Single Phase Transformer – Working Principle, Diagram, Construction & Applications

By BYJU'S Exam Prep

Updated on: September 25th, 2023

A Single Phase Transformer is a static device that transfers electrical energy from one circuit to another circuit without a change in frequency. Mutual induction is the working principle behind the operation of single phase or three-phase transformers. During single phase transformer construction, the yoke and limb carry the flux and these are designed in such a way that they carry maximum flux.

Generally, single phase transformer construction has two main parts i.e., core and windings. A core is generally made of a low reluctance material with high permeability to allow maximum flux to flow through it. The vertical portion of the core on which the coil is wound in the construction of single phase transformer is called the limb and the horizontal portion is called the yoke. Let us discuss the single-phase transformer along with its working principle, efficiency, construction, parts, and diagram of single phase transformer in the upcoming sections.

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What is Single Phase Transformer?

A single phase transformer is a type of transformer that only works with single phase power. This device is a passive electrical device that uses electromagnetic induction to transfer electrical power from one circuit to another. It is commonly used to lower (‘step down’) or raise (‘step up’) voltage levels between circuits.

A magnetic iron core serves as a magnetic component, while a cooper winding serves as an electrical component in a single phase transformer. Check out the single phase transformer diagram shown below:


Fig: Single Phase Transformer Diagram

Single Phase Transformer Working Principle

Single Phase Transformers are based on the principle of “mutual induction.” When current flows through a wire a magnetic field is produced. When a conductor passes through a magnetic field, a current flow will be induced through the wire. The method of transferring electrical energy by a transformer is done indirectly. Electrical energy is first converted into magnetic energy, then reconverted back into electrical energy at a different voltage and ampacity. Magnetism and electricity are closely related.

  • By altering the number of windings on the primary and secondary, we can alter the number of volts and amps between the source and the load.
  • The current in the secondary coil always changes by the inverse of the ratio by which the voltage changes.
  • If the voltage is raised to n times its original value by the transformer, the current in the secondary coil will be reduced to one-nth the value of the current in the primary coil.

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Efficiency of a Single Phase Transformer

The efficiency of a transformer will be generally between 95 to 99% as the construction of single phase transformer does not include any rotating parts. Hence, we say that a transformer is a static device. By bringing the two magnetic coils together during a single phase transformer construction, the efficiency of single phase transformer can be improved.

Transformers are generally- single phase transformer construction and third–phase constructions. Three-phase transformers can be further constructed in two ways.

  • By the bank of three single phase transformers.
  • By individual three-phase transformer.

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Construction of Single Phase Transformer

Generally, a transformer has two main parts: Core and Windings. There are many other parts in the construction of single phase transformer. But core and windings are the key parts in single phase transformer construction. By using core and winding, along with insulation, we can construct a hands-on single phase transformer.

Transformer Core

The core of a transformer is generally made of two materials. The first material is silicon steel. Silicon steel is used in order to reduce hysteresis losses. About 4 to 5% of silicon is added to increase the resistivity of the material and so to reduce the eddy current losses.

Wh= ηBxmaxfV


  • η = Steinmetz hysteresis coefficient
  • f = frequency of supply
  • V = volume of material

Here, x varies between 1.2 to 1.6 in silicon steel material

Also, the structure of silicon steel is body-centered cubic (BCC) structure. The permeability will be maximum along the edges and minimum along the diagonals of the cube.

Another material for transformers is cold-rolled grain-oriented (CRGO) steel. In the hysteresis loss equation mentioned above, the value of x for CRGO will be 1.8. Hence, we can conclude that maximum flux densities can be achieved with CRGO which reduces the size and cost of the machine. Practically, a transformer is made of CRGO steel.

Whatever the material it may be, the core is laminated in order to minimize the eddy current losses. As eddy current losses are directly proportional to the square of thickness, if the thickness is reduced, then we can see a good reduction in eddy current losses, which results in reduced core losses and in turn, increased efficiency. Eddy current loss can also be minimized by increasing the resistivity of the material (silicon steel case).

Hence, we can say that during construction itself, the core of a transformer is laminated into thin laminations, and insulating materials such as varnish, and impregnated paper is placed in between them in order to avoid short circuit situations. This process of bunching the core laminations is called core staggering. If we observe properly, this connection looks similar to that of a capacitor i.e., two conducting plates separated by an insulator. So, we can also experience dielectric losses in a transformer. But we neglect them as these are not in considerable amounts.

A core generally provides a low reluctance path for the flow of magnetic flux in the circuit. This core is generally of the following two types in a single phase transformer.

  • Core type: Windings surround the core in this type. It is generally used for high voltage and low KVA transformers. Also suitable for high voltage power transmission.
  • Shell type: Here, windings are surrounded by the core. These are generally used for low voltage, large KVA transformers. Also suitable for low voltage power transmission.

Transformer Windings

The second most important part in the construction of single phase transformer is the winding. Generally, the copper winding is preferred for a transformer. This winding is wound on both the primary and secondary. The winding which is connected to the supply is called primary winding and the winding which is connected to the load is called secondary winding.

Since copper is a conducting material and it has a certain resistance, so we can say copper losses also in a single phase as well as three-phase transformers. These two windings i.e., primary, and secondary are wound on the common core. Both the windings collectively experience copper losses.

Single Phase Transformer Parts

There are many other important parts in the construction of a single phase transformer. They are transformer oil, conservator tank, bushings, Buchholz relay, explosion vent, tap changers breather with silica gel crystals etc.

  • Transformer oil: Mineral oil or vegetable oil is used for transformer oil. This oil serves two purposes: It acts as a coolant and as an insulating medium between the transformer tank and core. The colour of fresh dielectric oil is pale yellow and after extreme use, it becomes black (scrap oil).
  • Conservator tank: It is an airtight cylindrical metallic drum just to store the oil temporarily and allow the transformer to expand and contract i.e., breathing conveniently during temperature variations.
  • Buchholz relay: It is a gas-actuated relay, and it is used for the protection of all the internal and incipient faults in a transformer. It is generally mounted between the main tank and the conservator tank.
  • Breather: The transformer while breathing will have certain moisture entering the tank. This moisture content is absorbed by silica gel crystals which are present in the breather. The colour of fresh silica gel is blue and after extreme use, it turns out to be pale pink. The colour of fresh silica gel is blue and in good condition. After a while, it turns out to be violet and after final extreme condition, it turns out to be pale pink. Then we can conclude that silica gel crystals need to be replaced with new ones for further operations.
  • Bushings: Bushings are just used to bring out the HV and LV windings through them. We can decide the LV and HV windings based on the length of the bushings. The lengthier bushing will indicate HV winding whereas the shorter bushing indicates LV winding.
  • Tap changer: Within a transformer, all the Voltage variations or changes are balanced using tap changers. These are of two types- on load and off load tap changers. If the tappings are changed without isolating the transformer from the supply, it is an on-load tap changer and vice versa is called an off-load tap changer.
  • Explosion vent: During internal faults, sometimes the transformer explodes due to the excessive flow of hot oil. This can be avoided by an explosion vent. When an internal fault occurs, the boiling hot oil expels out through the explosion vent, and thus exploding of the transformer can be avoided.

Single Phase Transformer Applications

Single phase transformers are commonly utilized as electronic components in commercial low voltage applications. They work as a step-down voltage transformer, lowering the household voltage to a level appropriate for powering devices. The applications of a single phase transformer are given below.

  • For voltage adjustment in television sets.
  • Inverters are used to boost energy in homes.
  • To provide power to non-urban areas.
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