transformer

A transformer is a static electrical machine that transfers power between two or more circuits using electromagnetic induction. It consists of two or more coils of wire wound around a shared iron or ferrite core. When alternating current flows through the primary coil, it creates a changing magnetic field in the core, which induces a voltage in the secondary coil. The transformer performs this transfer with no moving parts, no mechanical wear, and efficiency typically between 96 and 99 percent depending on size and design.

The fundamental reason transformers exist is to shift voltage levels. A generator might produce 20 kV; transmission lines operate at 138 kV or 765 kV to reduce resistive losses over distance; local distribution networks step down to 13.8 kV or 4.16 kV; and customer equipment expects 480 V three-phase or 120/240 V single-phase. Transformers make each of these transitions possible. The voltage ratio equals the turns ratio: a 10:1 step-down transformer with 1000 turns on the primary and 100 turns on the secondary will reduce voltage by a factor of 10, while stepping up current proportionally.

Transformers fall into broad categories by application. Power transformers rated above 100 kVA handle generation, transmission, and substation duties; they are large, often oil-cooled, and designed for continuous operation. Distribution transformers, typically 10 to 50 kVA, hang on utility poles or sit in pad-mounted enclosures serving neighborhoods. Small dry-type transformers rated below 10 kVA appear in control circuits, instrumentation, and machine tools. Specialty types include isolation transformers (primary and secondary have the same voltage but no direct electrical connection), autotransformers (primary and secondary windings share turns, saving copper), and instrument transformers (current transformers and potential transformers for metering and protection).

Common failure modes and operational stresses

Transformer failures stem from heat, moisture, and dielectric breakdown. Oil-immersed transformers depend on mineral oil both as a coolant and as electrical insulation; water contamination and oxidation degrade the oil, reducing its dielectric strength and leading to internal faults. Overloading causes winding temperature to rise; repeated thermal cycling fatigues insulation and accelerates aging. Voltage surges from lightning or switching transients can puncture insulation between windings or between a winding and ground. External mechanical damage, short circuits on the secondary side, and core saturation from DC bias or harmonic currents all shorten service life.

A three-phase power transformer typically weighs hundreds of kilograms and occupies significant floor space. The core is laminated from thin steel sheets to reduce eddy current losses. The windings are copper or aluminum wire, carefully wound to precise tolerances and insulated with paper, enamel, or synthetic materials. Temperature monitoring, oil sampling, and dissolved gas analysis are routine maintenance practices for high-value units. Nameplate data includes kilovolt-ampere (kVA) rating, primary and secondary voltages, impedance (percentage voltage drop under full load), cooling class (OIL-COOLED, SELF-COOLED, or DRY-TYPE), and tap settings for voltage adjustment.