Power transformers convert power-level voltages, as opposed to electrical signals, from one level or phase configuration to another. They are used to increase or decrease voltage to suit a specific application, and are widely used in electrical power distribution systems. For basic information about a transformer's construction and operation, please visit Engineering 360's Transformers Selection Guide.

The image above shows a typical power distribution system. Because power plants must ultimately provide electrical power to potentially remote locations, they generate high voltages to transmit the current to these areas. Multiple power transformers must be incorporated into to the system in order to gradually step down the voltage for safe residential use. Transformers may be used:

• to reduce voltage from 500 kV long distance transmission to 69 kV localized transmission
• to reduce transmission voltage to 12 kV distribution voltage at a substation
• to ultimately reduce distribution voltage to 110/220 V residential or commercial service

While most consumers are familiar with the transformer drums found on power line poles, power transformers represent a large variety of products suited to various applications. For example, alternating current (AC) adapters, also known as wall warts or plug packs, are simple rectifier transformers that convert AC mains electricity to lower voltage direct current (DC) for use with handheld devices. The image below shows a schematic diagram and DC output of a very simple AC adapter. The power transformer is the coiled diagram at left, while the rectifier is the diamond shape in the center.

Types of Power Transformers

Power transformers may be classified into several different types based on the device's construction and application. Common types include autotransformers, isolation transformers, flyback transformers, distribution transformers, and substation transformers.

Autotransformers vs. Isolation Transformers

Autotransformers are constructed with only one wound coil and can produce various selectable output voltages from a single input voltage. The single coil acts as both the primary and secondary coil, with the output voltage depending on where electrical connections on the coil are made. Autotransformers do not provide electrical isolation, but are smaller, lighter, and cheaper than dual winding transformers. These devices are often used to step voltages up from 220-240 V to 110-120 V for residential use.

Variable autotransformers (or variacs) provide adjustable output voltage by making an electrical connection using a sliding brush. Variacs are often fitted with adjustable control knobs and allow for very smooth voltage control.

A variable autotransformer (Variac)

Isolation transformers contrast with autotransformers in that they contain two distinct coils separated in such a way that they provide electrical isolation between coils. While isolation transformers may be used to step voltage up or down, they sometimes have two coils of equal windings, providing a one-to-one ratio and thus providing identical output voltage.

Single Phase vs. Three Phase

Single phase and three phase transformers are designed for use in single phase and three phase AC systems, respectively. Three phase transformers contain six total coils (three primary and three secondary) in order to accommodate all three signals.

Flyback Transformers

Flyback or line output transformers are specialized transformers originally designed to control cathode ray tubes (CRT) in televisions and monitors. Flyback transformers can sometimes be mistaken for specialized inductors in that, when input voltage is applied, current is stored in a magnetic core; when switched off, the secondary coil rapidly ramps up the output voltage, then safely ramps it down in a sawtooth wave (as opposed to the sine wave of the input voltage).

Unlike typical mains transformers designed to operate with 50-60 Hz AC, flyback transformers operate with switched currents at much higher frequencies, typically between 15 kHz and 50 kHz. In addition to their continued use with CRT devices, flyback transformers are used extensively in switched-mode power supplies and other wide-ranging applications.

A flyback transformer schematic

Distribution Transformers

Distribution transformers are pole-mounted devices that provide the final step-down action in a power distribution system. These devices supply relatively small amounts of power to residences and other locations connected to a power grid. They typically have high power and continuous voltage ratings.

A group of pole-mounted distribution transformers

Substation transformers are much larger devices installed at power distribution substations. They are used to step voltage up or down for transmission purposes.

Specifications

Electrical Specs

Buyers should consider important electrical specifications when selecting power transformers.

Power rating represents the rated power, in volts x amps (VA), of the transformer's secondary winding. Power rating is normally determined by the transformer's cooling method; air, oil, and water are common cooling methods.

Maximum primary voltage represents the maximum input voltage range. Buyers should determine the maximum voltage needed for their specific application and search using this number. Because a transformer may feature more than one input voltage, a datasheet will specify the maximum as well as all additional input voltages.

Maximum secondary voltage specifies the maximum output voltage range; this range should be queried in the same manner as the maximum primary voltage.

Direct current resistance (DCR) is the resistance of the transformer measured in DC current. Transformer manufacturers attempt to minimize DCR, therefore it is expressed and queried as a maximum value.

Core Construction

Power transformers may be constructed using various types of core materials.

Laminated transformers contain laminated steel cores insulated with a nonconducting material to reduce electrical loss.

Split core transformers are constructed using a unique hinge and locking snap to allow electrical connection without interrupting the current-carrying wire. They provide a low cost method for monitoring current.

Toroidal core devices consist of copper wire wrapped around a cylindrical core, preventing the magnetic flux that occurs within the coil from leaking.

References

IEEE - Power & Energy Society

Jochen Kronjäger - The High Voltage Page

Image credits:

HowStuffWorks | CNCCookbook | TipTemp | Wikimedia Commons | OSHA