A transformer is a static electrical machine that transfers energy between two or more circuits, typically at different voltage and current levels, without changing the frequency, through magnetic induction. Transformers, one of the fundamental pillars of the electrical system, enable the conversion of electrical energy from the high voltage levels at which it is generated in power plants to even higher voltages for efficient long-distance transmission, and then step-by-step reduction through distribution networks to safe and suitable levels for use in homes and industry.
History
The scientific foundations of the transformer were laid in 1831 with the discovery of the principle of magnetic induction by Michael Faraday. Faraday observed that a change in current in one coil induces a voltage in another nearby coil. However, the practical implementation of this principle and its adaptation for use with alternating current (AC) systems did not occur until the late 19th century.
The first practical transformer designs were developed in the 1880s by Hungarian engineers Károly Zipernowsky, Ottó Bláthy, and Miksa Déri in Budapest. They recognized the inefficiency of "open-core" transformers and invented "closed-core" transformers that directed the magnetic flux more effectively. These transformers could be connected in parallel, a significant advancement for electrical distribution networks.
In America, George Westinghouse recognized the potential of the transformer, licensed the technology, and collaborated with engineer William Stanley Jr. In 1886, Stanley established the first commercial AC power distribution system using transformers in Great Barrington, Massachusetts. These developments demonstrated the superiority of AC systems over Thomas Edison's direct current (DC) systems and played a decisive role in the AC victory in the "War of Currents."
Operating Principle
The operating principle of the transformer is based on Faraday's law of electromagnetic induction and Lenz's law. Fundamentally, it consists of at least two insulated conductive windings wrapped around a magnetic core: the primary winding and the secondary winding.
1. Application of AC to the Primary Winding: When an alternating voltage (AC) is applied to the primary winding connected to the transformer input, an alternating current (Ip) flows through it.
2. Generation of a Varying Magnetic Field: The primary current creates a time-varying magnetic flux (Φ) within the core. Due to the core's high magnetic permeability, this flux is largely confined and guided along the core.
3. Induction of Voltage in the Secondary Winding: The varying magnetic flux passing through the core also passes through the secondary winding wrapped around the same core. According to Faraday's law, when the magnetic flux around a conductor changes over time, a voltage (electromotive force — EMF) is induced in that conductor. The induced voltage in the secondary winding (Vs) is directly proportional to the rate of change of the magnetic flux through the core and the number of turns in the secondary winding (Ns). Similarly, a self-induced voltage (opposing EMF) is generated in the primary winding, opposing the applied voltage.
4. Voltage and Current Transformation: In an ideal transformer, the ratio of voltages between the primary and secondary windings equals the ratio of their number of turns: Vs/Vp = Ns/Np. Here, Vp is the primary voltage, Vs is the secondary voltage, Np is the number of primary turns, and Ns is the number of secondary turns. This ratio is called the turns ratio and is commonly denoted as a = Np/Ns.
In an ideal transformer, there are zero losses, meaning input power equals output power (Pinput = Poutput). Since AC power is given by P = Veff Ieff cos(ϕ) and transformers generally do not alter the phase angle (except with inductive loads), the relationship between voltage and current can be approximately expressed as Vp Ip = Vs Is. From this, the current ratio is found to be the inverse of the turns ratio: Ip/Is = Ns/Np. Thus, if voltage is stepped up (Ns > Np, Vs > Vp), current is stepped down proportionally (Is < Ip), and vice versa. Lenz's law states that the direction of the induced current opposes the change in magnetic flux that produced it.
Structure and Key Components
The fundamental structural components of a transformer are as follows:
- Core: Facilitates efficient transfer of magnetic flux between windings. Typically made from laminated silicon steel sheets with high magnetic permeability. To reduce eddy current losses under alternating current, the core is constructed from thin, insulated laminations. Special silicon steel alloys are used to minimize hysteresis losses. Cores may be shaped as E-I, U-I, or toroidal (ring-shaped), depending on the application. Ferrite cores are used for high-frequency applications.
- Windings: Conductive coils that form the input and output terminals of the transformer. Usually made from insulated copper or aluminum wire. Wound around the core as primary and secondary windings. In large transformers, windings are often arranged in cylindrical or disc-shaped layers, with gaps or channels left between them for cooling.
- Insulation Materials: Used to electrically isolate windings from each other, from the core, and from other metallic parts. Various materials are employed, including paper, pressboard, varnish, oil (mineral or synthetic), resin, and porcelain bushings. The durability of insulation is critical for the safe and long-term operation of the transformer.
- Tank and Enclosure: Protects internal components (core and windings) from environmental conditions such as moisture, dust, and contamination. In oil-filled transformers, it also contains the cooling oil. Typically made of steel and may include radiator fins to increase cooling surface area. Expansion tanks (conservators) or corrugated walls with expansion capability are used to accommodate oil expansion.
