Transformers may be classified by the core construction, winding arrangement or coil structure (core type, shell-type), voltage (step up, step down, autotransformer), type of supply (single or three-phase), it’s cooling medium (oil or air), and its purpose (or application). There are power transformers, distribution transformers (overhead and underground transformers), instrument transformers (current transformers, voltage transformers, or potential transformers), and many, many more.

A transformer is a system that allows an Alternating Current (or AC) to adjust voltage according to the application it is used for. It converts electrical energy (from primary side windings) to magnetic energy (in the magnetic center of the transformer) and back to electrical energy (on the secondary transformer side).

Many electrical and electronic instruments have transformers in them. Even though the construction and operation of these transformers are different, their sole purpose is pretty much the same.

Using transformers is a simple and cost-effective way to generate and transmit AC power at high voltages, this is why we use the Alternating Current (AC) in our homes. After generation, AC gets transmitted at higher voltages to the distribution network, and then it is converted to a safer and more usable lower voltage.

Now, let’s go through the transformer types. The types of transformers may be divided into the following sections:

1. Transformers by construction type.
2. Transformers by voltage conversion
3. Transformers by phases
4. Transformers by applications
5. Transformers by insulation and cooling

Transformers by construction type

These transformers can be divided based on how the core and the copper windings are arranged:

• Core-type transformer
• Shell-Type Transformer

Core-Type Transformer

In core-type transformers, windings are mounted on two limbs of the core and there is only one flux path in the core.

The core of core-type transformers is shaped like a hollow square made of steel laminations, and the windings surround it. These cylindrical coils are made up of layers, each of which is insulated from the others. Insulation materials such as paper, cotton, or mica may be used. Since low voltage windings are easy to insulate, they are mounted closer to the core.

In transformers, windings are the main current-carrying conductors wrapped around the laminated portions of the core, and they are an integral part of the transformer structure.

There are two windings in a single-phase core-type transformer

Feel free to download this PDF file.

The primary winding is attached to the voltage source and generates the magnetic flux, while the secondary winding is connected to the load, and voltage is induced in it by the means of a mutual inductance process.

Shell-type transformer

The shell-type transformer has three limbs or legs. Both the primary & secondary windings of the shell-type transformer are placed on a single leg (the central leg) while in a core-type transformer the windings are on two different legs.

Here is a PDF file of the above picture.

The central limb transports the entire flux, while the side limbs transport half of it. As a result, the central limb’s diameter is about twice that of the outer one.

To reduce the cost of insulation placed between the core and the winding in shell-type transformers, the low voltage winding is placed near the core, and the other after that.

The coils in a shell-style transformer are wound in the form of multi-layer disk-type. Paper insulates the various layers of multilayer disks from one another.

The entire winding is made up of stacked disks with insulation space between these coils (the gaps forming the cooling and insulating ducts that run horizontally).

Transformers by voltage conversion

The transformer is the most critical component of the electrical grid and it provides economical and efficient transmission and delivery of electricity. The operating frequency and nominal power on the main and secondary transformer sides are almost identical – while the voltage and current values are different.

Transformers have the ability to convert energy in both ways, from high voltage to low voltage, and vice versa. Because of this, they can be used as a voltage step-up or step-down transformer.

The design of either one of the transformers is identical and any of them can theoretically be used as both a step-up and a step-down transformer. It is solely dependent on the direction of energy flow.

If we compare windings of step-up or step-down transformers, low voltage windings will have a smaller number of turns than high voltage windings. The current will also be higher on the low voltage side and lower on the high voltage side.

Here are three types of voltage changing transformers:

1. Step-down Transformer.
2. Step-up Transformer.
3. Autotransformer

1. Step-up transformer

A “Step-up Transformer” has a secondary output voltage that is higher than the main input voltage. In this transformer, the voltage increases, and the current decreases.

Step-up transformer is a device that turns the low voltage (and high current) into high voltage (and low current)

Here is a schematic representation of a step-up transformer (please note the number of turns on primary and secondary windings):

Feel free to download the PDF file here.

Here is 4.5 star-rated (consumer-grade) Step Up & Step Down Transformer (paid link) sold on Amazon if anyone wants to try it.

2. Step-down transformer

Feel free to download a PDF file for greater detail or both schematics in PDF format.

