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Single Phase Transformers Information
- May 22, 2017 -

Single Phase Transformers Information

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Single-phase transformers accept single-phase 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.


Visit the IHS Engineering360 selection guide for transformers.


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A single-phase transformer is a type of power transformer that utilizes single-phase alternating current, 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. Single-phase transformers 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 DC current 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 shieldings.

  • Selecting variable variac autotransformersAutotransformer: 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: this type of transformer adjusts a voltage level to device specifications. They are commonly used as circuit isolators.

  • Constant-voltage transformer (CVT): these produce 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: to produce 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: are used to minimize 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: supply 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: a transformer with 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: convert 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: the 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.