What are the Different Types of Transformers? Explained

In this post I am going to explain different types of electrical transformers that are used in a wide variety of applications.

Transformers can be classified in many different ways, for example: based on their input / output character, voltage, frequency, application, power handling capacity etc. 

In this post we will be exploring all the major types of transformer which enables our modern life.

We are going to explore: 

• Transformers based on their input and output voltage.
• Transformers based on their material construction.
• Transformer based on their application. 

The above classification will be further subcategorized into several branches and we will illustrate with the help of block diagrams. But before we jump into the article let’s try to understand what a transformer is?

What is a Transformer?

A transformer is a passive component typically consisting of two or more windings wrapped up on a core material which transfers electrical energy from one winding to another winding or to multiple windings through electromagnetic induction and mutual induction.

The primary function of a transformer is to increase (step-up) or decrease (step-down) the output voltage.

Typically a transformer provides galvanic isolation between input and output circuits.

All types of transformer work primarily on alternating current (AC) and also can work on pulsating DC current. No transformer can operate on continuous / pure DC current. 

Transformer classification based on input and output voltage:

Step-Down transformer:

A step down transformer consists of a core material like iron, ferrite or toroid; the primary winding (where the input is applied) will have more winding than secondary winding (where the output is supplied).  

Due to the lesser number of turns of winding at secondary side the output voltage will be lower than the input, but the current will be higher than the input, ideally the power input and power output will be the same.

Step-up Transformer:

The step-up transformer does the inverse function of a step-down transformer.

The number of turns of winding in the primary side is less than the secondary winding, due this the output voltage (at secondary side) is higher than the input and the current is lower than the input. 

Still the input and output power is ideally the same, this is true for all types of transformer.

Isolation transformer:

The primary function of an isolation transformer is to provide galvanic isolation between input and output sides. 

Galvanic isolation means there is no direct contact between the input and output circuits, but still it can transfer electricity via other means, in a transformer the energy is transferred using alternating / pulsating magnetic fields. Galvanic isolation also exists in step-up and step-down transformers.  

The number of windings at primary and secondary sides are equal and hence input and output voltage, current, power are ideally the same but practically there will be some loss in the form of heat and this is true for all types of transformer. 

Auto transformer / Variac / Variable transformer:

An auto / variable transformer is a type of transformer that can vary its output voltage by rotating the provided knob. We can step-up or step-down the voltage as we wish. 

Unlike the conventional transformers that I have explained till now, the auto transformer has just single winding. The input and output act on the single winding, hence NO galvanic isolation between input and output. 

• Step-down configuration: 

The above illustrated schematic is of an auto transformer in step down configuration, it has single continuous winding and the output is taken in between the winding. 

• Step-up configuration:

If we reverse the input supply we can achieve a higher voltage than the input. Practically we don’t need to reverse the supply; instead we just need to rotate the armature which sweeps across the winding to give us the desired voltage.

This type of transformer is used in low and high power voltage stabilizers, where a servo motor controls the armature and provides a stable output voltage. 

Transformers Classification based Core Material

A transformer can be classified according to material used in the construction of the core. There are three major types of core material: iron / steel, ferrite and air. 

But, before we dive into the classification let’s understand why a core is necessary for a transformer?

The core of the transformer is responsible for constraining the magnetic field within the transformer and conducts the magnetic flux from one winding to another with minimal flux loss. It provides a low reluctance (resistance) path for the magnetic field to flow. 

Without the core, the magnetic field from the primary coil will spread into surroundings and only few magnetic flux lines reach the other coil winding. The core also provides structural strength to the transformer.

Soft Iron and silicon Steel core:

The common types of transformer core materials are iron and steel used in low frequency operation 50/60Hz. The soft iron and silicon steel has good magnetic permeability without getting saturated, meaning it can get magnetized easily but does not retain the magnetization after removal of the magnetic field.

The transformer core is not constructed with a single continuous block of material; instead thin sheets of insulated soft iron or silicon steel (illustrated below) are stacked and the coil windings are wound around it, this is to reduce eddy current effect which will waste energy as heat.    

Ferrite based Transformer Core:

Ferrite is a ceramic material made by blending iron oxide with oxides or carbonates of zinc, nickel, magnesium or manganese. 

The core of the transformer can be made using a continuous solid block of ferrite, since ferrite material is non-conductor, no eddy current losses like iron or steel cores.

Ferrite core based transformers are used in high frequency applications like switch mode power supplies and RF applications. 

