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What Are Electric Transformer Types?

Electrical transformers can be classified based on core medium, function or number of phases!

In my daily work life, as electrical engineer, I can use all  classifications as required. Some times, say, for instance, we need a current transformer for this job, or we need three phase transformer for other task.

The point is, it depends on what the task requires.

Transformer Types Based on Core Medium

Transformers different types based on the core medium placed between the primary and secondary windings are classified as, Air core and Iron core types.

Air Core Transformer

The flux between primary and secondary windings is linked through the air. Both the primary and secondary are wound on a non-magnetic strip.

Hysteresis and eddy current losses are eliminated in air-core type transformer, but compared to iron core the mutual inductance is less.

The reluctance offered to the generated flux is higher in the air transformer type than iron core type.

Iron Core type Transformer

three phase iron core transformer
three phase iron core transformer

Iron core transformer is a transformer with its windings mounted on a laminated iron core to make a perfect linkage path for the flux between primary and secondary.

Windings of primary and secondary are wound on multiple iron plate bunch.

Due to the conductive and magnetic property of the iron, less reluctance to the linkage flux is offered.

Iron core type transformers are widely used, in which the efficiency is high compared to the air core one.

Also, iron core transformers are smaller in size than air core ones of the same power rating.

Transformers Types Based on function

Electrical transformers main function is to transfer power from power source to load by stepping voltages up or down.

Transformers types based on function are, step up and step down transformers. Let’s discuss each of them.

Step Down Transformer

Step down transformer is this type of transformers that converts high voltage level applied on the primary winding to low voltage level on the secondary side. No matter what power rating the transformer is.

Step down transformer is used in electronic applications, and also in overhead power lines electrical distribution systems and household devices where low voltages are required for operation.

We can summaries applications of step down transformers as follows:

  • Chargers and adapters to step voltage level down to be suitable for charging cell phones and electronic devices.
  • Control circuits to step down voltage to feed control circuits. This application is widely used in industrial applications. We usually use step down transformer when we have a control circuit with voltage rating lower than the main source voltage.

For example at my work, we have a 3.3KV soft starter panel, while in the same panel we have low voltage control circuit, contactors and relays.

In this case, its clear we should use a 3.3KV/220V step down transformer to feed the low voltage control circuit.

  • Distribution substations to step down voltages from high to medium or low levels.
  • Before domestic loads and end users of electrical energy to step down voltages from medium to low values.

Step up transformer

The function of Step Up transformer is the opposite of the step-down transformer.

Step up transformer increases voltage applied at the primary windings to a higher voltage at the secondary windings.

In this type , the ratio of primary to secondary turns (n1/n2) is less than one. Step Up transformers are used in electrical energy transmission systems.

One of the main uses of step up transformers is electrical transmission through OHTL.

The power generation voltage is connected to power transformer, which is a step up transformer, to step up voltages before transmitting it through overhead transmission lines.

Why do we step up transmission voltage?

This voltage stepping up is essential because it gives the following advantages:

  • Increasing the transmission voltage decreases the current which in turns decreases the power losses during the long distances power lines.
  • The lower the transmitted current the smaller the wire size and the smaller the size of the wire carrying accessories. i.e lower cost.
  • The lower the wires and its accessories the smaller the towers i.e more money saving.

Also, step up transformers are used in case of voltage stabilizers. In case of voltage drop we use step up transformer to overcome this voltage drop issue.

Read my detailed article about step up and step down transformer and there uses here.

Power Transformer

A power transformer is a step up transformer connected to the power grid from one side, and to the OHTL from the other side. Its main job is to step up the generated voltage to be transmitted through OHTL.

The rating of the power transformer is 33 KV, 66KV, 110KV, 220KV,400KV, and so on. Power transformers are installed at power generation stations.

Power transformer loads aren’t variable. This transformer type usually works at its full load capacity.

I have created a YouTube video about what is power transformer. You can find it on YouTube below at the end of the page.

