Losses in a transformer | Types | Classification of losses in transformer

Classification of Transformer losses

There are mainly two types of losses in transformer. They are:

1. Core losses or Iron losses and
   1. Hysteresis loss and
   2. Eddy current loss
2. Copper losses

Apart from these two types there are other losses are present in

Transformer core Materials

In order to increase the efficiency of the transformer various core materials are being used during the developmental processes of transformer. The following are some of the popular core materials that are being used.

Air core transformer:

air core transformer circuit

It is the simplest form of the transformer without any core material or a non-magnetic core material. The two coils primary and secondary are placed near to each other and as the alternating current flows through primary conductor, a magnetic field is established around it and as this field links the second conductor, emf is induced in it. The percentage of the flux linking the secondary coil is small due to absence of low reluctance path. So such a transformer is inefficient.

Iron or steel solid core transformer:

The ability of iron or steel to carry flux is much higher than air. The ability to carry flux is called Permeability. Electrical steels have permeability in the order of 1500 compared to 1.0 for air. This means, the ability of a steel core to carry magnetic flux is 1500 times than air core. Steel core were used in transformers when alternating currents for distribution of electrical energy was first introduced. But still the no load losses, especially the eddy current losses are higher in these transformers and the efficiency is less with solid iron or steel core.

Laminated carbon steel core transformer:

In order to overcome the disadvantage of solid steel core transformers laminated carbon steel cores were introduced, to decrease the no load losses. The laminated carbon steel core gives about 6 watts/kg of core loss at a flux density of 1 tesla.

Hot rolled silicon steel core for transformer:

Switching, over further to hot rolled silicon steel core, it gave substantial reduction to core losses to 2 watts/kg at a flux density of 1.5 Tesla. The core material will have more permeability when magnetised in rolling direction and thus reduces the core losses.

Cold rolled grain oriented silicon steel core transformer:

Presently this is the most widely used core material and is highly efficient. It the cold rolled grain oriented silicon steel further reduced the core losses to 0.75 to 0.85 watt/kg at maximum flux density of 1.92 tesla.

Chemical composition of CRGO:

A part from the primary element (Fe) the other elements are as follows:

• Carbon = 0.01% max
• Manganese = 0.2% max
• Phosphorus = 0.03% max
• Sulphur = 0.01% max
• Silicon = 2.8% to 3.5%

Amorphous core transformers:

Energy efficiency in power sector has become even more relevant today than ever before due to increasing gap between demand and supply. This lead to the invention of new core material called amorphous core. The word “Amorphous” means lacking of symmetrical structure. This material has non crystalline molecular structure and often referred to as glassy metal. They exhibit very low hysteresis losses and are highly efficient and longer life. But these are expensive but can recover in the form of energy savings.

Also Read

Losses in Transformer

Trasformer
Construction of transformer

Construction of a Transformer

The basic parts of a transformer consist of two coils having mutual inductance and a core material. Apart from these there are other parts such as the outer container, insulating and cooling medium, bushings to obtain the terminals of the transformers out. During the construction of transformer first the windings are wound and then the core laminations are pushed through openings of coil and the core is prepared. Then the laminations are tightened with clamps and bolts.

Core of a transformer

The core is generally made of silicon steel laminations insulated from each other by a light coating of varnish. The most popular material for core is cold-rolled grain oriented sheet steel (C.R.G.O.). It reduces the core loss and has high permeability when magnetized in rolling direction. The laminations are used to reduce the eddy currents. For a 50 Hz transformer the thickness of laminations is around 0.35 mm while for a 25 Hz transformer it is 0.5 mm. The laminations are cut in L, T and I shapes and are assembled to form a core. The vertical parts of the core are usually called limbs or legs and the horizontal parts are called yoke. The core is assembled in such a manner that adjacent layers are staggered. It avoids continuous air gap, provide low reluctance and provide good mechanical strength.

For high frequency transformers which are employed in communication circuits the core is made of powdered ferromagnetic alloy.

Types of Core construction:

Generally there are two types of construction of core based on the manner in which the winding and the core are placed. They are

1. Core type
2. Shell type

Core type Transformer:

Core type transformer

In core type, the windings surround a considerable part of the core and require more conductor material but less iron material. For a single phase transformer core type has two legs. To reduce leakage flux half of the both low voltage (LV) and high voltage (HV) windings together are placed on one leg and the remaining halves are placed on the other leg. To minimise the insulation requirement LV windings which are easy to insulate are placed near the core and the HV winging is placed outside.

Shell type Transformer:

shell type transformer construction

In shell type transformer, the core surrounds the major part of the windings and require more core material and less conductor material. For a single phase shell type transformer there are three limbs. Both the LV and HV windings are wound around the central limb and are sandwiched over one another.

Air-core transformers:

This type of transformer is used in radio devices and certain measuring instruments. The core in these transformers is made on non-magnetic materials.

Cruciform of Core:

Two stepped core

For small transformers the limbs of the core are rectangular but it is not preferred for large transformers. In large sized transformers, circular cylindrical coils are used; for such transformers the cruciform core is used with multiple steps. It gives better space factor. To form the steps different sizes of the lamination are required. To form a single step core, it require to different sized laminations. Generally three stepped core is the most common.

Transformer tank:

The assembly of the power transformer is placed in a suitable container or casing and it can be called as a transformer tank.

Cooling:

Low power transformers are generally air cooled whereas large power transformers are immersed in oil for cooling. The circulating oil in the transformer apart from cooling the transformer provides better insulation. In some transformers the sides of the transformer are corrugated with radiators mounted on the sides which provide a path for better cooling.

Also Read:

Transformer
Different Core materials used in transformer

Transformer

---

Transformer is a static device or a machine that transfers electrical energy from one electrical circuit to another through medium of magnetic field without change in frequency. It is an electromagnetic energy conversion device and can raise or lower the voltage with a corresponding decrease or increase in current. A transformer consists of two conducting coils (or windings) having a mutual inductance. These coils are wound on a laminated core made of high permeability magnetic material. The coil which receives electrical energy from supply terminals is called