Ferrite cores can be manufactured into any shape or size we desire. Ferrite material is brittle like any other ceramic material, hence the care should be taken while transporting and dropping it on a hard surface will cause damage to the core material. 

Ferrite core transformers can handle a lot of power in a small footprint compared to iron core type, the drawback of ferrite core transformers is that they cannot operate efficiently on low frequencies like 50 / 60 HZ.

Air core:

Air core transformer as the name suggests it utilizes air as core for its operation. Air core transformer’s coil windings are wound on a plastic material and more stiff wires are used for its structural strength. 

Air core transformers have less inductance but have low core-loss compared to other types of transformer where core material causes some loss, however other losses like dielectric loss exist.  

Air core based transformers are used in radio frequency applications, the air core permits higher quality factor or Q-factor for the signal. 

Air core transformers are also used where high magnetic field strength is required (above 2 tesla) and due to the absence of a core material we can achieve zero magnetic saturation which is not possible with a core material. 

Transformer Classification based on usage:

Transformers are used in a wide variety of applications and particular names are given to such application specific transformers.

We are going to explore the topics that are mentioned in the above categories. There are few other types of transformers which are not covered in this post because of their limited usage. 

Instrument Transformer:

Instrument transformers are the category of transformers where they are used for measuring electrical parameters. Instrument transformer is further classified into three categories:

1. Voltage transformer.
2. Current transformer.
3. Combined instrument transformer.

Voltage Transformer:

A voltage transformer is used for measuring high voltages; it provides good galvanic isolation between high voltage line and measuring components. 

Voltage transformers are connected in parallel to the supply and they have high impedance primary winding so that they don’t load the supply.

The primary and secondary winding voltage ratio is made linear for accurate measurement and usually follows step-down winding configuration.

The voltage transformer can also have multiple secondary windings for measuring and protection circuits.

Current Transformer:  

The current transformer as the name suggest is used for measuring current and it will be connected in series with the power supply. 

The primary winding consists of just one for few windings and the secondary winding consists of several windings, the output at secondary winding is proportional and linear with primary winding’s input current. 

The core of the current transformer is usually a toroid (round shaped) where the primary winding just passes through it and the secondary winding will be wound on the toroid as illustrated below. The secondary winding may have several taps depending on the application. 

In measuring instruments it is common to find a current transformer in the form of a clamp that can open the core and close around a wire that needs to be measured. The above image shows a permanently attached current transformer. 

Combined Instrument Transformer:

A combined instrument transformer utilizes both voltage and current transformers in the same body and it is used widely in sub-stations for measuring electrical parameters for protection systems and even for estimating revenue. 

Using combined instrument transformers in substations reduces the physical footprint of the transformer. 

This concludes the discussion about the instrument transformer.

Trigger transformer / Pulse transformer:

A trigger transformer also known as pulse transformer is predominantly used for driving circuits with optimized parameters at the output side.

A pulse transformer is optimized for transmitting rectangular pulses, which essentially means fast rising and falling pulses.

Pulse transformers are used for driving MOSFET gates, IGBTs and digital circuits. A medium sized pulse transformer is used for controlling flash circuitry of cameras. High voltage pulse transformers are used for generating high voltage pulses in Tasers, radars, particle accelerators etc.   

RF Transformers:

RF transformers are the type of transformers used at radio frequencies. The RF transformer is further divided into 4 types as illustrated below: 

• Air core transformer:

In the previous section we saw what an air core transformer is, it is used in RF applications. The air core transformer consists of just a few turns of winding soldered directly on the PCB.

Since there is no core material its inductance will be very low and due to this air core transformers can achieve very high frequency and due to mutual induction like any other transformer primary winding energy is transferred to secondary winding depending on the transformer's winding ratio.

• Ferrite core transformer:

In one of the previous sections, we saw what a ferrite core transformer is and it is not only used in power electronics, it is also widely used in RF applications. 

Ferrite core transformer is employed in RF applications for impedance matching and the transformer consists of just a few turns.

Impedance matching is very important in RF applications, because if impedance is not properly matched between two RF circuit stages, it can cause inefficient power transfer and signal reflections which we must suppress as far as we can.

• RF transmission line transformer:

RF transmission line transformers are made using twisted pair cables, co-axial etc. which is wound on a ferrite core or other types of core material.

This type of transformer is utilized to achieve transmission of wide band signals where the transformer needs to efficiently transfer low and high frequency signals instead of being selective at one or few frequencies.

