windings surround the limbs of the core; in a shell type transformer the core surrounds the windings (the windings sit on the central limb, and the flux divides into two outer limbs).

   Core type (1-ph)          Shell type (1-ph)
   +--------------+          +---+------+---+
   | ##        ## |          |   |##  ##|   |
   |[##]      [##]|          |   |##  ##|   |
   |[##]      [##]|          |   |##  ##|   |
   | ##        ## |          |   |##  ##|   |
   +--------------+          +---+------+---+
  windings on 2 limbs     windings on centre limb

Basis	Core type	Shell type
Construction	Windings surround the core; one window per 1-ph unit	Core surrounds the windings; two windings in two windows
Magnetic circuit	Single flux path, limbs carry full flux	Flux divides; outer limbs carry half flux
Winding type	Concentric cylindrical coils	Interleaved sandwich (disc) coils
Core cross-section	Stepped (cruciform etc.) for round coils	Usually rectangular
Mechanical design	Coils less braced; need extra support against short-circuit radial/axial forces	Core and coils form a rigid block; better strength against short-circuit forces
Leakage reactance	Higher, since coils are concentric with fewer interleaves	Lower; can be controlled by number of sandwich sections
Cooling	Windings exposed, easy for oil to reach; better cooling of coils	Windings enclosed by core, coils cool less easily; core is better cooled
Repair	Easy to dismantle and repair coils	Difficult
Use	High voltage, most power and distribution transformers	Low voltage, high current, furnace and small transformers

In practice the core type is preferred for high voltages because insulating concentric cylindrical windings from the core is easier.

✓ Answer checked by Claude Sonnet 5.5

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