A transformer is a widely used device in the electrical and electronics domain. It is an electromagnetic device which follows the basic principle of electromagnetism discovered by Michael Faraday. We have covered about Transformers construction and operation in detail in previous tutorial. Here we will cover different types of transformers used in different types of applications. However, all types of transformers follow the same principles but they have different construction method.
Transformer Types based on Voltage Level
A Transformer can have multiple types of construction. Transformer does not have any electrical connection from one side to another; still, the two electrically independent coils can conduct the electricity by electromagnetic flux. A transformer can have multiple coils or windings on the primary side as well as on the secondary side. In several cases, multiple primary sides, where two coils are connected in series, often called as a center tapped. This center tapped condition can also be seen on the secondary side.
Transformers can be constructed in a way that it can convert the voltage level of the primary side to the secondary side. Depending on the voltage level, the transformer has three categories. Step Down, Step Up and Isolation Transformer. For the Isolation transformer, the voltage level is the same for both sides.
1. Step-Down Transformer
Step down Transformer is used in both Electronics and Electrical domain. A step-down transformer converts the primary voltage level to a lower voltage across the secondary output. This is achieved by the ratio of primary and secondary windings. For step-down transformers the number of windings is higher across the primary side than the secondary side. Therefore, the overall winding ratio of primary and secondary always remains more than 1.
In electronics, many applications run on 5V, 6V, 9V, 12V, 24V or in some cases 48V. To convert the single phase power outlet voltage 230V AC to the desired low voltage level, Step Down transformers are required. In instrumentation as well as in many electrical types of equipment, Step-Down transformer is the primary requirement for the Power section.
In electrical, step down transformers are used in electrical distribution system which works on very high voltage to ensure low loss and cost-effective solution for long distance power delivery requirements. To convert the high voltage to a low voltage supply line, Step down transformer is used.
2. Step-Up Transformer
Step Up transformer is exactly opposite of the step-down transformer. Step up transformer increase the low primary voltage to a high secondary voltage. Again it is achieved by the ratio of primary and secondary winding ratio. For the Step Up transformer, the ratio of the primary winding and the Secondary winding remains less than 1. That means the number turns in secondary winding is higher than the primary winding.
In electronics, step up transformers often used in stabilizers, inverters etc where low voltage is converted to a much higher voltage.
A step-up transformer is also used in Electrical power distribution. High voltage is required for power distribution related application. Step up transformer is used in the grid to step up the voltage level before the distribution.
3. Isolation Transformer
Isolation transformer does not convert any voltage levels. The Primary voltage and the secondary voltage of an isolation transformer always remain the same. This is because the primary and the secondary winding ratio is always equal to the 1. That means the number of turns in primary and secondary winding is same in isolation transformer.
The isolation transformer is used to isolate the primary and secondary. As discussed previously, the transformer does not have any electrical connections between primary and secondary, it is also used as an isolation barrier where the conduction happens only with the magnetic flux. It is used for safety purpose and to cancel noise transfer from primary to secondary or vice-versa.
Transformer Types based on Core material
The transformer transfers the energy by conducting electromagnetic flux through a core material. Different core materials produce different flux density. Depending on the core materials, several types of transformers are used in the power and electronics domain.
1. Iron Core Transformer
Iron core transformer uses multiple soft iron plates as the core material. Due to the excellent magnetic properties of iron, the flux linkage of the iron core transformer is very high. Thus, the efficiency of the iron core transformer is also high.
The soft iron core plates can be available in multiple shapes and sizes. The coils of the primary and secondary wound or wrapped on a coil former. After that, the coil former is mounted in soft iron core plates. Depending on the core size and shapes, a different type of core plates is available in the market. Few common shapes are E, I, U, L, etc. The iron plates are thin, and multiple plates are bunched together to form the actual core. For example, E type cores are made with thin plates with a look of letter E.
Iron core transformers are widely used and usually heavier in weight and shape.
2. Ferrite Core Transformer
A ferrite core transformer uses a ferrite core due to high magnetic permeability. This type of transformer offers very low losses in the high-frequency application. Due to this, ferrite core transformers are used in high-frequency application such as in switch mode power supply (SMPS), RF related applications, etc.
Ferrite core transformers also offer a different type of shapes, sizes depending on the application requirement. It is mainly used in electronics rather than electrical application. The most common shape in the ferrite core transformer is E core.
3. Toroidal Core Transformer
Toroidal core transformer uses toroid shaped core material, such as iron core or ferrite core. Toroids are ring or donut shaped core material and widely used for superior electrical performance. Due to the ring shape, the leakage inductance is very low and offers very high inductance and Q factors. The windings are relatively short and weight is much less than traditional, same rating transformers.
4. Air Core transformer
Air Core transformer does not use any physical magnetic core as the core material. The flux linkage of the air-core transformer is made entirely using the air.
In air core transformer, the primary coil is supplied with alternating current which produces an electromagnetic field around it. When a secondary coil is placed inside the magnetic field, as per the Faraday law of induction, the secondary coil is induced with a magnetic field which further is used to power the load.
