In the previous article we learned the basic concepts of a transformer. A transformer is simply a pair of inductors magnetically coupled to allow electromagnetic induction between them. With the help of transformers, AC voltages can be increased or decreased at low cost and without hassle. Increasing or decreasing DC voltages requires complex and expensive circuitry. This is why AC is used to distribute electrical energy, although most electronic devices use DC for their operation. Electronic devices convert the AC network into DC for their operation.
Transformers come in a variety of shapes, sizes and constructions. Transformers can be classified by core material, geometry and construction, voltage levels and use.
The main classifications are as follows:
- Laminated Iron Core
- ferrite core
- Iron Core Powder
- Air Core
The geometric classifications are as follows:
- Utility
- Solenoidal Core
- Toroidal Core
- Pot Core
The voltage level ratings are as follows:
- Step up
- Resign
- Isolation
Usage classifications are as follows:
- Power
- Measurement
- Distribution
- Pulse
- Audio
- IF
- RF
Transformer Cores
When building any transformer, manufacturers try to have maximum magnetic coupling between the two inductors. The magnetic coupling can be increased many times by using a ferromagnetic material or powdered iron as the core. A pair of inductors wound on a ferromagnetic core has a much better coupling coefficient compared to the air core transformer. However, the use of ferromagnetic core has its own limitations. Ferromagnetic cores have some power losses due to hysteresis and eddy currents and are also limited by current-carrying capacity. In addition to these limitations, the choice of core material also restricts the frequency range of a transformer. According to the type of core used, transformers are classified as follows
Rolled Iron Transformers – These transformers use silicon steel as the core material. Silicon steel is also called transformer iron or simply iron. Silicon steel is laminated in layers to prevent losses due to eddy currents and hysteresis. Eddy currents are circular currents that flow in magnetic material by magnetization. Eddy currents lead to loss of energy from the magnetic core in the form of heat. Hysteresis is the tendency of a magnetic core to be slow in accepting fluctuating magnetic flux. Due to hysteresis losses and eddy currents, these transformers are only suitable for 60 Hz frequency and other low frequencies in the audio range. As the frequency increases above a few kilohertz, internal core losses increase beyond viable limits.
Ferrite Core – Ferrite cores have high permeability and require fewer coil turns. However, at frequencies above a few megahertz, such cores begin to experience significant power losses due to eddy currents and hysteresis. This is why these transformers are suitable for frequencies above audio frequencies up to a few megahertz.
Iron Powder Core – Iron powder also has high permeability and lower losses due to hysteresis and eddy currents compared to ferrite cores. As the frequency increases, the need for high permeability decreases. Powder iron core transformers are suitable for very high frequencies up to 100 MHz. As there is no need for high permeability at very high frequencies above 100 MHz, air core transformers are more suitable due to their energy efficiency.
Air Core Transformers – In air core transformers, the primary and secondary coils are wound in a diamagnetic material. The magnetic coupling in these transformers happens through air. In these transformers, not only is the inductance of both coils low, but the mutual inductance is also very low, so there is very little magnetic coupling between the coils. These transformers have no power loss due to hysteresis or eddy currents and are also capable of moderating high currents. These transformers are suitable for high voltage applications where energy efficiency is a primary concern, such as distribution transformers. They are also suitable for very high RF applications above 100 megahertz. At high radio frequencies, the required inductance value is low, which can be easily achieved by air-core inductors, and energy efficiency is the main concern of VHF circuits.
It should be noted that the following symbol represents air core transformers:
Transformers with a magnetic core are represented by a symbol in which two parallel lines are added between the coil symbols as follows:
Transformer geometry and construction
Transformers can also be classified by their shape and geometry. The shape of a transformer depends on the type of inductor used in its construction and the shape of its core. Any transformer is essentially a pair of inductors wound on the same core. The classifications are as follows:
Utility Transformers – Utility transformers are power transformers in which laminated iron is used as the core material. These iron core transformers come with a variety of core shapes such as E, L, U, I, etc., and are bulky and heavy. The most common core shape used in these transformers is the E core or EI core because the laminated core is shaped like the letter 'E' with a bar placed at the open end of 'E' to complete the construction. The coils are wound on the core using the shell method or the core method. In the shell method, both coils are wound on the central bar of the 'E', one above the other. This ensures maximum magnetic coupling between the coils, but at the cost of high coil-to-coil capacitance. The shell method also limits the current carrying capacity of the transformer. In the core method, one coil is wound on the top bar of the 'E' and the other on the bottom. The magnetic coupling between the coils occurs only due to the magnetic flux through the core. The core method greatly reduces the coil-to-coil capacitance and makes it possible to handle high voltages. EI core utility transformers having a core or shell winding are most commonly used as 60 Hz transformers and other audio frequency transformers.
