Depending upon the type of construction used, the transformers are classified mainly into two categories i.e
- Core type transformer
- Shell type transformer
Core type Transformer
In core type construction ,the coils are wound around the two limbs of a rectangular magnetic core. Each limbs carries one half of the primary winding and one half of the secondary winding so as to reduce the leakage reactance to the minimum possible value. The lv winding is wound on the inside nearer to the core while the hv winding is wound over the lv winding away from the core in order to reduce the amount of insulation materials required.
Small Transformers may have cores of rectangular or square x-section with rectangular or circular coils but it is useless in the case of large capacity transformers. In the case of large-sized transformers stepped cruciform core with circular cylindrical coils is used. Such a core employs lamination of different sizes. The cost of manufacturing such a cruciform core is much greater, but the circular coils used are easier to wind and provide more mechanical strength when a short circuit occurs. Cruciform cores are employed because of the high space factor and reduced mean length of turns resulting in a reduced copper loss.
Shell type Transformer
In shell type, the coils are wound on the central limbs of a three-limb core. The entire flux passes through the central limb and divides into two parts going to the side limbs. The x-sectional area of the central limb is twice that of each of the side limbs. Sandwich-type winding is used in such construction.
Sandwich windings are most commonly employed in shell type transformers and allow easy control over leakage reactance. Leakage reactance can be reduced by subdividing the low and high voltage windings into a large number of sections or coils and arranging alternately the hv and lv sections with the lv section nearer to the yoke.
In comparison to concentric windings they have several drawbacks as they are more labour consuming in manufacture ,less stable in respect to short circuits and are more difficult to insulate from each other and form the yoke.so the core type transformer with concentric windings are more common.
If the core type and shell type transformers are compared, then the core type transformer has a longer mean length of iron core and a shorter mean length of coil turn. The core type transformer has smaller x-section of iron and therefore a greater number of turns. Hence, the core type construction provides more space for insulation making it best suited for EHV requirements.
But in the case of shell-type transformer core being used to protect the windings from mechanical damage. The shell-type transformer construction gives better support against electromagnetic forces between current-carrying conductors. These forces are of considerable magnitude under short-circuit conditions. The shell-type construction is commonly used for small transformers where a square or rectangular core cross section is suitable for economic consideration. The shell-type construction needs more specialized fabrication facilities than the core type, while the latter offers the additional advantage of permitting visual inspection of coils in the case of a fault and ease of repair at the substation site. Hence, the core type transformers are used in large high voltage.
Read Also: transformer-efficiency
Difference between Core Type and Shell Type Transformer
|S.N||Basic of comparison||Core Type Transformer||Shell Type Transformer|
|1||Definition||The coils are wound around the two limbs of a rectangular magnetic core.||The coils are wound on the central limb of a three core.|
|2||Cross-section||Cross-section may be rectangular, square, cruciform with circular cylindrical coils.||Cross-section is rectangular.|
|3||Copper||Requires more copper.||Less number of copper is required.|
|4||Laminations||Laminations are usually from alphabets letter L.||Laminations are usually from alphabets letter E & L.|
|5||Limb||It has two limb.||It has three limb.|
|6||Design||Easy in design and construction.||Needs more specialized fabrication facilities.|
|7||Flux distribution||Equally distributed on the side limbs.||Flux passes through the central limb and divides into two parts going to side limbs.|
|8||Leakage reactance||Leakage reactance is not easily possible.||Leakage reactance is highly possible.|
|9||Insulation||Provide space for insulation making best suitable for EHV requirements.||Provides less insulation.|
|12||Mechanical strength||Low||Core is used to protect the winding from mechanical damages.|
|13||Cooling||Better cooling as more surface are exposed to external atmosphere.||Fans are used for cooling.|
|14||Maintenance||Easier to repair as assembly can be dismantled easily.||Difficult to repair because both windings are on the same limbs.|
|15||Application||Used for EHV like power transformer , |
Auto Transformer, high voltage insulation.
|Used for low voltage application like transformers in electronic circuit, small transformer.|