However, air core transformer produces low mutual inductance compared to physical core material such as iron or ferrite core.
It is used in portable electronics as well as Radiofrequency related applications. Due to the absence of physical core material, it is very light in terms of weight. Properly tuned air core transformer also used in wireless charging solutions, where the primary windings are constructed inside the charger and the secondary windings are situated inside the targeted device.
Transformer Types based on Winding Arrangement
The transformer can be classified using winding orders. One of the popular types is Auto Winding Transformers.
Auto Winding transformer
Till now, the primary and secondary winding is fixed but in case of an auto-winding transformer, the primary and the secondary coil can be connected in series and the center tapped node is movable. Depending on the center tapped position, the secondary voltage can be varied.
The auto is not the short form of Automatic; rather it is to notify the self or single coil. This coil forms a ratio which consists of two parts, primary and secondary. The position of the center tap node determines the primary and secondary ratio thus varying the output voltage.
The most common use is the VARIAC, an instrument to produce variable AC from a steady AC input. It is also used in Power transmission and distribution related applications where the high voltage lines are needed to be changed frequently.
Types of Transformers based on Usage
There are several types of transformers also available which works in a specific domain. Both electronics and electrical sectors, several dedicated transformers are used as a step-down or step-up transformer based on the application of application. So, the transformers can be classified as below based on usage:
1. Power Domain
- Power Transformer
- Measurement Transformer
- Distribution Transformer
2. Electronics Domain
- Pulse Transformer
- Audio Output Transformer
1. Transformers used in Power domain
In Electrical, the Power domain deals with the power generation, measurement, and distribution. However, it is a very large field where transformers are an essential part to accommodate safe power conversion and successful power delivery to the substation and to the end users.
The transformers which are used in the power domain can be both outdoor and indoor but mostly outdoor.
(a) Power Transformer
Power Transformers are larger in size and used to transfer the energy to the substation or the public electricity supply. This transformer acts as a bridge between the power generator and the primary distribution grid. Depending on the Power rating and specification, Power transformers can further be classified into three categories: Small power transformer, Medium Power transformers, and the Large power transformers. The rating can be more than 30KVA to the 500-700KVA or in some cases that can be equal to or more than 7000KVA for small rated power transformer. The medium rated power transformer can be up to 50-100 MVA whereas large rated power transformers are capable to handle more than 100MVA.
Due to very high power generation, the construction of a power transformer is also critical. The construction includes solid insulating peripherals and well balanced cooling system. The most common power transformers are filled with oils.
The main principle of the power transformer is to convert the Low voltage high current to a high voltage low current. This is required to minimize the power loss in the power distribution system.
Another important parameter for the Power transformer is the phase availability. Typically Power transformers works in three phase system, but in some cases, Single phase small power transformers are also used. Three Phase Power transformers are the most costly and efficient than the single phase power transformers.
(b) Measurement Transformer
Measurement transformer is often referred to as an instrument transformer. This is another commonly used measurement instrument in the power domain. A measurement transformer is used to isolate the main power and convert the current and voltage in a smaller ratio to its secondary output. By measuring the output, the Phase, Current and Voltage of the actual power line can be measured.
The above image is showing the construction of the current transformer.
(c) Distribution Transformer
This is used in the last phase of the power distribution system. Distribution transformers are step down transformer, which converts High grid voltage to the end customer required voltage, 110V or 230V. It can also be single phase or three phases.
Distribution transformers can be smaller in shape as well as bigger, depending on the conversion capacity or ratings.
Distribution transformers can be further categorized into based on the type of insulation it uses. It can be a dry type or can be liquid-immersed. It is made using laminated steel plates mostly constructed in C shape as a core material.
Distribution transformer also has a different type of classification based on the location it is used. The transformer can be mounted on a utility pole, if so, it is called a pole mounted distribution transformers. It can be placed inside of an underground chamber, mounted on a concrete pad (pad mounted distribution transformer) or inside an enclosed steel box.
Generally, distribution transformers have a rating of less than 200kVA.
2. The transformer used in Electronics domain
In electronics, various small miniature transformers are used which can be PCB mounted or can be fixed inside the small product enclosure.
(a) Pulse Transformer
Pulse transformers are one of most used PCB mounted transformers that produce electrical pulses in a constant amplitude. It is used in various digital circuits where pulse generation is needed in an isolated environment. Therefore, the pulse transformers isolate the primary and secondary and distribute primary pulses to the secondary circuit, often digital logic gates or drivers.
Properly constructed pulse transformers should need proper galvanic isolation as well as small leakage and stray capacitance.
(b) Audio Output Transformer
Audio Transformer is another commonly used transformer in the electronics domain. It is specially used in Audio related application where impedance matching is required. Audio transformer balances the amplifier circuit and loads, typically a loudspeaker. The audio transformer can have multiple primary and secondary coils, separated or center tapped.
So we have covered various kinds of transformer, other than that there are some other special purpose transformer but they are out of scope of this article.