Single vs. Three Phase Transformers

The primary job of an electrical transformer is to convert the voltage while maintaining the frequency of an electrical current. Electrical transformers come in a number of different types, because they have a wide variety of applications.

However, within those types, there are two basic configurations — singe-phase and three-phase. We must first consider how transformers work to understand the differences between single-phase vs. three-phase transformers.

How Electrical Transformers Work

Your basic transformer converts power using a magnetic coupling between two sets of windings — primary and secondary — around a central magnetic core. Voltage enters the transformer on the primary side and wends its way through the first set of windings. This excites the core’s electrical field, which carries the voltage to the second set of windings and out of the transformer.

These devices can step voltage up or down, depending upon the number of windings on each side. Voltage is stepped up when the primary set has fewer windings than the secondary set. Voltage is stepped down if the secondary set is comprised of fewer windings than the primary.

Single Phase vs. Three-Phase Transformers

Electrical transformers using one set of windings are known as single-phase. These come in various models, which are rated based on their horsepower range, taps, mounting type (such as wall mount), dimensions, KVA and shipping weight.

A typical step-down single-phase transformer designation is 240/480 VAC Primary and 120/240 VAC Secondary, 1 PH, 60 Hz. This means it is configured to accept between 240 and 480 volts of AC current on the primary side, which it will reduce to between 120 and 240 volts on the secondary side. This single-phase transformer operates at 60 hertz.

Three-phase transformers use three sets of windings, each around their own core. The three windings are spaced apart by 120 degrees and generate power as they rotate through the magnetic field. The generated power is then sent out on three different lines, thus the “three-phase” designation.

The three windings must be connected in the proper order to match the incoming voltage and transform it to the level needed while maintaining proper polarity and phasing. The configuration of the three primary windings is referred to as “Delta” because they are triangular shaped. Secondary windings can be configured as either “Delta” or “Wye” (Y-shaped) depending on the placement of the windings and the voltage requirements.

A typical three-phase step down transformer designation is 460 VAC Primary, 230 VAC Secondary, 3 PH, 60 Hz. This one is configured to accept 460 volts of AC current on the primary side, which it will reduce to 230 volts on the secondary side. This three-phase transformer also operates at 60 hertz.

Single-Phase Applications

Frequently used for power distribution and voltage reduction in residential and commercial applications, long distance and localized transmission currents are stepped down into power levels more suitable for end-user applications with single-phase transformers.

The ones most people have seen are found on utility poles in neighborhoods where they reduce transmission voltage before conducting it into homes and businesses. Single-phase transformers can also be found in electrical appliances, as well as electronic devices such as computers and entertainment systems.

Three-Phase Applications

Heavy-duty industrial equipment is usually happier with three-phase power because voltage can be transmitted over longer distances using smaller conductors.

A three-phase transformer can provide approximately 170 percent more power than a single-phase system. For this reason, they are most often employed in electrical grids as power transformers and distribution transformers.

This is just a brief overview of single-phase vs three phase transformers. Within those two designations there are a multitude of types, all designed for specific purposes. And yet, regardless of their purpose, they all rely on the same basic architecture to perform their functions. Whether your application calls for a single-phase or a three-phase transformer, design and applications are carefully standardized and strict guidelines exist.

These include:

  • ANSI C57.12.21 for single-phase, high-voltage, pad-mounted distribution transformers
  • ANSI C57.12.25 for single- and three-phase, liquid-filled distribution transformers
  • IEC 62505-3-2 for traction railway systems using single-phase transformers

Following them will help ensure you spec the right equipment for your installations.