How Transformers Work: Step-Up and Step-Down, AC Fundamentals, and Power Calculations

Overview and Core Idea

Transformers are fundamental devices in modern electricity systems. They consist of two separated coils of wire wrapped around a ferromagnetic core. The primary coil connects to the source, and the secondary coil powers the load. The underlying principle is electromagnetic induction: a changing magnetic field in the primary coil induces a voltage in the secondary coil. This process transfers energy from the source to the load while allowing the voltage to be increased or decreased as needed. The video emphasizes that transformers require alternating current to function, because a constant magnetic field from direct current would not continuously induce voltage in the secondary coil.

Structure and How It Works

In most transformers the two coils share a common iron core. The core is made of many thin laminated sheets to reduce eddy currents, which would otherwise dissipate energy as heat. The windings are insulated with enamel to ensure current follows the entire length of the wire. When current in the primary coil changes sign, the magnetic field collapses and rebuilds with each half cycle, producing a changing magnetic flux that induces a voltage in the secondary coil. If the secondary is in close proximity, the magnetic field transfers energy efficiently, making the transformer work well. A small humming sound is commonly heard due to the mechanical expansion and contraction of the iron laminations as the current alternates.

AC, Frequency, and Utility Scale

The frequency of the input equals the frequency of the output in a transformer, so transformers do not change the frequency. In most parts of the world the standard frequency is 50 Hz or 60 Hz. The video notes that North American systems use a three-wire center-tapped secondary to provide both 120 V and 240 V, while many other regions use a two-wire system producing around 230 V. This distinction affects how loads are connected, for example to quickly boil a kettle in a home appliance. The transmission system uses high voltages to reduce current and thereby minimize losses over long distances, which is explained in the next section.

Why Increase Voltage for Transmission

Energy losses in cables depend on the current and the cable resistance. A high voltage with proportionally lower current reduces I squared R losses, enabling power to be transmitted farther with less waste. The video provides a numerical example: sending 10 kilowatts through a 5 ohm cable at 240 V would waste a large fraction of energy, while sending the same power at 400 000 V would dramatically cut losses to about 1 percent. This illustrates why power stations use very high voltages for transmission and step them down closer to homes for safety and practicality.

Power, VA and True Power

Transformers are rated in volt-amps (VA) or kilovolt-amps (kVA). The key idea is that the transformer transfers apparent power between coils, and the true power in kilowatts (kW) depends on the load's power factor. In an AC circuit the load can draw real power and reactive power; a portion is wasted as heat and does not do useful work. The video uses an analogy of beer and foam to explain foam represents reactive power. The transformer’s rating therefore reflects its ability to handle both voltage and current in an AC system, but the actual usable power depends on how efficiently the connected device uses that power.

Turns Ratio and Basic Calculations

Two essential formulas relate turns, voltages, and currents. The primary to secondary voltage ratio equals the turns ratio Vp/Vs = Np/Ns. The currents relate inversely as Ip/Is = Ns/Np, ensuring that the input power equals the output power in an ideal transformer. If we know turns and input voltage, we can determine the output voltage. If we know voltages and turns, we can calculate the necessary turns on either side. The video goes through practical examples: a 1000-turn primary with 100-turn secondary fed by 120 V yields 12 V on the secondary, confirming a step-down operation. Conversely, a 100-turn primary with 200-turn secondary fed by 120 V yields 240 V, a step-up example. It also shows how to compute primary and secondary currents given voltages and turns, and how to verify power balance by V × I is the same on both sides in an ideal transformer.

Core Losses and Efficiency

Real transformers experience copper losses due to winding resistance and iron losses in the core due to hysteresis and eddy currents. The laminated core reduces eddy currents by restricting their path, though some energy is still lost in the joints between laminations. The alternating current also causes tiny mechanical vibrations in the laminations, producing the characteristic hum. The video stresses that losses are inherent and that transformer efficiency depends on both coil resistance and core design.

Rectifiers and Power Supplies

Transformers are frequently used in rectifier circuits to convert AC to DC. A typical arrangement reduces the AC voltage with a transformer, then diodes convert to a rough DC, and a capacitor smooths the voltage to a stable DC supply. This is a common approach in power supplies and is explained in more detail in a linked prior video.

Practical Computations and Formulas

The video walks through a set of basic transformer calculations, assuming ideal behavior. It covers finding secondary voltage from a given primary voltage and turns ratio, determining primary or secondary turns from known voltages, and calculating currents when given power and voltages. The common thread is that power is conserved across the transformer so that Vp × Ip = Vs × Is, up to losses in a real device. The presenter also uses these relationships to discuss how a transformer in a rectifier circuit can be designed to meet a target output voltage and current.

Applications, Safety and Further Learning

Transformers come in many sizes, from tiny doorbell transformers to the large units that feed cities. The video notes that electricity is dangerous and advises against attempting transformer disassembly or experimentation without proper qualifications. It also references additional educational content on electric systems and frames the discussion within the broader context of electrical engineering and energy distribution. Viewers are encouraged to explore related videos for further learning and to follow the creator on various social platforms.