The Kynix Blog - Transformer

Power transformers are essential devices that adjust voltage levels to efficiently transfer electricity across long distances. They work by using electromagnetic induction to convert high voltage from power plants into lower voltage suitable for homes and businesses. Imagine them as bridges that connect different parts of the electrical system, ensuring smooth energy flow. Without reliable transformers, power outages and energy losses could disrupt daily life. In fact, transformer failures account for over 33% of prolonged outages, emphasizing their critical role in maintaining stable electricity supply.Power Transformers OverviewWhat Are Power TransformersPower transformers are static devices that transfer electrical energy between two or more circuits without changing the frequency. These devices use electromagnetic induction to move energy from one coil, called the primary winding, to another coil, called the secondary winding. The main purpose of power transformers is to change voltage levels, making it possible to send electricity over long distances and deliver it safely to homes and businesses.Electrical transformers do not create or use energy. They only transfer it from one place to another. This makes them passive devices in the power system.The basics of transformer operation rely on the turns ratio between the primary and secondary windings. When the number of turns in the coils changes, the voltage changes as well. This process allows transformers to step voltage up or down as needed. The magnetic core inside the transformer links the windings and helps induce voltage changes when current flows through the primary coil.Electrical transformers are essential for power generation, transmission, and distribution.They provide galvanic isolation, which means they separate different parts of the electrical system for safety.Transformers help match impedance and supply multiple voltage levels for different uses.Researchers have studied transformer basics to improve performance and safety. For example:Canola oil and other vegetable oils have been tested as eco-friendly insulating fluids for high-voltage transformers. These oils show good fire safety and stability at high temperatures.Some studies found that adding antioxidants to mineral oil and vegetable oil blends can improve insulation performance.New testing methods, like using ultrasound and artificial intelligence, help monitor transformer oil quality and predict faults.These research efforts support the development of safer and more sustainable electrical transformers for modern power systems.Role in Electrical SystemsTransformers play a vital role in every stage of the electrical grid. They step up voltage at power plants so electricity can travel long distances with less energy loss. When electricity reaches cities and neighborhoods, other transformers step the voltage down to safe levels for homes and businesses.Electrical transformers also help keep the power system reliable. Operators use advanced transformer infrastructure to collect real-time data from transformers. This data helps them spot overloaded or underused transformers and manage the system more effectively. By monitoring transformer basics, operators can prevent failures and reduce the risk of power outages.A study using survival analysis showed that spending more on preventive maintenance for power transformers lowers failure rates and outage costs. This means regular care and monitoring of transformers can keep the electrical system running smoothly, even in high-demand situations.Modern electrical transformers use advanced diagnostic tools, such as machine learning and big data analysis, to detect faults early. These tools help predict when a transformer might fail, allowing for timely repairs and better asset management.Electrical transformers support renewable energy systems, like wind farms, by handling unique stresses and helping detect faults.Sensor arrays and pattern recognition methods can analyze gases in transformer oil, giving early warnings of problems.These technologies make transformers more efficient and reliable, which is crucial for delivering electricity safely and consistently.Operating Principle of Power TransformersElectromagnetic InductionThe operating principle of power transformers centers on electromagnetic induction. This process allows transformers to transfer electrical energy from one coil to another without direct contact. When an alternating current flows through the primary coil, it creates a changing magnetic field. This magnetic field passes through the core and reaches the secondary coil. The changing magnetic field in the core induces a voltage in the secondary coil. This is the heart of transformer basics.A simple analogy helps explain this process. Imagine two people standing on either side of a fence. One person waves a magnet back and forth. The other person holds a coil of wire near the fence. The moving magnet creates a changing magnetic field, which passes through the fence and causes electricity to flow in the coil. In transformers, the core acts like the fence, guiding the magnetic field from one coil to the other.Most transformers achieve high efficiency in this process. Scientific experiments, such as heat run tests and computational fluid dynamics simulations, confirm that transformers can transfer about 99% of the input power to the output. Only about 1% is lost as heat, which is known as transformer losses. These experiments also show that the temperature inside a transformer changes with the load. The thermal time constant, which measures how fast the transformer heats up, depends on the amount of current flowing. This helps engineers design transformers that stay safe and reliable, even during overloads.The efficiency of electromagnetic induction in transformers depends on several factors. The core material, the number of turns in each coil, and the frequency of the alternating current all play a role. The equation for induced voltage is e = -N dφ/dt, where N is the number of turns and dφ/dt is the rate of change of magnetic flux. This equation shows how transformer basics rely on the relationship between the coils and the magnetic field.Note: Electromagnetic induction allows transformers to change voltage levels without changing the frequency of the electricity. This makes them ideal for power grids, where frequency must stay constant.Voltage TransformationVoltage transformation is the main function of power transformers. The operating principle of power transformers uses the turns ratio between the primary and secondary coils to change voltage levels. If the secondary coil has more turns than the primary, the transformer increases the voltage. If it has fewer turns, the transformer decreases the voltage. This process is called voltage conversion.The relationship between the number of turns and the voltage is simple. The ratio of the secondary turns to the primary turns equals the ratio of the output voltage to the input voltage. For example, if a transformer has 200 turns on the primary coil and 25 turns on the secondary coil, it can change 120 volts on the input side to 15 volts on the output side. This is a key part of transformer basics.Transformers do not change the frequency of the electricity. They only change the voltage. This feature is important for the stability of the electrical system. The operating principle of power transformers ensures that the power delivered to homes and businesses matches what is needed for safe operation.A table can help summarize the relationship between coil turns and voltage:Primary Turns (Np)Secondary Turns (Ns)Input Voltage (Vp)Output Voltage (Vs)20025120 V15 V100200110 V220 VTransformer losses, such as heat, occur mainly in the core and windings. However, these losses are small compared to the total power transferred. Most transformers operate at about 99% efficiency, making them very effective for voltage transformation in power systems.Tip: The ability to change voltage levels safely and efficiently makes transformers a key part of modern electrical networks.Components of Power TransformersImage Source: unsplashCore and WindingsThe core and windings form the heart of any transformer. The core consists of thin laminated steel sheets, each less than 1 mm thick, with a carbon content below 0.1%. Engineers add silicon to the steel to reduce energy losses from eddy currents. The core has two main parts: limbs, which hold the windings, and yokes, which connect the limbs at the top and bottom. This structure helps guide the magnetic field efficiently.Windings are made from copper or aluminum wire. The number of turns in each winding determines the voltage transformation. High-voltage windings use more turns of thinner wire, while low-voltage windings use fewer turns of thicker wire. Insulation materials, such