Single-phase transformers accept single-phase alternating current (AC) power and output single-phase AC power, typically at a higher or lower voltage level. Energy is transferred from one circuit to one or more circuits via electromagnetic induction.

(Find single phase transformers by Specification or see the IEEE GlobalSpec Directory of suppliers.)


A single-phase transformer is a type of power transformer that utilizes single-phase AC, meaning the transformer relies on a voltage cycle that operates in a unified time phase. They are often used to step-down long distance and localized transmission currents into power levels more suitable for residential and light-commercial applications. The ratio of primary (input) windings to secondary (output) windings determines the change in current. Single-phase transformers with a 1:1 ratio can be used to isolate circuits. They abide by Ohm's law, and outside of minor inherent loss due to heat, do not create or remove power.

Single-phase transformers are more popular than three-phase transformers in non-urban areas, as the cost of a three-phase distribution network is much higher, and the overall electrical demand is lower. The highest voltage available in a single-phase network is regulated by utility infrastructure and industrial regulations. A single-phase transformer is frequently used for power distribution and voltage reduction for residential and commercial applications. When used with appliances, the lower voltage output is often rectified into direct current (DC) before powering appliances, such as a computer.

High-voltage systems typically employ three-phase transformers to power apartment buildings, retail centers, factories, offices, and other large-scale structures, as well as electric motors—single-phase power supplies do not produce the rotating magnetic field required to induce rotation. Three-phase power systems are more common in cities, where dense power supply demands require transformers that route hundreds or thousands of kVA.


The following transformer types are commonly manufactured to accept and output single-phase AC power.

  • Audio transformer: removes ground noise from audio signals by encasing the transformer in magnetic shielding.
  • Autotransformer: typically used in low power applications to connect circuits with different voltage classes. It contains only one winding, cannot isolate circuits, and is usually smaller, lighter, and cheaper than other transformers. The voltage source and electrical load are connected to two taps, and voltages are determined by tapping the winding at different points. An autotransformer with an adjustable tap is known as a variac or variable transformer, and an example is pictured at right.
  • Buck-boost transformer: adjusts a voltage level to device specifications. They are commonly used as circuit isolators.Variable autotransformerVariable autotransformer
  • Constant-voltage transformer (CVT): produces a relatively constant output voltage, despite potentially large variances in the input voltage.
  • Constant-current transformer: also called a regulator, it has a self-adjusting secondary winding that provides a consistent output current for any load within its dynamic range. It is common for streetlight applications.
  • Distribution transformer: this is the oft-seen, pole-mounted transformer that steps down current for light-duty electrical applications.
  • Flyback transformer: produces a high-voltage output, the transformer stores energy in its magnetic windings for a short period of time.
  • Generator step-up transformer: steps up voltage levels to a suitable long-distance transmission voltage level.
  • Harmonic mitigating transformer: utilizes phase-shifting, electromagnetic flux cancellation, and source impedance to decrease harmonic currents in distribution systems, which ultimately lowers transformer operating temperature.
  • Impedance matching transformer: minimizes signal reflection from an electrical load and often have a 1:1 turn ratio. A common example of an impedance-matching transformer would be a balun, which is used to connect two circuits of mismatched impedance, such as a balanced line of two conductors carrying equal currents in opposite directions that is connected to an unbalanced line of one conductor carrying current and a ground.
  • Industrial control transformer: supplies power to constant-current or constant-voltage devices that may be sensitive to variations in electrical supply, such as solenoids, relays, or other electromechanical devices.
  • Interface transformer: isolates communication signals.
  • Isolation transformer: is not used to step up or step down voltages, but rather to buffer circuits from each other.
  • Leakage transformer (stray-field transformer): maintains a high leakage inductance by loosely coupling the magnetic fluxes of the primary and secondary windings. This makes the transformer short-resistant, an important characteristic of transformers for welding operations.
  • Lighting transformer: supplies low voltages for lighting and other light-duty applications.
  • Medical transformer: leakage current, high potential requirements, temperature class, and current and thermal fusing are the primary concerns of medical transformers due to the sensitive environments in which they are employed. They are carefully regulated by law and industrial standards.
  • Multi-ratio transformer: contains several outputs with each output tap corresponding to a different transformer ratio.
  • Neutral grounding transformer: protects power transformers and generators from harmful fault currents. When a fault occurs, a voltage is induced in the open delta and there is a voltage drop in the connected resistor.
  • Power transformer: converts voltages from one level or phase to another for widespread power distribution.
  • Rectified transformer: converts AC to DC.
  • Resonant transformer: a capacitor is placed across one or both windings to function so the circuit can be tuned.
  • Solar power transformer: can be incorporated as part of a single-phase string inverter, or as a step-up transformer to connect PV plants to a grid.
  • Substation transformer: a step-down transformer that converts transmission-level voltages to distribution-level voltages.


