Working Principle of a Transformer

Posted on September 30, 2017 in Transformers by With 1 Comment
Working Principle of a Transformer

Working Principle of a Transformer (Credit: exportersindia)

A transformer is a static piece of apparatus used for transferring power from one circuit to another circuit without change in frequency. It can raise or lower the voltage with a corresponding decrease or increase in current. The principle of transformer action is also applicable in many ways to motors, generators, and control apparatus.

The principle of transformer action is based on the work of Michael Faraday, whose discoveries in Electromagnetic Induction showed that, given two magnetically coupled coils, a changing current in one coil will induce an electromotive force in the other coil. Such electromagnetically induced EMF are called transformer voltages, and coils specifically arranged for such purposes are called transformers.

The most important tasks performed by transformers are:

  • Changing voltage and current levels in electric power systems
  • Matching source and load impedance’s for maximum power transfer in electronic and control circuitry
  • Electrical isolation (isolating one circuit from another or isolating dc while maintaining ac continuity between two circuits).
Transformers are used extensively in ac power systems because they make possible power generation at the most desirable and economical level (10–20 kV), power transmission at an economical transmission voltage (as high as 400–1000 kV) and power utilization at most convenient distribution voltages (230/400 V) for industrial, commercial and domestic purposes but in industrial applications voltages may have to be as high as 3.3, 6.6 or 11 kV for large motors. In communication and electronic systems where frequency ranges from audio to radio and video, transformers are used for a wide variety of purposes.

Working Principle of a Transformer

The transformer operates by applying an alternating voltage to the primary winding. As the voltage increases, it creates a strong magnetic field with varying magnetic lines of force (flux lines) that cut across the secondary windings. When these flux lines cut across a conductor, a current is induced in that conductor. As the magnitude of the current in the primary increases, the growing flux lines cut across the secondary winding, and a potential is induced in that winding. This inductive liking and accompanying energy transfer between the two windings is the basis of the transformer’s operation.

Diagram showing magnetic circuit and windings of a transformer

Diagram showing magnetic circuit and windings of a transformer

The magnetic lines of flux “grow” and expand into the area around the winding as the current increases in the primary. To direct these lines of flux towards the secondary, various core materials are used. Magnetic lines of force: much like electrical currents, tend to take the path of least resistance. The opposition to the passage of flux lines through a material is called reluctance, a characteristic that is similar to resistance in an electrical circuit.

When a piece of iron is placed in a magnetic field, the lines of force tend to take the path of least resistance (reluctance), and flow through the iron instead of through the surrounding air. It can be said that the air has a greater reluctance than the iron. By using iron as a core material, more of the flux lines can be directed from the primary winding to the secondary winding, this increases the transformer’s efficiency.

If the secondary voltage is greater than the primary value, the transformer is called a step-up transformer, if it is less, it is known as a step-down transformer; if primary and secondary voltages are equal, the transformer is said to have a one-to-one ratio. One-to-one transformers are used to electrically isolate two parts of a circuit. Any transformer may be used as a step-up or step-down depending on the way it is connected.

In order to ensure the largest and most effective magnetic linkage of the two windings, the core, which supports them mechanically and conducts their mutual flux, is normally made of highly permeable iron or steel alloy (cold-rolled, grain oriented sheet steel). Such a transformer is generally called an iron-core transformer. Transformers operated from 25–400 Hz are invariably of iron-core construction. However, in special cases, the magnetic circuit linking the windings may be made of nonmagnetic material, in which case the transformer is referred to as an air-core transformer. The air-core transformer is of interest mainly in radio devices and in certain types of measuring and testing instruments.


  1. Electric Machines by D. P. Kothari &  I. J. Nagrath

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Hello, I'm Kalpesh, An Electrical Engineer and the founder of Substation System.

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