as electrical-grade paper and transformer oil, protect the windings and prevent short circuits. All copper and aluminum transformers must meet strict DOE efficiency standards. These standards ensure that the components of power transformers operate with minimal energy loss.ComponentSpecification / Measurement DetailsCoreLaminated steel sheets < 1 mm thick; carbon < 0.1%; silicon alloyingCore StructureLimbs (vertical), Yokes (horizontal)WindingsCopper or aluminum; HV: more turns, thinner wire; LV: fewer turns, thicker wireInsulationElectrical-grade paper, pressboard, transformer oilCooling MethodsONAN (Oil Natural Air Natural), ONAF (Oil Natural Air Forced)Note: Transformer design programs adjust to meet national and international standards, such as IEC and IEEE, to guarantee reliable performance.Primary and Secondary CoilsThe primary coil receives the input voltage, while the secondary coil delivers the output voltage. The ratio of turns between these coils sets the voltage transformation. Engineers optimize coil design to reduce energy losses, such as copper losses (I2R) and iron losses. They select the wire gauge and coil shape carefully to balance efficiency and cost.Researchers use advanced algorithms to find the best design for both coils. They consider scenarios like minimizing copper use in the primary or secondary coil, or finding a compromise between the two. This approach helps create efficient and reliable components of power transformers for every application.Three-phase transformers use star (Y) or delta (Δ) winding configurations.Cooling methods, such as ONAN and ONAF, keep the coils at safe temperatures.The transformer equation, Vs = (Vp / Np) × Ns, links voltage to the number of coil turns.Types of Power TransformersImage Source: pexelsStep-Up and Step-Down TransformersEngineers use two main types of power transformers to manage voltage: the step-up transformer and the step-down transformer. A step-up transformer increases voltage from the primary to the secondary coil. This type is essential for power transmission over long distances because higher voltage reduces energy loss. A step-down transformer does the opposite. It lowers voltage to safe levels for homes and businesses. Both types play a key role in the electrical grid.The table below compares the main features of step-up and step-down transformers:AspectStep-Up TransformerStep-Down TransformerTurns RatioSecondary has more turns than primary (Ns > Np)Secondary has fewer turns than primary (Ns < Np)Voltage EffectIncreases output voltageDecreases output voltageCurrent EffectDecreases output currentIncreases output currentMaintenanceRequires less maintenanceRequires more maintenanceApplicationsPower transmission, X-ray machinesHomes, offices, power adaptersEfficiencyAbout 98%About 98%A step up transformer is often found at power plants. It prepares electricity for high-voltage transmission lines. A step down transformer is common in neighborhoods and buildings, making electricity safe for everyday use.Distribution and Transmission TransformersDistribution and transmission transformers serve different roles in the power grid. Transmission transformers handle high-voltage transmission, moving electricity from power plants to substations across long distances. Distribution transformers lower the voltage again, making it usable for homes, schools, and businesses.Real-world data shows the importance of these types of power transformers. Transmission and distribution transformers each account for over 40% of the global installed transformer capacity. Power grids worldwide use about 4.7 million kilometers of transmission circuits and up to 104 million kilometers of distribution lines. Utilities rely on real-time data from distribution transformers to manage changing power flows and keep the grid stable.Market research groups power transformers by voltage level, application, phase, insulation, core type, and rating. The table below shows these categories:CategorySubcategories / TypesUsage and Performance ContextVoltage LevelLow Voltage, Medium Voltage, High VoltageResidential (low), industrial (medium), transmission (high)ApplicationResidential, Commercial, IndustrialHomes, businesses, heavy industryPhaseSingle Phase, Three PhaseThree-phase for industry and large-scale useInsulationOil, Solid, Gas, AirImpacts safety and performanceCore TypeShell, Closed, BerryAffects cooling and efficiencyRating (MVA)100-500, 501-800, 801-1200Linked to industrial and utility needsImage Source: statics.mylandingpages.coMedium voltage transformers hold the largest revenue share in 2024. Industrial applications lead in growth, while high-voltage transformers are expected to grow fastest in the coming years.Tip: Choosing the right type of power transformer ensures safe, efficient, and reliable electricity for everyone.Applications of TransformersPower DistributionElectrical transformers play a central role in power distribution systems around the world. Cities and towns rely on these devices to deliver electricity safely and efficiently. In urban areas, substation transformers help manage the flow of electricity through complex networks. For example, studies in China have shown that transformer capacity can limit how much electricity a city can supply. When a transformer reaches its limit, it becomes a bottleneck for the entire network. Utility companies use these findings to decide when to upgrade transformers and improve network security.In the United States, the scale of power distribution is massive. There are between 60 and 80 million distribution transformers in use as of late 2024. These electrical transformers help move electricity from transmission lines to homes and businesses. The demand for transformers is rising quickly. By 2050, experts expect the need for transformer capacity to grow by up to 260% compared to 2021. Many transformers in use today are over 40 years old, which means utilities must plan for replacements and upgrades. Supply chain issues and long manufacturing times add to the challenge.Note: Distribution transformers in the U.S. lose nearly 2% of all electricity generated, mostly due to core losses at low loads. Improving efficiency could save billions of dollars over time.Efficiency and ReliabilityEfficiency and reliability are key factors in the performance of electrical transformers. In industrial settings, high-efficiency power transformers help companies save money and reduce their impact on the environment. The table below shows how different types of transformers perform:Transformer TypeEfficiency RangeMaximum Load Resistance (Ω)General Market Transformers95% – 98.5%300High-Power TransformersUp to 99.7%N/AStep-Down TransformersNot specified80Most electrical transformers in industry operate between 95% and 99% efficiency. Regular maintenance and balanced loading help keep these numbers high. Tools like Distribution Transformer Monitoring Units allow operators to check transformer health in real time. Power Factor Correction methods, both passive and active, also improve efficiency and voltage stability.Reliability matters because transformers support critical infrastructure. When a transformer fails, it can cause power outages and disrupt daily life. Utilities monitor transformer performance and replace aging units to keep the grid stable. As more renewable energy sources and electric vehicles connect to the grid, the need for reliable and efficient electrical transformers will only increase.Power transformers keep electricity flowing safely and reliably in homes and businesses. Statistical models, such as the Weibull distribution, help experts predict transformer lifespan and plan maintenance. Studies show that insulation issues and overloads cause most failures, which highlights the need for regular checks. Researchers use advanced simulations and experiments to improve transformer design and performance. These efforts help everyone enjoy stable power every day. For those interested, exploring recent research on transformer reliability and thermal modeling can offer deeper insights.FAQWhat is the main job of a power transformer?A power transformer changes voltage levels to move electricity safely and efficiently. It helps send power over long distances and delivers the right voltage to homes and businesses.Why do transformers need cooling?Transformers heat up during use. Cooling systems, like oil or fans, keep the temperature safe. This prevents damage and helps the transformer last longer.Can a transformer work with direct current (DC)?No, a transformer only works with alternating current (AC). The process of electromagnetic induction needs a changing magnetic field, which DC does not provide.How do people know if a transformer is failing?Operators use sensors and monitoring tools.They check for unusual sounds, heat, or oil leaks.Early signs help prevent bigger problems.

