How Does a Transformer Work? Changing Voltage Without Moving Parts
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Two coils of wire wound on a shared iron core, with no electrical connection between them and nothing that moves, will take alternating current in at one voltage and deliver it at another with efficiency above ninety-nine percent in large units. That device is the reason electricity is distributed as alternating current, and the reason a power station can be a hundred miles from the city it supplies.
The principle
A changing current in a coil produces a changing magnetic field, and a changing magnetic field through a coil induces a voltage in it, which is Faraday's law of induction. A transformer places both coils on a common magnetic circuit so that nearly all the field produced by one passes through the other. Because both coils experience the same changing flux, the voltage induced in each is proportional to its number of turns, so the ratio of output voltage to input voltage equals the ratio of the turns. A hundred turns against ten gives a tenth of the voltage. Since the device cannot create power, and power is voltage multiplied by current, reducing the voltage tenfold raises the available current roughly tenfold, minus losses. The essential requirement is change: a steady direct current produces a steady field, no changing flux and therefore no output at all, which is why transformers work only with alternating current and why connecting one to a battery does nothing except overheat it.
Why the grid depends on it
Transmitting power over distance wastes energy in the resistance of the conductors, and that loss depends on the square of the current flowing. Since a given amount of power can be delivered as a high voltage with a small current or a low voltage with a large current, raising the voltage cuts the loss dramatically: transmitting at ten times the voltage means a tenth of the current and a hundredth of the resistive loss. Long distance transmission therefore runs at hundreds of thousands of volts, which is lethal and completely unsuitable for a building, so the voltage is stepped down in stages through substations to distribution levels and finally to the couple of hundred volts delivered to a socket. Only a transformer makes that conversion cheaply and efficiently, and this is the substance of the dispute in the 1880s between Edison's direct current and the alternating current promoted by Westinghouse and Tesla. Alternating current won because it could be transformed, and direct current could not, a limitation only removed by modern power electronics.
Where the losses are
Efficiency is high and not perfect, and the losses fall into categories that shape the design:
- •Copper losses, the resistive heating in the windings, which rise with load and are reduced by thicker conductors
- •Hysteresis loss, energy spent repeatedly reversing the magnetisation of the core, reduced by using silicon steel or amorphous metal that magnetises easily
- •Eddy current loss, circulating currents induced in the core itself, which is why the core is built from thin insulated laminations rather than a solid block, since the laminations interrupt those loops
- •Leakage flux, the portion of the magnetic field that fails to link both windings, which behaves as an unwanted series inductance
- •Core losses continue whenever the transformer is energised regardless of load, which matters because distribution transformers sit connected for decades
- •Heat removal sets the practical limit, so large units are immersed in oil that both insulates and carries heat to external radiators
The varieties
The same principle is specialised heavily. A step-up or step-down power transformer changes voltage; an isolation transformer uses equal turns to provide no voltage change at all, purely so that the output has no electrical connection to the supply, which is a safety measure. An autotransformer uses a single tapped winding, sharing part of it between input and output, which is smaller and cheaper for modest voltage changes and gives up the isolation. Instrument transformers produce a small, accurately scaled version of a large current or voltage so that instruments and protective relays can measure safely. Audio and impedance matching transformers transfer signals between circuits of different impedance, since impedance transforms as the square of the turns ratio. Switch mode power supplies, which is what almost every phone charger is, rectify the mains and then switch it at tens or hundreds of kilohertz through a tiny ferrite transformer, which works because the size of core needed falls as frequency rises, and that is why a modern charger is a fraction of the weight of the heavy mains-frequency adaptors it replaced.
The takeaway
A transformer links two coils through a shared magnetic core, so a changing current in one induces a voltage in the other in proportion to the turns ratio, with power conserved so that raising voltage lowers current. It requires alternating current, since a steady current produces no changing flux. Raising transmission voltage cuts resistive loss by the square of the current, which is why the grid is built on transformers and why alternating current won the argument in the 1880s. Losses are resistive heating, hysteresis, eddy currents and leakage.