Distribution transformer

This type is used to step down the voltage level. Yup, it’s a step down transformer. Normally is used at the consumer end.

Therefore, the rating of this type of transformer is lower than power transformer, for example 230 V, 440V, 6.6 KV, and 11KV.

The main uses are domestic as well as industrial purposes.

Power transformer vs Distribution Transformer

These two types of transformers are the same construction and working principle. The differences are:

  1. The rating of power Transformer is higher than that of distribution Transformer. Although there is no standard that specify the rating of each type. But some references consider that Power Transformers are higher than 500KVA. 
  2. Usage of distribution Transformer is in the distribution system, while power Transformer is mainly used in the power generation plants.
  3. Distribution transformers work on different load values, i.e the load is not fixed and varies during the day intervals. While power transformer loads are usually fixed.
  4. Power transformer works on full load, while distribution transformer can work on no load.

Its also important to know that IEC standard consider that the two types, I mean power and distribution, are one type.

Also, the American standard consider that both are power type Transformers, and the only difference is the rating as follows:

  • Small power Transformer, rating from 500-7500KVA
  • Medium power Transformer, rating 7500KVA-100MVA
  • Large power Transformer, rating 100MVA and up.

Instrument transformer

An instrument transformer is a high accuracy step down transform that isolates or transforms current or voltage level, to use this lower level in measurement or protection devices.

Instrument transformer is used in measuring instruments and protection devices such as voltage meters, current meters and protection relays.

Instrument transformers are mainly designed to operate at low voltage and low current.

These transformers also called isolation transformers because, when there is a high voltage at the primary end, It isolates the secondary windings.

The purpose of the isolation is to protect the measuring device which is attached to the secondary side.

We can divide instrument transformer into two types. Current transformer (CT), and potential transformer (PT).

Current Transformer (CT)

AC Current Meter
AC Current Meter requires a current transformer to measure electric current

Current transformer is an instrument transformer that steps down AC current to use it for overcurrent protection and measuring of electric current.

It converts the high current into the desired value of the current required in the circuit.

Current transformer is used in swichgear, control panels, electrical generators and other applications where the current readings should be visible instantly.

The CT is connected to an AC current gauge and shows the current value.

For more information aboutcurrent transformer, read my article here.

Potential Transformer (PT)

Potential transformer is a step down transformer that is used to measure voltage when the primary side is connected with medium or high voltages. It’s also used to step down voltage into a safe limit.

Potential transformer also is used to energize control and utility circuits with lower voltage than the main bus voltage.

As I told you above in this article, in my work we have a 3.3KV soft starter panel. Inside the panel we have some low voltage contactors and control devices.

The solution of feeding these devices is to use a potential transformer. In this case if the low voltage control devices are 110v.

Then the potential transformer should be 3.3/110V.

Uses of instrument transformers:

  • In measuring instruments to work as a ratio device between high voltages and current circuits and the low voltage and low current measuring device.
  • To isolate testing devices and the person who is using it from high voltages and currents.

A good example of this transformer type is medium voltage current and voltage measuring devices.

I use a medium voltage voltmeter in my work. The over head power line voltage is 11KV.

With the help of this medium voltage voltmeter I can make my work safely.

Transformers types according to phases number

Electrical loads and circuits are one of two types, single and three phase. Single phase loads are like domestic loads and small applications single phase motors.

While industrial loads use 3 phase induction motors, heaters and other machines working on 3 phase power. Besides some single phase loads.

Single Phase Transformer

Single phase transformer transforms AC power from one single phase circuit to another.

The primary winding connects the AC power supply and the secondary side is connected with the load, both is a single phase connection.

It is mostly used in electronic circuits like televisions set for voltage regulations, chargers and adapters. Other uses are residential and commercial lighting and heating equipment.

Three-phase Transformer

Three-phase transformers are used to either step up or step down the voltage level in the power transmission three phase system. It is also used in power distribution grids.

I have written a detailed article about step up and step down transformer, I highly recommend reading it. You can find it here.

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