Introduction Can a step-down transformer be used as a step-up transformer? This involves not only the principle of the transformer, but also the specific components and their functions in circuit. In terms of working principle, the transformer can step down and step up. Does this mean they can be converted? But it is worth noting that the voltage grade, impedance characteristics, impedance voltage characteristics and winding current, etc. all determine whether the step-down transformer can be used for step-up. So here we will explain it in detail. Step-up and Step-down Transformers Working & Applications Catalog Introduction Ⅰ Electrical Transformer Working Principle Ⅱ Differences between Step-down and Step-up Transformers Ⅲ Example Analysis Ⅳ Theoretical Analysis Ⅴ FAQ Ⅰ Electrical Transformer Working Principle Transformer is a common electrical equipment that can be used to transform a certain value of alternating voltage into another one with the same frequency. A step-up transformer is a device used to transform a low alternating voltage into another higher value with the same frequency. While the step-down transformer is a very important equipment in the power transmission and transformation system. That is, its normal operation is not only related to its own safety and reliable power supply for users, but also directly affects the stability of the power system.Transformers generally have two functions, one is the buck-boost function, and the other is the impedance matching function. Let me talk about the former. Usually we use a variety of voltages in applications. For example, the life lighting power is 110V, the industrial safety lighting is 36V, and the voltage of the welding machine needs to be adjusted. These are inseparable from the transformer. For example, according to the principle of mutual inductance, the transformer passes through the main and auxiliary coils to reduce the voltage to the voltage we need. Figure 1. EMF Formula The main parts of the transformer are the iron core and the windings on it. The two windings are only magnetically coupled but not electrically connected. Add an alternating voltage to the primary winding to generate alternating magnetic flux that links the primary and secondary windings, and induce the electromotive force (EMF) in the two windings respectively. As long as the number of turns of the primary and secondary windings is different, the purpose of voltage transformation can be achieved by transformer.   Ⅱ Differences between Step-down and Step-up Transformers 1) The step-down transformer converts the higher voltage at the input of the power supply into a lower voltage for our normal use to achieve the purpose of step-down.2) The step-up transformer can convert a low voltage into a higher voltage. (Additionally, the inverter transformer is also a kind of step-up transformer). In principle, the step-down transformer and the step-up transformer are the same, the specific difference is the inductance, copper consumption, and winding capacity of the high-voltage side and the low-voltage side. The same transformer, no matter it is used for step-up or step-down, the iron loss is the same. Under no-load conditions, the high-voltage side winding of the step-down transformer has many turns, large impedance, large inductance, small current and low copper loss, in addition, the high-voltage side winding has a larger capacity. At this time, it becomes a step-up transformer, the iron loss is the same, but the low-voltage side winding has a small number of turns and a small impedance. The inductance is small and the copper loss is small, and the primary side capacity is smaller than the secondary at this time.But there is a question. When the step-down transformer is converted to a step-up transformer, can the rated parameters of the low-voltage side coil withstand the loss under no-load conditions? If so, how much power is left for the high-voltage side.Whether to increase or decrease voltage depends on the ratio of the number of turns of the primary coil and the secondary coil. 1:1 is only for isolation. Therefore, the step-down transformer can be used as a step-up transformer, but it may not work in practice. Figure 2. Transformer Voltage Conversion Ⅲ Example Analysis As above mentioned, step-up transformer and step-down transformer cannot be used as a reverse conversion. Because the step-up transformer is equivalent to stepping up low-voltage power into high-voltage power. For the system, its low-voltage side is equivalent to absorbing electric energy, and the high-voltage side sending electric energy is equivalent to the power source. That is, the load of the system accepts the standard rated voltage, and the voltage output on the power supply side takes into account the voltage drop of the circuit and the transformer itself, about 10%. In order to ensure that the voltage delivered to the user is exactly the rated voltage, the voltage output on the high voltage side is 10% higher than the rated voltage.For example, if the rated voltage of the low-voltage side of a step-up transformer is 20kV and the high-voltage side is 110kV, the receiving voltage of the low-voltage side is 20kV, and the high-voltage side is 10% higher, about 121kV. If you consider the transformation ratio, suppose that the low-voltage side has 20 turns, and the high-voltage side cannot be 110 turns but 121 turns. If this step-up transformer is used as a step-down transformer, its high-voltage side can be regarded as a load from the system and can only receive a rated voltage of 110kV, and meanwhile the output voltage of the low-voltage side cannot reach 20kV, which can’t work normally. Similarly, the step-down transformer cannot be used as a step-up transformer. In the actual application process, the structure and protection part of the step-down transformer is different from that of the step-up. So this action will slowly reduce the stability of the transformer and may affect its service life.Of course, there is also a case where a step-down transformer can be used as a step-up one, as long as the voltage does not exceed the primary and secondary voltage. Figure 3. Transformer Phase Change Ⅳ Theoretical Analysis Nowadays, it is very common that the voltage instability fluctuates during our usual mains electricity use. Therefore, each family needs to install a power supply device for its own power line. Considering that some people often use low voltage, and some people's home voltage is always high, so there are step-up transformers and step-down transformers.We first look at the rectifier transformer. We found that the secondary wire on its surface is particularly thick, which is due to the larger current in the secondary circuit. It can be imagined from this that if the secondary circuit is used as the primary side, its impedance must be very small, and the power supply must provide a large current to obtain the required voltage on the secondary side of the transformer, resulting in low conversion efficiency. Ordinary transformers do have this possibility. For example, the electric