- Cooling System: Dissipates heat generated within the transformer to maintain the temperature of windings and core within permissible limits. Cooling methods vary depending on the transformer's size and type:
- Bushings: Insulators that allow the ends of transformer windings to exit the tank safely and with proper insulation. At high voltage levels, they are made from special porcelain or composite materials and feature a fluted design to increase the creepage distance and prevent flashover.
- Protective Devices: Large transformers include protective devices such as Buchholz relays, which detect abnormal conditions like overheating, pressure buildup, or gas accumulation and trigger alarms or disconnect the transformer.
Types of Transformers
Transformers are classified into various types based on different criteria:
- Power Transformers:
- Definition and Use: Used at power generation stations (to step up voltage from generators) and in main transmission lines (for conversion between high voltage levels). They operate at the highest power levels, ranging from several MVA to thousands of MVA.
- Distribution Transformers:
- Definition and Use: Used in distribution networks to reduce high voltage from transmission lines or main substations to lower levels suitable for end users (residential, commercial, small industrial). Power ratings typically range from a few kVA to several MVA.
- Instrument Transformers (Measurement Transformers):
- Definition and Use: Used to reduce high voltage and current values in electrical networks to standard, safe levels (typically 100 V or 5 A) for measurement instruments (voltmeters, ammeters, energy analyzers) or protective relays. They isolate measurement and protection circuits from high-voltage/high-current networks.
- Isolation Transformers:
- Definition and Use: Used to provide high electrical isolation between primary and secondary windings. Typically have a 1:1 turns ratio, meaning they do not change voltage. Their primary functions are to ensure safety by breaking galvanic (direct electrical) connections between input and output circuits (reducing earth leakage risks) or to isolate sensitive equipment from network noise.
- Autotransformers:
- Definition and Use: Unlike other transformers, they have a single winding that serves both primary and secondary functions. A portion of the winding is shared between both circuits. Voltage transformation is achieved by tapping the winding at different points.
- Audio Frequency Transformers:
- Definition and Use: Used for processing or transmitting audio signals (typically in the 20 Hz to 20 kHz range).
- Pulse Transformers:
- Definition and Use: Used to transmit, step up, or isolate rectangular or short-duration pulse signals without distortion.
Losses and Efficiency
Real transformers, unlike ideal models, have energy losses. These losses cause a portion of the input power to be converted into heat, reducing efficiency (η). Efficiency is the ratio of output power to input power, usually expressed as a percentage: η = Poutput / Pinput × 100% = Poutput / (Poutput + Ploss) × 100%. The main types of losses are:
- Core Losses (Iron Losses):
- Hysteresis Losses: Result from internal friction in the core material as the magnetic field reverses direction. They depend on frequency and magnetic flux density.
- Copper Losses (Winding Losses):
- Heat generated due to the resistive (ohmic) losses in the primary and secondary windings (Rp, Rs) as current (Ip, Is) flows through them (Ip²Rp + Is²Rs). These losses are proportional to the square of the load current and increase with load.
- Leakage Flux Losses:
- Occur because not all magnetic flux links both windings completely. Some flux leaks outside the core or between windings. This creates leakage inductance, causing voltage drops and phase shifts under load. While not a direct energy loss, it contributes indirectly to losses through AC impedance.
- Dielectric Losses: Losses occurring in insulation materials; generally very small.
- Stray Losses: Losses caused by currents induced in metallic structures outside the core (tank, frame) by stray magnetic fields.
Transformers are among the most efficient electrical machines. In large power transformers, efficiency can exceed 99% at full load. Efficiency is typically highest near full load.
Importance and Applications
Transformers are the technology that enables alternating current to form the foundation of modern electrical systems. Efficient voltage conversion is difficult in DC systems, making long-distance transmission impractical. Transformers allow AC voltage to be easily stepped up and down, enabling energy to be transmitted over thousands of kilometers with minimal losses. Major applications include:
- Electrical Power System: Voltage step-up at power plant outputs, voltage conversion between levels in transmission networks, voltage step-down at distribution centers, and provision of suitable voltage for end consumers.
- Industry: Machines and motor drives requiring different voltage levels, welding equipment.
- Electronic Devices: Power supplies (adapters), audio systems, televisions, computers. The first stage in power supplies that convert mains voltage to low DC voltage for electronic circuits is typically a transformer.
- Renewable Energy Systems: Used to connect energy from wind turbines or solar panels to the grid.
- Transportation: Electric trains, trams, and some electric vehicles.
- Medical Applications: Isolation transformers are used for safety.
In conclusion, transformers are machines that efficiently convert voltage and current levels of electrical energy using electromagnetic induction. They play a critical role at every stage of electrical energy, from generation to final consumption, forming a fundamental pillar of modern life.