The core of the transformer increases inductance by several fold, hence increases Q factor. The RF transformer is also used for matching impedance between two RF stages.

• Balun Transformer:

Balun transformer is made for interfacing between balanced and unbalanced RF circuits.

The Balun is placed between source and the load and it is a two port component and it can convert unbalanced signals to balanced signals and the opposite of this is also true. It can also provide impedance matching between source and load. 

This concludes our discussion about RF transformers.

Power Transformer:

Power transformer is the type of transformer involved in transmission and distribution of high voltage / high energy supplies for commercial and residential purposes.

Power transformers include: iron core transformers, autotransformers, isolation transformers etc. which we already explored in one of the previous sections.

In this section we are going to explore specifically about transformers that are used in power generation and distribution and here are the types:

1. Polyphase transformer.
2. Phase shifting transformer.
3. Grounding transformer.
4. Variable frequency transformer.

• Polyphase Transformer:

Polyphase transformers are the type that can handle multiple phases in one transformer for stepping down or to stepping up the voltage.

Polyphase transformer consists of multiple single phase winding and below is the illustration of a polyphase transformer with cutout view. 

The common type polyphase transformer is a three phase transformer who’s primary and secondary windings are connected in delta and wye configurations as illustrated below.

The transformer’s primary and secondary windings can have the above configurations in any combinations like wye-wye, wye-delta, delta-wye, delta-delta. 

Wye configuration has 4 terminal points; the 4^th point is the middle where three windings meet; it is also used as return path (neutral) in unbalanced loads.    

• Grounding transformer / earthing transformer:

Grounding transformer also known as earthing transformer is a supplementary transformer used in three phase power systems for grounding. 

It is used for providing ground (return) path for delta and ungrounded wye system.

Since delta configuration does not offer a neutral point for a load, we need this type of transformer working alongside with a three phase transformer to provide a neutral point, so that a load can be connected between phase and neutral. They also provide a low impedance (resistance) path to ground.

• Phase shifting transformer:

Phase shift transformer also known as quadrature booster transformer that can shift phase angle of its output. It is used to control the flow of active power / real power of a three phase transmission system.

Let’s consider a grid that distributes power to two places namely A and B, when power requirement at A is more compared to B, more power can be sent to area A by manipulating phase angle of alternating current and reliving overload situation. 

When there is more power required at area B when compared to A, phase angle manipulation can be done to satisfy the need of B.

In conclusion, the phase shift transformer balances the power on a grid by distributing it.      

• Variable frequency transformer:

VFT or variable frequency transformer is used for transmitting power between two unsynchronized AC systems.

Unlike DC systems where two similar voltages can be connected in parallel directly with the help of diodes, an AC system cannot do this without proper synchronization; otherwise we will end up short circuit. 

Unlike conventional transformers, VFT has a stator and a rotor which need to be rotated for its normal operation, so it is not solid-state. VFT is a new technology as of now. 

This concludes about power transformers.

Audio Transformers:

An audio transformer is a type of transformer that is designed to handle signals ranging from 20 Hz to 20 KHz.

Audio transformers can eliminate DC components in the audio signal, it can also eliminate RF frequency interferences and they provide impedance matching for source and speaker which is essential for delivering maximum power at best efficiency.

Audio transformers can split an audio source into many and also can combine multiple audio sources to one.

Since audio transformers are magnetic components they can be susceptible to external magnetic interferences like power lines that carry 50 or 60 Hz mains power, this can be heard as humming noise in the speaker.

Audio transformers can affect audio quality because their magnetization at the core is not linear and also does not have linear frequency response. 

Audio transformers are classified into few types:

1. Loudspeaker transformer.
2. Output transformer.
3. Coupling transformer.

• Loudspeaker transformer:

A loudspeaker transformer is for minimizing transmission losses over a long distance where the audio source is far from the speaker (example public address system). 

At the audio source, the signal voltage is stepped-up and far away at the speaker a step-down transformer is used that matches the speaker’s voltage specifications.

• Output transformer:

Output transformers are found in vacuum tube based amplifiers where it is used to match impedances. The tube requires a high impedance load, whereas the speaker is a low impedance load. 

Coupling transformer:

Coupling transformers are used to interface different stages of an audio amplifier circuit. Their primary task is to connect two different circuit stages and yet electrically isolate them from each other.

Coupling transformers can be used for inverting a signal by flipping its output to the next stage and it is also for impedance matching. 

This concludes our discussion about audio transformers.