energy generated by the user's self-provided low-voltage generator may pass the power transformer (step-down) back to the grid. So once the self-provided generator starts, you need to open the circuit breaker connected to the grid. Even with this possibility, it is not arbitrarily that the electric energy can be fed back to the grid through the transformer.Let's look at the expression of AC voltage: . Note that U on the right side of the equal sign is the effective value of the voltage, and this voltage must meet the specified rated value, f is the frequency (which must also meet the condition of the standard value), and Φ is the phase difference.We call these three parameters on the primary side of the transformer consistent with the grid requirements on the secondary side of the transformer, which is called synchronous operation. It is a necessary operation that must be performed for the power supply and the power grid to be combined. And the same period value must fully comply with the specific specification value given by the specification standard.Since the synchronization parameters of the power grid are fixed, the generator must adjust its own synchronization value. The adjustment process of the same period is not very easy. The synchronous period can only be satisfied in an instant. We can only achieve as close as possible, that is, quasi-synchronous. If it is found that the quasi-synchronization is completed, immediately close the circuit breaker, and the electric energy generated by the generator can be boosted by the transformer and sent to the grid. It can be seen that this is not easy, and it can only be achieved by supporting a synchronous measuring instrument or a relay.Pay attention to the wiring problem of the transformer, that is, the connection group of the transformer. Generally, the phase of the high-voltage side of the transformer is deviated from the low-voltage side. Standards and specifications are vividly expressed using a clock. For example, Y11 and Y0, respectively indicate the connection at 11 o'clock and 0 o'clock (11 o'clock means that the difference between the two is 30 degrees in electrical angle, and 0 o'clock has no deviation). Therefore, when doing synchronous operations, we must also consider what time the transformer wiring is. In U.S, many households have solar power generation devices as auxiliary power supplies to generate electricity for own use. When the electricity is enough, it can be fed back to the grid and get benefit. Obviously, there are synchronization devices and power transformers here. Figure 4. Phase Deviation   Ⅴ FAQ 1. What is a step up transformer used for?In the National Grid, a step-up transformer is used to increase the voltage and reduce the current. The voltage is increased from about 25,000V to 400,000V causing the current to decrease. Less current means less energy is lost through heating the wire. 2. What is difference between step up and step-down transformer?The main difference between the step-up and step-down transformer is that the step-up transformer increases the output voltage, while the step-down transformer reduces the output voltage. 3. How does a step up transformer work?Generally, a step-up transformer comes with more turns of wire in the secondary coil that increases the received voltage in the secondary coil. ... Hence, in simple words, a step up transformer increases the electricity voltage from lower to higher in the secondary coil according to the requirement or the application. 4. What is an example of a step up transformer?As an example, a 10:1 step-up transformer requires ten times the turns on the secondary winding: In this formula, we converted the voltage from 5V to 50V (step-up) in a transformer with ten turns on the primary winding, and 100 turns on the secondary winding. 5. What appliances use step up transformer?While this is done to make it suitable for general use, there are certain appliances like electrical motors, microwaves, X-ray machines etc. that require a high voltage to start. A step-up transformer is used to convert the existing power supply to the desired voltage. 6. What is the formula for step up transformer?Using this formula, P = E x I, and its direct derivatives, I = P / E and E = P / I, all transformer attributes can be calculated. For example, if the transformer's rating is 10 KVA and has a 240-volt output, it has a current capacity of 41.67 amperes (10,000 watts / 240 volts = 41.67 amps). 7. What is the main function of a step down transformer?Transformers are classified by their function, which is either step up or step down. Step-up transformers increase the voltage of the incoming current, while step-down transformers decrease the incoming current's voltage. 8. How does a step down transformer work?Primarily, a step-down transformer works on the basic principle of electromagnetic induction. According to Faraday's first law of electromagnetic induction, a conductor when placed in a varying electromagnetic field will see an induced current based on the rate at which the flux changes. 9. Why do we use a step down transformer?The higher the current, the more heat is lost. To reduce these losses, the National Grid transmits electricity at a low current. This needs a high voltage. ... These high voltages are too dangerous to use in the home, so step-down transformers are used locally to reduce the voltage to safe levels. 10. Where do we use step up and step down transformers?Step-up and step-down transformers use electromagnetic induction to convert voltage between two circuits. We use both types in the distribution of power from supply stations to the end user, as well as to ensure that the appropriate voltage goes into a circuit on many personal devices. 11. Why do we need to step down voltage?Increased voltage allows decreased current which dramatically reduces power loss. Once the power completes its journey, we decrease its voltage at a step-down transformer to make it safer and more useable in the neighborhood. 12. What is transformer explain step up and step down transformer?A transformer that increases the voltage from primary to secondary (more secondary winding turns than primary winding turns) is called a step-up transformer. Conversely, a transformer designed to do just the opposite is called a step-down transformer. 13. How does a transformer step down voltage?The concept of a step-down transformer is actually quite simple. The transfer has more turns of wire on the primary coil as compared to the turns on the secondary coil. This reduces the induced voltage running through the secondary coil, which ultimately reduces the output voltage. 14. Does step down transformer consume electricity?Thus, if you plug a 300W load into a step-down transformer (assuming the transformer is rated for more than 300W), expect it to draw a little more, perhaps 325W - 375W depending on quality of construction. 15. Does step down transformer increase current?A step-up transformer increases voltage and decreases current, whereas a step-down transformer decreases voltage and increases current.

