Core Type Transformer

There are several types of transformer core based on its construction. In general it is preferable to accommodate the windings of all the 3Φ in a single core frame. 3Φ Transformers are more economical over three 1Φ transformer. Main advantage of 3Φ transformer is that component of the third and its multiple harmonics of mmf cancel each other, consequently the secondary voltage wave shape are free from distortions due to the third harmonics in mmf. Though 3Φ transformer ratings are large enough, it is difficult to transport, in such cases 1Φ transformers are widely prefered.

The cores of 1Φ and 3Φ are broadly classified as

• 1Φ three-limbed core
• 1Φ two-limbed core
• 3Φ three-limbed core
• 3Φ five-limbed core

1Φ three-limbed core

Here, the windings are placed around the central limb, also known as main limb. Thus the main magnetic flux generated in the central limb gets splited into two parallel return paths provided by the yokes and auxiliary limbs. For the magnetic flux density as that in the central limb, the auxiliary limbs and the yokes need to have the cross section only half of the main limb. Only 1Φ transformer prefer this type of transformer core, as it is more economical than two limbed core construction.

1Φ two-limbed core

The single phase power rating of the transformers are so large that if the winding of full power ratings were to be placed on the central limbs. As 1Φ three-limbed core have 3 limbs, its width would become too large to transport. To overcome such difficulties the windings are splited into two parts and placed around two seperate limbs, called 1Φ two-limbed core. In 1Φ two-limbed core the cross-sectonal area of the limbs and the yokes are identical. Cores of 1Φ two-limbed are bulkier than the 1Φ three-limbed core. The percentage leakage reactance for 1Φ two-limbed core are higher as compared to 1Φ three-limbed core Hence, 1Φ three-limbed cores are highly prefered.

1Φ Three-limbed core vs 1Φ Two-limbed core

1Φ three-limbed core	1Φ two-limbed core
Difficult to transport	Easy to transport
Less bulkier	More bulkier
Percentage of leakage reactance is less	Percentage of leakage reactance is more

3Φ three-limbed core

3Φ three-limbed core typed constructions are mostly prefered by 3Φ power transformer of small and medium power rating. Each phase of the winding is placed around one limb. For each phase of magnetic flux appearing in a limb, the yokes and the remaining two limbs provide the return path. At any instant of time, the phase fluxes Φ_A + Φ_B + Φ_C = 0. In 3Φ three-limbed core typed construction, all limbs and yokes have identical cross section.

3Φ five-limbed core

3Φ five-limbed core typed constructions are mostly prefered by 3Φ power transformer of larger power rating. As 3Φ five-limbed core has five legs , its more difficult to transport. Here, the cross sectional area of the auxiliary limbs and auxiliary yokes are reduced to 40% or more as compared to the main limbs and main yokes.

3Φ three-limbed core	3Φ five-limbed core
Easy to transport	Difficult to transport
Limbs have same diameter	Auxiliary limbs and Main limbs have different diameter
Yokes have different diameter	Auxiliary yokes and Main yokes have different diameter