--- title: "Core type versus shell type transformer" subject: "Electric Machine Design" chapter: 3 key: "semester_5/electric_machine_design:core-vs-shell-transformer" source: https://www.bctengineer.com/q/semester_5/electric_machine_design/core-vs-shell-transformer site: BCT Engineering --- # Core type versus shell type transformer Electric Machine Design · Chapter 3: Transformer Design - Asked in: 2080 Bhadra, 6 marks; 2074 Asoj, 6 marks; 2074 Chaitra, 6 marks (asked 3 times) - Video: [Core Type vs Shell Type Transformer | Complete Construction Guide with 3D Animation](https://www.youtube.com/watch?v=9DVBhmfHGfs) — Circuit Masterclass - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/core-vs-shell-transformer - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-3 ## Question Differentiate between core type and shell type transformer on the basis of construction, mechanical design, leakage reactance and cooling. ## 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. ## Answer 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). ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a shell type transformer: A. Windings surround the limbs with two flux paths B. Only high voltage windings are used C. The core surrounds the windings, and the flux divides in outer limbs D. Windings are placed on both outer limbs Answer: C. Shell type has windings on the central limb and a core around them, with flux splitting into two paths. (Key point: Construction) 2. Which has lower leakage reactance? A. Core type, with concentric coils B. Both are equal C. Neither, it is zero D. Shell type, using interleaved sandwich coils Answer: D. Sandwich coils reduce leakage flux, and the number of sections can control it. (Key point: Leakage reactance) 3. Which type resists short-circuit forces better? A. Shell type, as core and coils form a rigid block B. Core type, as coils are fully braced C. Core type, as it has more limbs D. Neither needs bracing Answer: A. The shell type is more rigid; core type coils need extra support. (Key point: Mechanical strength) 4. How do the two compare in cooling? A. Shell coils cool easily and the core cools poorly B. Core type coils are exposed so oil reaches them easily, while shell coils are enclosed C. Both cool identically D. Core type cools poorly because the coils are hidden Answer: B. Core type windings are exposed and cool well; in shell type the windings are enclosed by the core. 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