Ⅰ Introduction A flyback converter has the function of a simple switch-mode power supply, which is usually applied in either AC or DC applications. This low- to mid-power device with multiple outputs transfers power from the input to the output during off-time. It can be found in a television set, a plasma lamp, and a variety of other electronic devices that require high voltage.   flyback transformers which I have met with." width="455" height="240" />  6 different kinds of flyback transformers which I have met with   Catalog Ⅰ Introduction Ⅱ Basics of Flyback Transformers 2.1 What are Flyback Transformers? 2.2 What is a flyback? Ⅲ How Does a Flyback Transformer Work? Ⅳ Design of Flyback Transformer 4.1 Key Components of Flyback Transformer 4.2 Design of Flyback Transformer Ⅴ Advantage  and application of Flyback Transformers 5.1 Advantages of Using Flyback Transformers 5.2 What are typical flyback transformer applications? Ⅵ Practical Projects of Flyback Transformers Ⅶ FAQ       Ⅱ Basics of Flyback Transformers   2.1 What are Flyback Transformers?   A flyback transformer is a gapped-core coupled inductor. When the input voltage is applied to the primary winding during each cycle, energy is stored in the gap of the core. It is then transferred to the secondary winding, where it is used to power the load. Flyback transformers are used in flyback converters to provide voltage transformation and circuit isolation.   Flyback transformers are the most common choice for low-cost, high-efficiency isolated power supply designs up to 120 Watts. They offer circuit isolation, the ability to have multiple outputs, and the ability to have positive or negative output voltages. They can also be controlled across a wide range of input voltage and load conditions. Because the energy is stored in the transformer, the flyback topology, unlike the other isolated topologies, does not require a separate output filter inductor. This reduces the number of components required and simplifies the circuit requirements. This article goes over flyback transformers and the applications that they are best suited for. What is a flybackTransformers ?     2.2 What is a flyback?   In the condition of flyback topologies, energy is kept in the magnetic field of the transformer during the first half of the switching cycle, but in the second half of the cycle, it is released to the secondary winding(s) connected to the load. Flyback transformers have a gapped-core design that allows for high energy storage without oversaturating the core. This aspect of energy storage distinguishes flybacks from other topologies such as forward-mode, in which energy is transferred directly from primary to secondary. Flyback transformers are also known as coupled inductors as the gapped core and stored energy.   The circuit of a flyback transformer？       Ⅲ How Does a Flyback Transformer Work?   A flyback transformer circuit resembles other isolated transformer circuits in appearance and has many of the same components, such as a switch, output rectifier or diode, and input and output capacitors. However, compared with other isolated transformers, flyback transformers store energy within the core and do not require a separate output inductor. By ensuring circuit isolation and allowing both positive and negative output voltage, this highly efficient construction facilitates cost-effective power usage.   Switch The metal oxide semiconductor field-effect transistor (MOSFET) is the most common switch design for flyback converters, consisting of three terminals that modify the intensity of and redirect electronic signals. Flyback converter switches can also consist of bipolar transistors, gallium nitride (GaN), or silicon carbide (SiC). Current flows are stored in the core through the primary coil when the switch is closed (or in the on position). When the switch is turned off, current flows through the secondary coil and is transmitted to the output load. The output voltage is adjusted by varying the duty cycles and turn ratios of the primary and secondary coils.   Coupled Inductor A coupled inductor is made up of the coils that transmit and store energy in a flyback transformer. Mutual electromagnetic inductance connects the two coils—when energy flows into the primary coil, it creates a magnetic link and generates a voltage in the second coil. The function of coupled inductors is to change the voltage. They can also be used to isolate circuit components to improve electrical flow efficiency. When the switch is turned on, energy enters the primary winding and is stored in the core. When the switch is in off status, the stored magnetic flux flows into the secondary winding, and the energy is distributed via a diode.   Output Diode As current flows from the second coil, it can be increased, decreased, or modified. Diodes ensure that current flows unidirectionally toward the output and that the voltage remains constant to meet the application's requirements. Because the current from the transformer can fluctuate depending on the input voltage, the diode, and output capacitor help to maintain a constant outflow of current.   Input and Output Capacitors In flyback transformers, capacitors can be installed on both the input and output ends of the current flow. They are used to reserve energy to release it in controlled amounts. Input capacitors manage the flow of energy into the primary coil, while output capacitors manage the flow of energy to ensure a smooth flow at the desired voltage and current.     Fig. 2: Current when switching on and off       Ⅳ Design of Flyback Transformer   4.1 Key Components of Flyback Transformer Primary switchMutually coupled inductorOutput rectifierInput capacitorOutput capacitor     4.2 Design of Flyback Transformer It is made up of a few electrical components. A switching device, consisting of transistors or MOSFETs, is present at the input to turn on and off the primary coil's input voltage. In reverse bias, the secondary coil contains a diode that restricts current flow.       Fig 1. Construction of Flyback transformer   A flyback transformer, unlike other types of transformers, is designed to be excited by direct current voltage. A switch powered by a DC supply drives the primary winding. The magnetic flux is transferred to the secondary coil when the primary coil is activated. Because the secondary coil is connected to a diode in reverse bias., the current is stored. However, As a result, the next incoming primary winding pulse adds to the stored current (i.e the energy stored in the previous pulse). The result of this subsequent addition is a very high pulse of electricity at the output.       Ⅴ Advantage  and application of Flyback Transformers   5.1 Advantages of Using Flyback Transformers Flyback transformers have several advantages over other types of converters, including:   Circuit isolation. The circuit isolation provided by flyback transformers prevents electrical hazards and improves safety, especially for those working near high-energy electrical systems.   Compact size. Flyback transformers are smaller, lighter, and easier to install than comparable transformers due to their simple design, making them a better fit within your overall electrical system.   Cost-effectiveness. Flyback transformers typically cost the same as other transformers. However, because they contain fewer components, flyback converters – of which flyback transformers are an important component – are typically less expensive than comparable converters. This is important to remember when thinking about flyback transformers and their use in flyback converters.   Convenience Flyback transformers can be used to isolate and manipulate multiple output voltages from a single control.     5.2 What are typical flyback transformer applications? Flyback transformers can be used in a variety of applications, including:   DC-DC power suppliesTelecomLED LightingPower over Ethernet (PoE)Capacitor chargingBattery chargingSolar MicroinvertersAC-DC power supplies Flyback transformers are commonly used when the output current is less than 10 amps and the output power is less than 100 watts. Coilcraft sells standard, off-the-shelf flyback transformers ranging in power from a few Watts to around 120 Watts. Forward-mode, push-pull, and half-bridge / full-bridge topologies are more efficient when higher current and power are required.     Ⅵ Practical Projects of Flyback Transformers The drivers used to drive flyback transformers in flyback mode, or push-pull topology, are featured in this project because they are not conventional transformers.   Projects circuit   These flyback transformers can be found in anything with a CRT, such as TVs or monitors, and are responsible for generating high voltage to create an electric field. In turn, electrons are accelerated toward the screen, where they excite phosphors and form images. Flyback transformers are made of coupled inductors that are driven differently than iron-cored main transformers because they use a ferrite core that requires different operating conditions.   building drivers for flyback transformer   To avoid finding the built-in primary, you may need to consider winding your primary during construction. The primary turns can be adjusted to achieve the desired output voltage or drive voltage. The 555 astable modes is used in one flyback driver design.   Ⅶ FAQ 1. What is the function of flyback transformer? A flyback transformers, also known as a line output transformer, stores energy from input voltage in switched-mode power supplies. It is useful for multiple electrical applications. At one time, these transformers were used to meet high voltage needs at high frequencies.   2. Why is it called a flyback transformer? The reason it is called a flyback transformer is because the primary winding uses a relatively low-voltage saw-tooth wave. The wave gets strengthened first and then gets switched off abruptly; this causes the beam to fly back from right to left on the display.   3. What is a flyback transformer in CRT? A flyback transformer, sometimes called a line output transformer, is used in older CRT TV's and computer monitors to produce the high voltage required to drive the CRT and electron gun. They also have auxiliary low voltage windings which the TV designers use to power other parts of the TV.   4. What type of transformer do I have? In order to do this, simply look for the “W” on your device's label. This will help you determine which transformer you need. If the device is 300 watts, then you will need to buy a transformer that is also 300 watts.   5. Why flyback converter is used? Flyback converters are often used in power supplies requiring low to medium output power at several output voltages. With a flyback, multiple outputs incur little additional cost or complexity—each additional output requires only another transformer winding, rectifier and output filter capacitor.   6. How do I select a core for a flyback transformer? The ideal core material should have a maximum available ΔB and low core losses (proportional to the shaded area). Powder cores are made of tiny insulated particles, hence, the air gaps are distributed evenly through the core structure.        