The ratio of primary (input) coils to secondary (output) coils determines if the voltage is increased or decreased after routing through the transformer. Some transformers have an adjustable turn ratio, while others maintain 1:1 (or near 1:1) ratio to simply isolate circuits. Other transformers utilize a single coil, and voltage is transmitted by tapping the coil at an intermediate point.

Winding configuration

  • Single: one primary winding that accepts one nominal voltage
  • Dual: dual primary windings that accept two nominal voltages
  • Quad (2+2): dual primary windings and each winding accepts two nominal voltages
  • 5-lead: primary winding that can accept five nominal voltages
  • Ladder: consists of cascading windings that create a series of inductances between adjacent windings


  • Oil-filled transformers rely on a strong dielectric oil to insulate components and dissipate heat. Mineral oil, synthetic ester, and silicone-based fluids have replaced PCBs. Some transformers may utilize radiators, filters, fans, pumps, or heat exchangers to manage transformer oil, depending on the application.
  • PCB-filled transformers are replaced by other cooling methods when the fluid expires, as PCB has been identified as a carcinogen for more than 50 years. Many PCB transformers remain in operation today, however, and may still be sought in less regulated markets.
  • Transformers that use water to cool components are immersed in oil, but cool water is fed through copper pipes below the oil surface to improve circulation and heat exchange. Another method is to pump heated oil out of the transformer through tubes that are doused in water.
  • Dry/air-cooled transformers contain two windings that face each other but do not contain a core. The windings are cooled via convection, which might be supplemented with a ventilated enclosure and blowers or fans.
  • Encapsulated transformers utilize a dielectric, thermal management resin to seal the transformer components from contaminants.

Types of coresTypes of cores


The overall size and weight of a transformer ultimately determines how it should be installed.

  • Chassis: integral structures enable the transformer to be installed via fasteners.
  • Chip: typically manufactured with thin-film technology, these transformers are incorporated into integrated circuits (ICs) and are often used as isolators.
  • Dish/disk: toroidal core transformers can be mounted with hardware that includes a bolt through the middle of the torus.
  • H-frame: a mounting style that mitigates the effects of vibration and shock.
  • Modular jack: typically a modular connector with a built-in transformer.
  • Pad: the transformer is installed upon a structural foundation, such as substation transformers that are affixed to a concrete pad.
  • PC/PCB: also known as board-mount transformers, these transformers transfer voltage between two circuits for circuit board applications. They consist of windings, core, casing, a mounting method (through-hole or surface mount), and connection terminals. Some PCB transformers are ICs made via semiconductor processing.
  • Pole: these ubiquitous transformers affixed to roadside utility poles step-down voltage from localized transmission levels to residential- and commercial-appropriate voltages.
  • Skid/trailer: large transformers can be easily relocated to meet changing electrical needs. They accommodate temporary increases in local electrical needs.


The accompanying parameters are important when considering single-phase transformers.

  • Operating frequency range: transformers with high operating frequencies tend to be smaller, as fewer windings are needed to match Electric  power substation transformerElectric power substation transformerimpedances.
  • Primary voltage rating: input voltage range; several nominal voltages represent more than one primary winding.
  • Secondary voltage rating: output voltage range
  • Secondary current rating: rated output current
  • Power rating (VA): maximum voltage suitable for a transformer, expressed in volts-amps.
  • Operating temperature: safe temperature range of a transformer in operation; transformer temperatures rise while in use.


  • Current limiting protection: an overcurrent protection mechanism.
  • Flameproof: the transformer has enhanced fire resistance, which is useful in potentially-reactive environments such as mines.
  • NEMA enclosure: the transformer casing or container conforms to a NEMA rating, a standard regarding ingress protection for various industrial and environmental contaminants.
  • Indoor/outdoor rated: the transformer is specified for certain operating environments. Oil-filled transformers are almost always installed outside.
  • Waterproof: transformer has a sealed case to prevent water intrusion.
  • Submersible: transformer can be submerged.
  • Tamperproof: the transformer cabinet retains a lock or other vandal-resistant mechanism.


Transformer design and applications are meticulously standardized. Many guidelines exist regarding single-phase transformer applications and manufacture. Prominent standards include:

- ANSI C57.12.21 Single-phase, high voltage, pad-mounted distribution transformers

- ANSI C57.12.25 Single- and three-phase, liquid-filled distribution transformers

- IEC 62505-3-2 Traction railway systems using single-phase transformers

To contact the author of this article, email