Transformer Fundamentals

A transformer is an electrical device that uses the principle of electromagnetic induction to transfer energy from one electric circuit to another. It is designed to either increase or decrease AC voltage between the circuits while maintaining the frequency of the current. Transformers do this with no conductive connection between the two circuits. This is possible through the application of Faraday's Law of Induction which describes how a magnetic field will interact with an electric circuit to produce electromotive force (EMF).

How do transformers work?

The basic transformer comprises three parts: the magnetic core, the primary winding, and the secondary winding. The primary winding is connected to a live source of AC power. This produces an alternating magnetic field that surrounds the winding. This induces an EMF in the secondary winding. If the circuit of the secondary winding is closed, then AC current will flow through it. These windings share the magnetic core, which is usually made out of laminated steel sheets and provides a low reluctance path for the magnetic field. The ratio between the output voltage and input voltage is the same as the ratio of the number of turns between the two windings. In a step-down transformer, the secondary winding will have fewer turns than the primary, and in a step-up transformer, it will have more.

transformer diagram
Basic transformer electrical diagram                   (Shutterstock)

History of Transformers

The first transformer was invented in 1884 in England, and it revolutionized the way AC power could be used. This transformer was first used in the first AC power station, the steam-powered Rome-Cerchi power plant, in 1886. Using the transformer, AC power could be generated and supplied at a high voltage (1,400 to 2000 volts) and then stepped down to a safer, usable voltage for use in homes and businesses.

While this original basic design is still used today, modern transformers are used for a wide variety of applications. Those used in electrical power stations can be several stories high and are used to transmit power at high voltages, which is more efficient than doing so at low voltage because it reduces power loss due to heat. Signal and audio transformers are much smaller and are used to match the output of microphones and other audio devices to the input of amplifiers. Measurement transformers convert the power of a main power line to lower voltage that can then be measured to determine its power output without damaging sensitive equipment. Pulse transformers deliver pulses from a primary circuit to a secondary circuit for transmitting digital information to logic gates or drivers in electronic devices.

Three Phase Transformers

Most of the applications above describe single-phase transformers. This type of transformer has one primary winding and one secondary winding. However, transformers also come in the three-phase variety. Three-phase transformers have three sets of windings. These transformers are used to power industrial loads and generate three-phase power.

For review, here are the basics of a transformer:

  • A transformer uses electromagnetic induction to transfer AC current from one circuit to another while increasing or decreasing voltage.
  • The basic transformer has three parts: the primary winding, the secondary winding, and the magnetic core.
  • There are all types of sizes and uses for a transformer, from massive transformers used by power stations to tiny transformers used in electronics.


transformer depictions
Different ways to depict transformers                    Getty Images

What does a transformer do?

Transformers transfer electrical energy between two or more circuits through electromagnetic induction. Their primary function is to increase (step up) or decrease (step down) voltages. This capability makes them essential for efficiently transmitting electrical power over long distances and for providing the appropriate voltage levels for various applications, from industrial equipment to household appliances.

Why do transformers blow?

Transformers can blow due to overloading, electrical surges, short circuits, aging, poor maintenance, manufacturing defects, or environmental factors. Overloading causes overheating, while surges from lightning or the grid can damage components. Short circuits increase current flow, leading to damage. Aging, lack of maintenance, and exposure to harsh environments degrade the transformer over time. Manufacturing defects might also lead to failures, resulting in power outages and the need for repairs or replacements.