The transformer is an essential part of electrical equipment. So it is necessary to know and master the basic knowledge of it. Is a necessary skill of every electric design. Catalog I. What is a Transformer? II. How does Transformer Works? III. What types of transformer are there? IV. What are the components of the transformer? V. What are the losses of transformers in operation? How to reduce them? VI. What is the nameplate of the transformer? What are the main technical   data on the nameplate? VII. How to choose a transformer? VIII.Why transformer cannot run when overload? IX. What kinds of tests should be done for transformers in operation? FAQ I. What is a Transformer? The transformer is a device that uses the principle of electromagnetic induction to change the AC voltage. The main components are primary coil, secondary coil, and core (magnetic core). The main functions are voltage conversion, current conversion, impedance transformation, isolation, voltage stabilization (magnetic saturation transformer), and so on. It can be divided into a power transformer and special transformer (furnace transformer, rectifier transformer, power frequency test transformer, voltage regulator, mine transformer, audio transformer, intermediate frequency transformer, high-frequency transformer, impulse transformer, instrument transformer, electronic transformers, reactors, voltage, and current transformer, etc.) The role of the core is to strengthen the magnetic coupling between the two coils. In order to reduce the eddy current and hysteresis loss in the iron, the iron core is formed by the superposition of the painted silicon steel sheet; there is no electrical connection between the two coils, and the coils are wound by insulated copper wire (or aluminum wire). One coil connected to the AC power supply is called the primary coil (or the primary coil) and the other coil is the secondary coil connected to electrical appliances. The actual transformers are very complicated, so there may be problems that exist to concern, such as copper loss (coil resistance heating), iron loss (core heating), magnetic flux leakage (air-closed magnetic induction line), and so on.  To simplify the discussion, an ideal transformer is introduced. An ideal transformer requires some necessary conditions: ignoring the flux leakage, ignoring the resistance of the primary and secondary coils, ignoring the loss of the iron core, and ignoring the no-load current (the current in the primary coil which supplies the secondary coil). For example, the power transformer is close to the ideal condition when it is running at full load (the output with a rated power of the secondary coil). The transformer is a static electrical appliance made by the principle of electromagnetic induction. When the primary coil of the transformer is connected to the AC power supply, the core produces an alternating flux, which is represented by φ. The φ in the primary and secondary coil is the same, and φ is also a simple harmonic function, and φ = φ msinωt. According to Faraday's law of electromagnetic induction, the induction electromotive force in the primary and secondary coils is e1=-N1d φ/dt, e2=-N2d φ /dt. N1, N2 is the number of turns of the secondary coil. From the diagram, we can see that U1=-e1, U2=e2(the primary coil physical quantity is represented by the subscript 1, the secondary coil physical quantity is indicated by the subscript 2), and the complex-effective value is U1=-E1=jN1 ω Φ, U2=E2=-jN2 ω Φ, and makes transformer ratio k=N 1 /N 2. From the upper formula, we can get U1 /U2=-N1 /N2=-k. that is, the voltage effective value of the transformer to that of two coils, which is equal to its coil-voltage ratio, and the phase difference of the voltage of two coils is π. Further More Based On Above Mentioned U1/U2=N1/N2 Under the condition that the no-load current can be neglected, there is I1 /I2=-N2 /N1, that is, the effective value of the coil's current is inversely proportional to the number of turns, and the phase difference is π. On the contrary, under the condition of no-load current, I1/ I2=N2/N1 The power of the ideal transformer is equal to that of the subsoils, that is P1=P2. It shows that the ideal transformer itself has no power loss. But there is always a loss in the actual transformer, and its efficiency is η= P2 /P1, for example, although power transformer efficiency is very high, can reach over 90%, still has a little loss. In an AC circuit, the equipment that increases or decreases the voltage is called a transformer. The transformer can transform any voltage into the value we need at the same frequency to meet the requirements of transmission and distribution. For example, the power generated by a power plant has a lower voltage level, which must be increased the voltage to transmit to a far distance, and the power area must reduce the voltage to a suitable voltage level for power equipment and daily use. II. How does Transformer Works? This video gives a detailed animated illustration on the working of electrical Transformers. Here the basic working principle and construction of transformer, step-up transformer, step-down transformer, transformer winding and core construction are well illustrated. Transformers are based on electromagnetic induction. It consists of an iron core made of silicon steel sheet (or silicon steel sheet) and two sets of coils around the core. The core and the coil are insulated from each other without any electrical connection. The coils connected to one side of the transformer and the power supply are called primary coils (or primary sides), and the coils that connect transformers and electrical equipment are called secondary coils (or secondary sides).  When the primary coil of the transformer is connected to the AC power supply, the changing magnetic field line in the core appears. Because the secondary coil is wound on the same iron core, the magnetic field line cuts the secondary coil, and the inductive electromotive force must be generated on the secondary coil, finally, the voltage at both ends of the coil generated. Because the magnetic line is alternating, the voltage of the secondary coil is also alternating. And its frequency is exactly the same as the frequency of the power supply. It is proved by the theory that the voltage ratio between the primary coil and the secondary coil is related to the turns of coils. It can be expressed as follows: Primary coil voltage / secondary coil voltage = primary coil turns / secondary coil turns, the higher the number of turns, the higher the voltage. Therefore, it can be seen that the turns of the secondary coil are less than the primary coils, that is, a step-down transformer, otherwise, it is a step-up transformer. III. What types of transformer are there? According to the number of phases, there are single-phase and three-phase transformers;  according to thefunction, there are power transformers, special power transformers, voltage regulating transformers, measuring transformers (voltage transformers, current transformers), small power transformers (for small power equipment), safety transformers;  according to the structure, there are core type and shell type; according to the coil, there has double winding and multi-winding transformers, auto-transformer; according to the cooling mode, oil-immersed type and air-cooled type transformers. IV. What are the components of the transformer? Transformer components are mainly composed of iron core, coil, also have other parts, such as oil tank, oil pillow, insulating sleeve and splice, etc. What’s the function of transformer oil? The functions of transformer oil are:  (1) insulation;   (2) heat dissipation;  (3) elimination of arc. What is autotransformer? The autotransformer has only one set of coils, and the secondary coils are tapped from the primary coils, and its electricity can transmitted. It not only has electromagnetic induction, but also the transmission of electricity. There are fewer silicon steel sheets and fewer copper wires in this kind of transformer than in ordinary transformers, often used to voltage regulator. How voltage regulator works? The voltage regulator is constructed the same as the autotransformer, but the iron core is made into a ring