How Transformers Work

What Transformers Do

A transformer is an electrical device that transfers energy between two circuits through electromagnetic induction, changing the voltage level in the process. It consists of two coils of wire, called the primary and secondary windings, wrapped around a shared magnetic core made of laminated iron or ferrite. When alternating current flows through the primary winding, it creates a changing magnetic field in the core, which induces a voltage in the secondary winding. No electrical connection exists between the two windings; energy transfers entirely through the magnetic field.

The voltage ratio between primary and secondary depends on the ratio of turns in the two windings. If the secondary has twice as many turns as the primary, the output voltage is twice the input voltage (a step-up transformer). If the secondary has half as many turns, the output voltage is half the input (a step-down transformer). This relationship is expressed as V_secondary / V_primary = N_secondary / N_primary, where N represents the number of turns.

Transformers work only with alternating current because they require a constantly changing magnetic field to induce voltage in the secondary winding. A steady DC current would create a constant magnetic field, producing no induced voltage after the initial moment of connection. This fundamental requirement is the primary reason alternating current became the standard for electrical power distribution.

The Physics of Transformer Operation

Faraday's law of electromagnetic induction is the physical principle behind transformer operation. When the magnetic flux through a coil changes over time, a voltage is induced that is proportional to the rate of change. In a transformer, the alternating current in the primary winding produces a magnetic flux that varies sinusoidally, and this varying flux links through the secondary winding via the shared core, inducing a proportional voltage.

An ideal transformer conserves power: the power input to the primary equals the power output from the secondary. Since power equals voltage times current (P = IV), stepping up the voltage necessarily steps down the current by the same ratio, and vice versa. A transformer that doubles the voltage halves the current. This conservation of power is what makes transformers so valuable for electrical distribution: voltage can be raised for efficient transmission without any net gain or loss of power.

Real transformers are not perfectly efficient, though large power transformers come remarkably close, with efficiencies of 98 to 99 percent. Losses come from several sources: resistance in the copper windings (copper losses or I-squared-R losses), energy wasted in magnetizing and demagnetizing the core material each cycle (hysteresis losses), and circulating currents induced in the core itself (eddy current losses). Laminating the core into thin sheets separated by insulation greatly reduces eddy current losses, which is why transformer cores are never made from solid blocks of iron.

Types of Transformers

Power transformers are the large units found in electrical substations, handling voltages from thousands to hundreds of thousands of volts and power ratings from megawatts to gigawatts. They are filled with mineral oil that serves as both coolant and electrical insulation. These transformers are designed for maximum efficiency at their rated load and are among the most reliable and long-lived pieces of electrical equipment, often operating for 40 years or more.

Distribution transformers are the smaller, more numerous units that step voltage down from medium-voltage distribution lines (typically 4 to 35 kilovolts) to the 120/240 volts used in homes and small businesses. In North America, these are the cylindrical devices mounted on utility poles or the green metal boxes at ground level in residential neighborhoods. Millions of distribution transformers operate across the electrical grid.

Instrument transformers are specialized devices used for measurement and protection rather than power delivery. Current transformers (CTs) produce a small current proportional to the large current flowing in a power line, allowing ammeters and protective relays to monitor the line without being directly connected to dangerous high-voltage conductors. Voltage transformers (VTs or PTs) similarly step down high voltages to safe levels for metering equipment.

Core Design and Construction

The magnetic core is critical to transformer performance. Most power and distribution transformers use cores made from grain-oriented electrical steel, a silicon-iron alloy processed so that its magnetic domains align preferentially in one direction. This orientation reduces hysteresis losses and allows the core to carry higher magnetic flux density, resulting in smaller, more efficient transformers.

Core geometry falls into two main categories: core type and shell type. In a core-type transformer, the windings surround the core legs, and the magnetic circuit forms a simple rectangle. In a shell-type transformer, the core surrounds the windings, providing better magnetic shielding and mechanical support. Both designs are used in practice, with the choice depending on the specific voltage, current, and power requirements.

High-frequency transformers, used in switch-mode power supplies and electronic circuits, replace steel cores with ferrite, a ceramic material with much lower eddy current losses at high frequencies. These transformers are much smaller than their line-frequency counterparts because higher frequency allows the core to transfer more power per cycle. The power adapter that charges your laptop contains a high-frequency transformer operating at tens or hundreds of kilohertz, which is why it can be so much smaller than a line-frequency transformer of the same power rating.

Transformers in the Power Grid

The electrical grid relies on transformers at every stage of power delivery. At the generating station, step-up transformers raise the generator output (typically 11 to 25 kilovolts) to transmission voltages of 115 to 765 kilovolts. These high voltages allow power to travel hundreds of kilometers with acceptable losses. At regional substations, step-down transformers reduce the voltage to distribution levels, and finally, neighborhood transformers step it down to the voltages used in buildings.

Three-phase transformers handle the three-phase power used throughout the electrical grid. Three-phase power uses three conductors carrying AC voltages offset by 120 degrees, providing constant total power delivery and more efficient use of conductor material. Three-phase transformers can be built as a single three-legged unit or assembled from three single-phase transformers, each approach having tradeoffs in cost, weight, reliability, and ease of replacement.

Transformer failures on the grid can have serious consequences because large power transformers are expensive, heavy (some weigh hundreds of tons), and have long manufacturing lead times. Utilities carefully monitor transformer health using dissolved gas analysis of the insulating oil, thermal imaging, and electrical testing to detect developing problems before they cause failures. Protecting the grid's transformer fleet is a critical aspect of maintaining reliable electrical service.