There are two types of transformers, categorized by the shape of magnetic core of the transformer - (i) Core Type Transformer and (ii) Shell Type Transformer.
Table of Contents
✔ Core Type Transformer
Table of Contents
✔ Core Type Transformer
✒ Winding
✒ Winding
Definition of Core Type Transformer
The transformer's magnetic core is made up of laminations, which form a rectangular frame. The laminations are cut in the form of L-shaped strips shown in the following figure. For single phase core type transformer we use single window core. The alternating layer is stacked differently to eliminate continuous joints in order to avoid the high reluctance at the joints where laminations are butted against each other.
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| L shaped core |
The primary winding and the secondary winding are interleaved on both core to reduce the leakage flux. Half of each winding is wrapped side by side or concentrated on the one limb and another half is wrapped on another limb as shown in below figure. For simplicity, primary winding and secondary winding are placed on the separate limbs of the core.
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| Core Type Transformer |
Winding of Core Type Transformer
Between the core and low voltage winding and between the primary and secondary winding, the insulation layer is provided. The low voltage winding is always placed close to the core for reducing the insulation. The winding is cylindrical, and the lamination is later inserted.
In the core type transformer, the core is surrounded by winding. Since the flux is constant in the entire core, the reluctance of each and every limb connected in series. Then the series magnetic circuit is like as -
Properties of Core Type Transformer -
- Less Insulation
- High voltage rating.
- Require more amount of copper(Cu) for winding, that means the resistance of winding increases. But we want I²R losses should to be less. So it has low current rating.
- Transformer has equal cross section area.
- It has interleaved winding.
Application of Core Type Transformer
Core type transformers are common in high voltage applications such as for distribution transformers, power transformers and obviously auto transformers. The reasons are -
- As high voltage corresponds to high flux. and to reduce the iron losses we have to use thicker core. So core type transformer is better option.
- In high voltages application needed heavy insulation, but in the core type putting insulation is easier.
Disadvantages
Leakage flux is the main drawback of core type transformer. In this arrangement the leakage flux is greater than that of the shell type transformer. This affects in the performances of the transformer, but this arrangement is still better for large-scale transformers due to easier access during maintenance to the windings.
To know what is Transformer - click here
Definition of Shell Type Transformer
'E's and 'I's shape laminations are used to make the core of shell type transformer. To reduce the reluctance at the joint where the laminations are butted against each other, the alternating layers are stacked differently.
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| E and I shaped Core |
The single phase shell type transformer has three limbs. This design improves the core's mechanical strength. It also enhances the protection of windings against outside mechanical shocks. The windings of the HV and LV are wrapped around the central limb and the outer limbs are empty. The total flux(φ) is carried by the central limb whereas the side limbs carries half of the total flux(φ/2). That's why to accommodate the flux, the width of the central limb is about to twice of the outer limbs.
The magnetic flux flows through two closed magnetic paths that reduce the core losses and thus increasing the transformer performance. So, the shell type transformer gives more output than similar core type transformer. Using shell type transformer, we get one of the main benefit is that the core provides a good support against the electromagnetic forces that occur between the current carrying conductors in the short-circuit case.
Winding of Core Type Transformer
Both the HV and LV windings are placed on the central limb. The low voltage (LV) winding is placed nearer to the core, and the high voltage(HV) winding is placed beyond the low voltage winding to reduce the insulation costs between the core and the low voltage winding. That means the high voltage coil is sandwiched between two low voltage coils.
As the HV and LV coils both are wounded on the central limbs, the quantity of conductor is needed for wounding is less compare to similar core type transformer. But the requirement of the insulation materials is more as the windings are sandwiched on each other to avoid the risk of short circuit faults. That's why due to its winding structure, the design of cell type transformer is little bit complex. Any fault in the inner windings can only be attended after all outer windings have been removed, hence the winding maintenance in shell style transformer are very difficult.
Cooling system of Shell Type Transformer : In shell style transformer, Forced oil / forced air cooling is necessary as heat produced during working, can not be easily dissipated from windings due to the surrounding yokes and limbs.
Due to flux distribution, we can say it is a parallel circuit because only in the parallel circuit flux can be divided. So the parallel circuit is like as -
Properties
- Winding is surrounded by core.
- More insulation is required.
- Since it already has more insulation, So we can not increase the insulation and can not increase the voltage rating. So, it has less voltage rating.
- Less copper is required that means resistance is less.
- Since resistance is less, we can apply higher current. It has higher current rating.
- If central limb cross sectional area = A then outer limb cross sectional area = A/2. ∴ Outer limb cross section = 50% of inner limb cross section
Applications of the Shell Type Transformer
Shell type transformers are mainly used in low voltage applications and are very commonly used in low voltage power circuits as well as in the electronic circuits. Such transformers are often used to optimized the cost of a circuit because these transformers have a rectangular or square cross-sectional core which costs less.
Disadvantages
Because of its complexity, special manufacturing facilities are needed for the construction of a shell-type transformer which leads to higher production and labor costs.
Differences between Core Type Transformer and Shell Type Transformer
Basis for Comparison
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Core Type Transformer
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Shell Type Transformer
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Definition
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Core is surrounded by winding.
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Winding is surrounded by Core.
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Lamination Shape
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‘L’ shaped
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‘E’ and ‘I’ shaped
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Cross Section Area
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Cross section may be square.
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Cross section is rectangular in
shape.
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Required Copper
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More
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Less
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Limbs
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Two
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Three
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Insulation
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More
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Less
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Flux Distribution
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Equally distributed on the two limbs
of the core.
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Inner limb carry total flux and
outer limb carry half of the flux.
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Winding Placement
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The HV and LV windings are placed on the side limbs.
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Both HV and LV windings are placed on the central limbs.
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Magnetic Circuit
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Series
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Parallel
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Losses
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More
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Less
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Maintenance
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Easier
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Difficult
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Mechanical Strength
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Low
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High
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Output
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Less
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High
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Cooling System
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Oil is used to cool down the system
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Forced air is used.
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Others name
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Cylindrical Winding or Concentric
Winding
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Sandwich or Disc Winding
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