Solenoidal Coil Transformers – Solenoidal core transformers are generally used as loopstick antennas for radio frequency circuits. These transformers have primary and secondary windings on a cylindrical core (ferrite or iron powder). The coils are wound on top of each other or separated. In these transformers, the primary captures the radio signals and the secondary provides impedance matching to the first amplifier stage of the radio circuit. Such transformers have been quite common in portable radio communications equipment.
Solenoid Coil Transformer vs Toroidal Coil. (Image: Leets Academy)
Toroidal Core Transformers – Toroidal core transformers have a primary and secondary winding on a toroidal core and the coils can be wound on top of each other or separated. Toroidal cores are a better alternative to solenoidal cores in radio frequency circuits. They contain the magnetic flux within the core, so these transformers can be directly mounted without any additional shielding as long as the coils are insulated. In addition to no electromagnetic interference, toroidal cores also provide greater inductance around the coil. As the magnetic flux remains contained within the core, toroidal core transformers feature better magnetic coupling between the coils.
Pot Core Transformers – Pot Core transformers have the primary and secondary wound on one half, one on top of the other, or next to each other. The potentiometer cores provide the highest possible inductance with the obvious advantage of self-protection. One of the main disadvantages of pot core transformers is the coil-to-coil capacitance. Due to the coil-to-coil capacitance and the exceptionally high inductance of both coils, potentiometer core transformers are only suitable for low frequencies. At high frequencies, the required value of inductance is low and the capacitive reactance needs to be essentially minimized.
Transformer voltage levels
The most common application of a transformer is to moderate AC voltages. A transformer can increase, decrease, or leave AC voltage levels intact. This is the easiest but most important classification of transformers. They are the following:
Step-up transformer – In a step-up transformer, the secondary has a greater number of turns than the primary. Since the primary to secondary turns ratio is less than 1, the voltage applied to the primary is increased to a higher voltage on the secondary. Consequently, this comes at the cost of lower current levels in the secondary winding. Step-up transformers are used in stabilizers and inverters where lower AC voltages need to be converted to higher voltages. They are also used in power grids to increase AC voltage levels before distribution.
Voltage level architecture of a step-up transformer, where the secondary has a greater number of turns than the primary. (Image: top-ee.com)
Step-down transformers – In a step-down transformer, the primary has a greater number of turns than the secondary. Since the turns ratio of the primary to secondary winding is greater than 1, the secondary voltage is less than the primary voltage. Step-down transformers are commonly used in electronic applications. Electronic circuits typically require 5V, 6V, 9V, 12V, 18V or 24V for their operation. Step-down transformers are commonly used in power supply circuits before rectifiers to step down the 120V or 240V AC mains to the required low voltage levels. In power distribution, step-down transformers are used to reduce high voltages to supply electrical power to the poles. This ensures energy efficiency and savings in electrical energy distribution.
Voltage architecture of a step-down transformer where the primary has a greater number of turns than the secondary. (Image: top-ee.com)
Isolation Transformers – Isolation transformers have the same number of turns on the primary and secondary. Since the ratio of the number of turns from primary to secondary is exactly 1, voltage levels remain the same in both windings. These transformers are used to provide electrical isolation between electronic circuits or to cancel the transfer of noise from one circuit to another. Isolation transformers need to have high inductive coupling and minimal capacitive coupling. This is why these transformers are designed to have a minimum number of turns in separate coils wound on a highly magnetic, self-protected core.
Isolation transformers are also used to connect balanced and unbalanced circuits. Balanced circuits are those that can be connected in any way through one port. Unbalanced circuits are those that need to be connected in a specific way through a port. Balanced and unbalanced loads can be connected via isolation transformer by grounding the center tap on the balanced side. If the balanced and unbalanced loads have the same impedance, then the isolation transformer must have a turns ratio of 1. If a balanced and unbalanced load have a different impedance ratio, the turns ratio must be the square of the impedance. Isolation transformers are also used to couple amplifier stages in radio frequency transmitters and receivers.
In the next article we will continue with the classification of transformers by use. By usage, transformers largely belong to the electrical or electronic domain. In the electrical domain, transformers are generally classified by their respective applications. In the electronic domain it is easy and obvious to classify transformers by the frequency of their operating signal.