When power is generated and transferred across power lines to your local electrical station, it is at a high voltage level, therefore it needs to have a way to “step down” and supply a much lower voltage before it enters your house. Step-down transformers do exactly that and they can be found anywhere from mounted on electrical poles to underground locations.

Creating AC power at high voltages, transmitting it over a distribution network, and stepping the voltage down to a level where it becomes useful in our lives, is the most efficient way to go.

3. Auto-Transformer

This is a unique kind of transformer with just one winding and a secondary output known as a “tap.” This tap is usually customizable, allowing you to change the output AC voltage, similar to a voltage divider.

Here is a PDF file for this picture.

In standard transformers, the primary and secondary windings are positioned in opposite directions, but in autotransformer windings, the primary and secondary windings are connected in series.

The difference between autotransformers and traditional two or three-winding transformers is that they only have one winding (it acts as a primary as well as secondary winding). As a result, this type of transformer is capable of both conduction and induction.

The voltage of the autotransformer is varied according to the location of secondary tapping on the coil. If there is a voltage loss, an autotransformer with several output taps may be used to compensate and adjust the power line to the appropriate voltage.

One example of a device that implements an autotransformer is an Automatic Voltage Regulator (or AVR). It automatically switches the different taps of the autotransformer through relays to compensate for changes in line voltage. Here is one form Norstar (paid link) sold on Amazon.

Autotransformers can act as Step-up and Step-down transformers!

In a step-down transformer mode, the entire winding serves as the main, while only a portion of it serves as the secondary, resulting in a lower voltage induced in the secondary part of the winding. While in a step-up transformer mode, exactly the opposite happens.

Autotransformers are often used as stators for the stable start of synchronous motors, induction motors, and other electrical devices. They are also commonly found in furnaces.

Transformers by Phases

A transformer itself cannot be used as a phase changer to convert single-phase to three-phase or three-phase to single-phase power. In order to make the transformer connections compatible with three-phase supplies, we must link them in a specific way.

We can use three single-phase transformers on a three-phase supply if we link their primary windings to each other and their secondary windings to each other in a fixed configuration.

The two most commonly used types of transformers are:

1. Single-Phase Transformer. A single-phase transformer is a very popular type of transformer due to the many applications that it’s used in. It has a main and secondary winding, and it can be used to lower or boost the secondary voltage.
2. Three-Phase Transformer. This is the most popular type of transformer for industrial environments. Three primary or main windings and three secondary windings are connected to form a three-phase transformer.

1. Single-phase transformer

A single-phase transformer is a type of electrical system that operates on a single-phase Alternating Current. It takes in and out single-phase power that many household appliances and devices use.

Single-phase means that there are two wires, one is denoted as live wire and the other is denoted as neutral wire

You can download a PDF file of the above picture.

This type of transformer is extremely useful for retail consumer devices due to its high performance and compact design. It is also commonly used in non-urban electricity distribution areas due to lower power demand and overall savings in cost.

These transformers are used to “step down” or reduce the voltage to an acceptable level while maintaining the same frequency. Another common use for a single-phase step-down transformer is in electrical equipment and appliances mostly used in homes.

Each winding of a basic single-phase transformer is wrapped cylindrically on a soft iron limb separately to provide the required magnetic circuit. It provides a way for the magnetic field to pass between two windings, inducing a voltage.

2. Three-phase Transformer

A three-phase transformer operates on a three-phase Alternating Current (it takes in and out three-phase power).

This type of transformer has three primary as well as three secondary windings

Each leg of the transformer core has one set of primary and secondary windings wound on it. We can say that it is a set of three single-phase transformers combined to form a three-phase transformer.

In three-phase transformers, three primary windings and three secondary windings can connect in 4 different ways.

1. Star – to – Star connection (Y – Y)

The three primary windings are connected in star topology and the three secondary windings are also connected in a star topology.

2. Star – to – Delta connection (Y – Δ)

The three primary windings are connected in star topology and the three secondary windings are in delta topology.

3. Delta – to – Star connection (Δ – Y)

The three primary windings are connected in delta topology and the three secondary windings are in a star topology.

4. Delta– to – Delta connection (Δ – Δ)

The three primary windings are connected in delta topology and the three secondary windings are also connected in delta topology.

