What is a single and three phase transformer and what are the difference between them? Three phase transformer connection

What is a transformer

Transformers is used for power transmission, they are used throughout alternating current systems from generating plants to our home. Transformers can increase or decrease the voltage. Voltage is increased for the transmission of power for long-distance power lines, it is then reduced by other transformers while the power enters our house. Transformers transfer electrical energy indirectly, electrical energy is converted into magnetic energy, then it is reconverted back into electrical energy at a different voltage.

What is an ideal transformer

An ideal transformer is a transformer that has no losses, its winding won’t have any ohmic resistance or magnetic leakage, and therefore no core lose. But it is impossible for practical, yet the appropriate characteristic of an ideal transformer will be used in characterized the practical transformer.

What is a single-phase transformer

Single-phase transformers mostly have a two-legged iron frame, with one half of the primary and secondary winding wounded on each leg. It consists of two highly inductive coils wound on an iron or steel core, the winding which is connected to the AC supply is called the primary winding and the other one is secondary. The AC supply is connected to the primary winding and the load is connected to the secondary. The primary and secondary windings are isolated from each other and because of that, there is no physical connection between the primary and secondary windings. The low voltage and high voltage windings are concentric with each other with low voltage winding placed on the inner side nearer to the core. The operation of the single-phase transformer, when the primary winding is connected to the single-phase AC-supply then AC current will start flowing through it, the AC primary current produces an alternating flux in the core, most of this changing flux gets linked with the secondary winding through the core. The varying flux will induce a voltage into the secondary winding according to the faraday’s laws of electromagnetic induction. Because of the primary current, there is an induced voltage in the secondary winding due to the mutual induction so the EMF induced in the secondary winding is called mutually induced EMF.

What are the advantages of three-phase transformer over single-phase transformer

Three-phase transformers are used to power larger motors and other heavy loads. A three-phase transformer is more capable to handle heavy loads and they are economical while comparing to the coast and application of single-phase transformer. Three-phase power can be transmitted over long distances with smaller conductor sizes.

What is a three-phase transformer

A three-phase transformer is constructed by winding three single-phase transformer on a single core. Transformers will be placed in an enclosure and after that, it will be filled with dielectric oil. Dielectric is a non-conductor of electricity, so it provides the electrical insulation between the windings and the case. A three-phase transformer is either constructed by connecting together three single-phase transformer and thereby it forms a three-phase transformer bank, or by using a one-preassembled and balanced three-phase transformer which consists of three pairs of single-phase windings mounted onto one single laminated core. In a three-phase transformer, most power is distributed in the form of three-phase AC. From the generator a the transformer receives a three-phase power in order to receive it, the three-phase transformer must have three coils or windings and that must be connected perfectly so that it can match the incoming power.

What are the advantages of three-phase transformer over three single-phase transformer

Three-phase transformer occupies less space for the same rating, compared to a bank of three single-phase transformers. It weighs and costs less. Only one unit is required to be handled that makes it easy to operate, it can be transported easily, the core will be a smaller size and the material required for the core is less, single three-phase unit is more efficient. In the case of three single-phase units, six terminals are required to be bought out, while in the case of the three-phase unit, only three terminals are required to be bought out.

What is a transformer bank

It is the connection of two or more single-phase transformer, and they have connected either delta or wye. There are certain advantages for three single-phase transformer bank. Sometimes it is possible to have one transformer in a bank with a higher power rating so an unbalanced load can be supplied. If one of the transformers is out of service then the system operation is possible with the remaining transformers.

What are the connections of three-phase transformer

Three-phase transformer connection can be made with three, single-phase transformers or with multiple windings installed on a common magnetic core of a single, three-phase transformer. The two most common three-phase transformer connections are called delta and wye.


In this type of connection, both three-phase primary and secondary windings are connected in delta. In the delta connection line to line, voltage is the same as phase voltage. If a balanced load is connected to lines, then the resulting line current will be equal in magnitude and this produces balanced line currents in the incoming lines. This is used for industrial purpose, it is used in systems which carry large currents on low voltages. The advantage of this type of connection is that there is no phase displacement between the primary and secondary voltages. Then there is no distortion of flux because the third harmonic current can flow through delta connected primary windings without flowing in the line wires. The cross-section of the conductor is reduced and there will be no difficulty experienced due to the unbalancing of the loads on the secondary side. This type of connection can be used to supply large three-phase motor loads.


In a delta wye connection, the primary is connected in delta and the secondary is connected in star. The incoming line voltage is equal to the primary winding. A delta wye connection produces a 30-degree phase shift between the line voltages of the incoming and outgoing transmission lines. An advantage of a wye connection is that it reduces the amount of insulation needed inside the transformer. They are used for commercial and industrial purposes.


In this type of connection, the primary is connected in wye, while the secondary is connected in delta. The current and voltage in a wye-delta connection are the same as in the delta-wye connection, the primary and secondary connections are interchanged and the result will be a 30-degree phase shift between the voltages of the incoming and outgoing lines. This connection is used for high voltage transmission.


In this type, a bank of three transformers will be connected in “Y” on both primary and secondary sides. The neutral of the primary is connected to the neutral of the source, mostly by way of the ground, this connection provides each transformer with a third winding. In this type of connection, there is no phase shift between the incoming and the outgoing line. Mostly this connection is not used because of harmonics and balancing problems. This type of connection is used for small current rating and high voltage transformers. The advantage of this type of connection is the number of turns per phase and the amount of insulation is minimum. There is no phase displacement between primary and secondary voltages.