coil. The secondary coil tap uses a sliding brush contact to make the surface of the ring along the contact slip in a circular way to achieve voltage regulation smoothly. What is the current relationship between the primary coil and the secondary coil of the transformer? When the transformer operates with load, the current change of secondary coil will cause the corresponding change of primary coil current. According to the principle of magnetic potential balance, it is deduced that the current of the primary and secondary coil is inversely proportional to the number of turns of the coil, the current is small with more turns, and the current with less turns is large. The following formula can be expressed: primary coil current / secondary coil current = secondary coil turns / primary coil turns. What is the voltage change rate of a transformer? The voltage change rate of the voltage regulator is one of the main indexes of transformer performance. When the transformer supplies power to the load, the voltage at the load end of the transformer will inevitably decrease. Comparing the reduced voltage value with the rated voltage value, the percentage is the rate of voltage change. It can be expressed by the formula: voltage change rate = [(secondary rated voltage-load terminal voltage) / secondary rated voltage] ×100%. Generally, for the normal power transformer, when connected to the rated load, the voltage change rate is 4% to 6%. How to ensure that the transformer has a rated voltage output? Too high or too low voltage will affect the normal operation and service life of the transformer, so the voltage must be adjusted. The method of voltage regulation is to draw out several taps in the primary coil and connect them to the tap beginning, which changes the number of turns of the coil by turning the contact. In addition, the required rated voltage can be obtained by rotating the position of the tap switch. It also needs to note that voltage regulation usually occurs after the load of the transformer is cut off. What kind of small transformers are usually used? Where are they applied? Small transformers refer to single-phase transformers with a capacity below 1k VA, mostly used as power transformers for electrical equipment control, electronic equipment and safe lighting equipment. V. What are the losses of transformers in operation? How to reduce them? The loss of transformer in operation includes two parts. (1) One is caused by the iron core. When the coils are electrified, the magnetic field lines are alternating and cause eddy current and hysteresis loss in the core.  (2) Another loss is caused by the resistance of the coil itself. When the primary and secondary coils of the transformer have current passing through, some electrical energy may lose. The sum of iron loss and copper loss is the transformer loss, which is related to transformer capacity, voltage, and equipment utilization. Therefore, in the selection of transformers, the capacity of the equipment and the actual usage should be as consistent as possible, in order to improve the utilization rate of the equipment, pay attention not to make the transformer lies in light load operation. VI. What is the nameplate of the transformer?  The nameplate of the transformer should indicate the transformer's performance, technical specifications, and use occasions to meet the needs of the user. The main technical data usually selected are as follows: (1) The number of rated capacity. The output capacity of the transformer is rated. For example, the rated capacity of a single-phase transformer is Uline × I line, and the capacity of a three-phase transformer is also the U line × I line. (2) Rated voltage volts. Indicate the terminal voltage of the primary coil and the secondary coil (when the load is not attached). Note that the terminal voltage of the three-phase transformer refers to the line voltage U-line value. (3) Rated current amperes. It means LineI current value that allows long-term passage of primary and secondary coils at rated capacity and allowable temperature rise. (4) Voltage ratio. It is the ratio between primary coil rated voltage and secondary coil rated voltage. (5) Line connection mode. Single-phase transformers have only a set of coils of high and low voltage, only for single-phase use, and three-phase transformers have Y/△type. In addition to the above technical data, there are transformer rated frequency, phase number, temperature rise, impedance percentage of the transformer, etc. VII. How to choose a transformer?  First of all, it is necessary to investigate the power supply voltage of the place where the electricity is used, the actual power load of the user, and the conditions of the place where it is located, and then select one by one according to the technical data indicated by the nameplate of the transformer, generally from the capacity and voltage of the transformer. Considering the current and environmental conditions, the capacity selection should be based on the capacity, nature, and service time of the user's power equipment to determine the required load, and then select the transformer capacity. In normal operation, the power load of the transformer should be about 75% ~ 90% of the rated capacity of the transformer. When the actual load of the transformer is less than 50%, the small capacity transformer should be used, and the large transformer should be replaced immediately if the rated capacity of the transformer is greater than that of the transformer. At the same time, when selecting the transformer to determine the primary coil voltage of the transformer according to the line power supply and the voltage value of the secondary coil according to the electrical equipment, it is best to select the low-voltage three-phase four-wire power supply system. This can provide a power supply for the entire operation. For the selection of current, attention should be paid to that the load can meet the requirements of the motor when it starts (because the starting current of the motor is 4 ~ 7 times larger than that of the sinking operation). VIII. Why transformer cannot run when overload? Overload operation refers to the transformer operating in excess of the currency specified on the nameplate. Overload is divided into normal overload and accident overload. The former refers to the increase of power consumption under the normal power supply, and it often makes transformer temperature rise, impels transformer insulation to age, and reduces service life. Therefore, transformer overload is not allowed. In special cases, the overloading of transformers in a short period of time should not exceed 30% of the rated load in winter, and not more than 15% in summer. For the latter, the accident overload and allowable time requirements are as follows: Multiple of Rated LoadReasonable Time of Overload Multiple of Rated Load Reasonable Time of Overload Indoors Outdoors 1.30 2 hours 1 hour 1.60 30 minutes 15 minutes 1.75 15 minutes 8 minutes 2.00 7.5 minutes 4 minutes IX. What kinds of tests should be done for transformers in operation? In order to ensure the normal operation of the transformer, the following tests should be carried out regularly. (1) Temperature test. Whether the transformer is running normally, the temperature is very important. The regulations stipulate that the upper oil temperature shall not exceed 85℃(that is, the temperature rise is 55℃). General transformers are equipped with special temperature measuring devices. (2) Load measurement. In order to improve the utilization rate of transformers and reduce the loss of electric energy, it is necessary to determine the real power supply capacity of transformers in the operation of transformers, the measurement is usually carried out during the current peak period and is measured directly with a clamp ammeter. The current value shall be 70%~ 80% of the rated current of the transformer. (3) Voltage measurement. The regulation requires that the voltage range should be within ±5% of the rated voltage. If beyond this range, taps should be used to adjust the voltage to reach the specified range. Voltmeters are generally used to measure the terminal voltage of the secondary coil and the terminal voltage of the user. (4) Insulation resistance measurement. In order to keep the transformer in normal condition, insulation resistance must be measured to prevent insulation aging and accidents. When measuring the transformer, the transformer should stop running and the insulation resistance of the transformer should be measured by using the tramegger. The resistance measured should not be less than 70 percent of the previously measured value. When using tramegger, the low-voltage coil may adopt a voltage grade of 500 volts. FAQ 1. What is the use of transformer? Transformers are employed for widely varying purposes; e.g., to reduce the voltage of conventional power circuits to operate low-voltage devices, such as doorbells and toy electric trains, and to raise the voltage from electric generators so that electric power can be transmitted over long distances. 