Three-phase power transformers are best suited for electrical systems that need efficient power distribution and high-power transmission capabilities.

Unfortunately, I cannot post big pictures here, but this PDF file will show more details.

Multiple waveforms (or phases) can be used to make power flow smoother and more effective, even though they are all at the same frequency.

Three-phase power transformers have many advantages in industrial settings. They’re cheaper, lighter, and easier to work with, so they’re perfect for industrial machinery.

Power generation stations and electrical distribution networks both use three-phase transformers. High-power demanding industrial loads and applications use them as well.

Transformers by applications

Transformers are devices that move electricity from one circuit to another and their primary function is to raise (step-up) or decrease (step down) voltage. They may be used for a wide range of purposes.

Some transformers can be as tall as a building (such as those used at generating stations), while others are quite tiny (and used for electronic applications). Regardless, the transformer’s function remains the same, and that is:

To convert electrical power from one kind to another

Some of the transformers by applications (or uses) include:

• Power transformer. Commonly used at power generating stations and for high-voltage power transmission applications. Its degree of insulation is very high.
• Distribution transformer. These transformers are used in the distribution system. Distribution transformers are generally not as big as power transformers and are self-cooling oil-filled.
• Instrument transformer. Used in various industrial devices for relay and safety purposes.  There are the Current Transformers and Potential (or Voltage) Transformers.

Power Transformer

Power transformers are used to step down or step up the voltage at generating stations, substations, and power transmission lines. At the power generation stations, the voltage level is raised by using a step-up power transformer (while lowering the current), so it can go down the transmission lines.

As a result, transmission line losses are minimized. Depending on the device (and where it is used), the power transformer may be single-phase or three-phase.

Autotransformers and distribution transformers all belong to the power transformer category.

Distribution Transformers

At the generating stations, the voltage is stepped up to lower the transmission losses. For any household application, this power is excessive, this is why we have distribution step-down transformers to bring it to the normal level of 110v (in the USA) or 240v (formerly 240V in the UK, 220V in the rest of Europe).

Distribution transformers function in the same way as big transformers in a substation, only this transformer reduces the voltage to service the voltage needs of the consumer (or homeowner):

The distribution transformers are sized to handle a quite large load of customers that are connected to the system.

Businesses and manufacturing facilities, in general, do not share transformers with other consumers and usually need bigger transformers. To minimize expenses, very often industrial companies use 440-volt or 240-volt services for their equipment.

Single-phase distribution transformers are more common in non-urban areas and it is very seldom that a shell-type transformer is used. There are two most common types of distribution transformers:

• Pole-mounted (or overhead) transformer. This type of transformer provides 110/240-volt “split-phase” power for residential and some commercial operations. It is the most commonly used transformer in the USA for distribution purposes. They can be of different sizes.
• Underground (or pad-mounted) Transformers. These types of transformers don’t differentiate from the pole-top units, but they are housed in an oil-filled metal structure that looks like a box and mounted to the ground-level concrete base, or “pad.” The cables of this device are hidden underground.

Instrument Transformer

An instrument transformer is a type of electrical equipment of high precision that in conjunction with protective relays is commonly used in power measuring devices. Its job is to separate or transform voltage or current values which are further measured by measuring devices like voltmeters or ammeters.

The main winding of the transformer is connected to the high voltage circuit, while the relay is connected to the secondary circuit. Here is a nice document in PDF with details on instrument transformers.

There are two primary kinds of instrument transformers:

• Current Transformer (CT)

A current transformer is used to reduce the current in a power grid to a degree that it could be used by an Ammeter of a low rating.

Here is a PDF file for “CurrentTransformer” from the picture above.

• Potential transformer (PT) or Voltage transformer (VT).

A potential transformer is used to reduce the voltage of a power source to a degree that can be used by a Voltmeter of a low rating.

And this is a PDF file for the “Potential Transformer”.

** Schematics Disclaimer! The above, illustrated images, depicting various parts of transformers and their cooling mechanisms which include radiators, fans, etc., and the direction of flow of air, oil, and water are employed for the purpose of providing clarity and easy, conceptual understanding. However, in reality, the arrangement and appearance of such parts are completely different.

ALL schematics you can download here in a single PDF file.

Transformers by insulation and cooling

Feel free to read an article that fully covers transformers by cooling methods and insulation.