A transformer is a device that raises or lowers the voltage of alternating current (AC) electricity. It has no moving parts and works through electromagnetic induction, transferring electrical energy from one circuit to another using a shared magnetic field. Transformers are the reason electricity can travel hundreds of miles from a power plant to your home without losing most of its energy along the way.
How a Transformer Works
A transformer has two separate coils of wire wrapped around a shared metal core. The first coil, called the primary winding, connects to the incoming power source. The second coil, the secondary winding, connects to whatever needs the power on the other end. These two coils are not electrically connected to each other. Energy passes between them entirely through a magnetic field.
When alternating current flows through the primary coil, it creates a constantly changing magnetic field in the metal core. That changing magnetic field then induces a voltage in the secondary coil. This is the same principle Michael Faraday discovered in the 1830s: a changing magnetic field near a wire will push electrons through it, generating electricity. The key insight is that the voltage produced in the secondary coil depends on how many loops of wire it has compared to the primary coil.
The Turns Ratio
The relationship between the two coils follows a simple formula. The ratio of the output voltage to the input voltage equals the ratio of the number of wire loops (called “turns”) in each coil. If the secondary coil has twice as many turns as the primary, the output voltage will be double the input voltage. If it has half as many turns, the output voltage will be halved.
There’s a tradeoff, though. A transformer doesn’t create energy out of nothing. When voltage goes up, current goes down by the same proportion, and vice versa. The total power (voltage multiplied by current) stays roughly the same on both sides. Modern transformers manage this exchange with about 95 to 97 percent efficiency, meaning only 3 to 5 percent of the energy is lost in the process.
Step-Up and Step-Down Transformers
Transformers come in two basic types based on what they do to voltage. A step-up transformer increases voltage by having more turns on the secondary coil than the primary. A step-down transformer decreases voltage by having fewer turns on the secondary side. The same physical device could do either job depending on which side you feed power into, but in practice they’re designed and optimized for one direction.
Both types are essential to the power grid. At the power plant, electricity is generated at relatively low voltages, typically between 5 and 34.5 kilovolts. Step-up transformers then boost this to much higher levels for long-distance transmission. Common transmission voltages include 115, 230, 345, 500, and even 765 kilovolts. Higher voltage means lower current for the same amount of power, and lower current means far less energy wasted as heat in the transmission lines.
Once that high-voltage electricity reaches your area, step-down transformers at substations reduce it in stages. Sub-transmission networks carry power at 34 to 69 kilovolts over shorter distances. Distribution systems, rated below 34 kilovolts, bring it closer to neighborhoods. The final step-down transformer, often the cylindrical canister you see on a utility pole, drops the voltage to the 120 or 240 volts your home uses.
What’s Inside a Transformer
The core is made from thin, stacked sheets of silicon steel rather than a single solid block of metal. This layered (laminated) design exists to reduce a specific type of energy waste. When the alternating magnetic field passes through a conductive core, it induces small swirling currents within the metal itself, called eddy currents. These currents generate heat and waste energy. Laminating the core into thin, insulated sheets interrupts those currents and keeps losses low.
The wire coils wrapped around the core are made from either copper or aluminum. Both work well as conductors. Copper carries current more efficiently for its size, while aluminum is lighter and cheaper. Copper windings are about 20 percent heavier than aluminum ones for the same capacity, so the choice often depends on the application’s weight and cost constraints.
Where Energy Gets Lost
Even at 95-plus percent efficiency, transformers do lose some energy. These losses fall into two categories.
Core losses happen inside the metal core itself. Eddy currents, as described above, are one source. The other is hysteresis loss, which occurs because the steel resists the constant back-and-forth magnetization caused by alternating current. Every time the magnetic field reverses direction (120 times per second on a standard 60 Hz system), the core’s molecular structure has to realign, and that process converts a small amount of energy into heat. Core losses are constant whenever the transformer is energized, regardless of how much power is being drawn from it.
Coil losses (also called copper losses) happen in the windings themselves. Any time current flows through a wire, the wire’s natural resistance converts some electrical energy into heat. The amount of heat increases with the square of the current, so coil losses grow significantly as the load on the transformer increases.
Cooling Methods
Because transformers generate heat, they need cooling systems. The two main approaches are oil-filled and dry-type designs.
Oil-filled transformers submerge their coils and core in a special insulating oil. The oil absorbs heat from the windings and core, then circulates through ducts and radiator fins to release that heat into the surrounding air. These are the workhorses of the power grid, used in transmission lines, distribution substations, and power plants. Because the oil is flammable, oil-filled transformers are almost always installed outdoors, whether on the ground, on a concrete pad, or mounted on a utility pole.
Dry-type transformers use air circulation instead of oil. They’re housed in ventilated enclosures and rely on natural or forced airflow to stay cool. Without flammable oil, they’re much safer for indoor use. You’ll find them inside hospitals, shopping malls, airports, subway systems, office buildings, and high-rises, anywhere a fire risk from oil would be unacceptable.
Specialized Transformer Types
Beyond the standard step-up and step-down models, two specialized designs show up frequently.
- Isolation transformers have a 1:1 turns ratio, meaning they don’t change the voltage at all. Their purpose is to physically separate two circuits so that no direct electrical connection exists between them. This protects sensitive equipment from electrical noise and voltage spikes. They’re commonly used with computers, medical devices, and telecommunications equipment.
- Autotransformers use a single winding that serves as both the primary and secondary coil, with a tap at some point along its length. This makes them smaller, lighter, and cheaper than a standard two-winding transformer, but they don’t provide electrical isolation between the input and output. They’re often used to boost voltage at the end of a long transmission line, to provide reduced starting voltage for large motors, and in fluorescent lighting systems.
Why Transformers Only Work With AC
Transformers require alternating current because electromagnetic induction only occurs when the magnetic field is changing. Direct current (DC) produces a steady, constant magnetic field, which won’t induce any voltage in the secondary coil. This is a major reason AC became the standard for power grids in the late 1800s. The ability to easily step voltage up and down with a simple, efficient, no-moving-parts device gave AC a decisive practical advantage over DC for transmitting electricity over long distances.