Shell Type Transformer: What is it? (& Applications)

Contents

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Key learnings:

Shell Type Transformer Definition: A shell type transformer is defined as a transformer with a magnetic core using ‘E’ and ‘L’ shaped laminations.
Core Structure: The core has three limbs, with the central limb carrying all the flux and side limbs half the flux, enhancing strength and protection.
Winding Design: HV and LV windings are alternately wound around the core, needing less conductor but more insulation.
Cooling System: Requires forced air and/or oil cooling to dissipate heat effectively from the windings.
Applications: Shell type transformers are used for low voltage applications and help optimize circuit expenditure.

A shell type transformer has a magnetic core that surrounds the windings. Depending on that core construction, we can categorize transformers into two types:

Shell Type Transformer
Core Type Transformer

Core of Shell Type Transformer

We use ‘E’s and ‘L’s shape laminations to make the core of the shell-type transformer. That stacked E and L pack is common on small units; large shell-form cores use other punchings too.

Limb of Shell Type Transformer

A single-phase shell type transformer has a core with three limbs. The closed core braces the windings against shock. The HV and LV windings are on the central limb, which carries all the flux, while side limbs carry half the flux (φ/2). Thus, the central limb’s cross-section is twice that of the side limbs.

Magnetic flux in a shell type transformer flows through two closed paths. Core loss and efficiency of transformer still depend on flux density and steel grade. The same core does give strong support against electromagnetic forces during short-circuits.

Winding of Shell Type Transformer

HV and LV windings are wound longitudinally along the core alternately. The HV coils are sandwiched between two LV coils as shown in the figure below.
Since both HV and LV coils are wound on the central limb, shell type transformers often need less conductor material than core type transformers. They need more insulation bulk because of the sandwich. An inner-coil fault means removing the outer windings, so repair is slower.

Cooling System of Shell Type Transformer

Heat is harder to shed because the yoke surrounds the coils. Small dry shells still use natural air. Larger units may need forced air or forced oil.

Applications of Shell Type Transformer

Shell type transformers are common in low-voltage power and electronic circuits. Large shell-form power transformers also exist. Square or rectangular punchings can be cheaper to stamp than a circular core, but that is a factory choice, not a universal saving.