2. What are the 3 types of transformers? There are three primary types of voltage transformers (VT): electromagnetic, capacitor, and optical. 3. What is the basic principle of transformer? A transformer consists of two electrically isolated coils and operates on Faraday's principal of “mutual induction”, in which an EMF is induced in the transformers secondary coil by the magnetic flux generated by the voltages and currents flowing in the primary coil winding. 4. Does a transformer convert AC to DC? A transformer is built to transfer the energy from one circuit into another circuit by way of magnetic coupling. ... An alternating current creates a magnetic flux in the core on its way through the first winding, inducing the voltage in the others. It can convert high and low voltages, it cannot convert AC to DC. 5. What are the main parts of transformer? There are three basic parts of a transformer: a. an iron core which serves as a magnetic conductor, b. a primary winding or coil of wire and. c. a secondary winding or coil of wire. 6. What are the classification of transformer? Depending upon the type of construction used, the transformers are classified into two categories viz.: (i) Core type, and (ii) Shell type. Depending upon the type of service, in the field of power system, they are classified as: (i) Power transformers, and (ii) Distribution transformers. 7. Can a transformer work on DC? As mentioned before, transformers do not allow DC input to flow through. This is known as DC isolation. This is because a change in current cannot be generated by DC; meaning that there is no changing magnetic field to induce a voltage across the secondary component. 8. How do you convert a transformer? This conversion is made by winding two separate conductors around a common iron core. Applying an alternating voltage to the primary conductor produces current which sets up a magnetic field around itself. This is known as mutual inductance. 9. What are two components of no load current in transformer? The no-load current of a transformer consists of two components: The Magnetization Current iM is the current required to produce the flux in the transformer core. The Core-loss Current ih+e is the current required to make up for hysteresis and eddy current losses. 10. Which type of transformer core is most efficient? SHELL CORE. The most popular and efficient transformer core is the SHELL CORE, as illustrated in figure (4). As shown, each layer of the core consists of E- and I-shaped sections of metal. These sections are butted together to form the laminations. 11. What is the power factor of transformer? The power factor of a distribution transformer is between (0.75 to 0.80) when secondary is connected to u.p.f loads. 12. Why do we need Transformers? Transformers help improve safety and efficiency of power systems by raising and lowering voltage levels as and when needed. They are used in a wide range of residential and industrial applications, primarily and perhaps most importantly in the distribution and regulation of power across long distances. 13. What is the difference between a step up transformer and a step down transformer? A transformer that increases the voltage from primary to secondary (more secondary winding turns than primary winding turns) is called a step-up transformer. Conversely, a transformer designed to do just the opposite is called a step-down transformer. 14. Are transformers dangerous? There is no established evidence that the exposure to magnetic fields from powerlines, substations, transformers or other electrical sources, regardless of the proximity, causes any health effects. 15. Why transformer rating is in kVA not in kW? Copper losses (I²R) depends on current which passing through transformer winding while Iron losses or core losses or Insulation losses depends on Voltage. ... That's why the transformer rating may be expressed in VA or kVA, not in W or kW.

Warm hints: The word in this article is about 3000 words and  reading time is about 10 minutes.   The transformer is a static electrical device, mainly composed of an iron core (or magnetic core) and coil. The coils have two or more windings, of which the ones connected to the power are called primary coils, and the rest are called secondary coils. Transformers are widely used in electrical equipment such as household appliances, electronic equipment, switching power supply, and so on. Circuit symbols commonly used T as the beginning of the number, for example, T01, T201.    This article covers the construction, functions, classification, and design of transformers and materials used for building magnetic cores in transformers.     Catalogs   I. The Composition of Transformer II. The Construction and Functions of Transformer III. High-frequency Transformer Design Program 3.1 Program structure 3.2 Matters needing attention when doing the core material   selection 3.3 Ferrite magnetic material requirements IV. Power Transformer Classification V. Principle and method of Transformer Design FAQ   I. The Composition of Transformer 1）The primary side 2）The secondary side 3）Magnetizing inductance 4）Leakage inductance 5）Open-circuit or short-circuit measurement of the primary side leads to the  Magnetic inductance and the leakage inductance turns ratio respectively:  K=Np/Ns=V1/V2   II. The Construction and Functions of Transformer 1) Electrical isolation 2) Energy storage 3) Voltage change for same power input.   III. High-frequency Transformer Design Program   3.1 Program structure (1) Core material (2) Core structure (3) Core parameters (4) Transformer Winding Parameter (5) package assembly (6) Temperature rise check   (1) Core material Soft magnetic ferrite is widely used in switching power supply because of its own characteristics. It has the advantages of high resistivity, low AC eddy current losses, low price, and easy to be machined into magnetic cores of various shapes. The disadvantages are low working magnetic flux density, low permeability, large magnetostriction, and high sensitivity to temperature changes. Which kind of soft magnetic ferrite material can satisfy the design requirement of a high-frequency transformer more fully, only when it is carefully considered and the transformer design can reach the high-cost performance.   (2) Magnetic core structure The factors considered in the selection of magnetic core structure are as follows: reducing magnetic leakage and leakage inductance, increasing the area of coil heat dissipation, which is beneficial for shielding and makes it easier to wind coils, more convenient to wire for assembly and so on.   The magnetic leakage and leakage inductance are directly related to the magnetic core structure . If the magnetic core does not need air gap, then a enclosed ring-like or square type magnetic core may be used as far as possible.   (3) Magnetic core parameters In the design of core parameters, special attention should be paid to the operating flux density only limited by the magnetization curve, but also by the losses, and also related to the working mode of power transmission. When the flux changes in one direction, there is ΔB=Bs-Br, which is not only limited by the saturation flux density but also mainly by the losses (Losses cause temperature rise, which in turn affects magnetic flux density). The operating flux density Bm=0.6～0.7ΔB.   An air gap can decrease Br and therefore increase the flux density ΔB. The exciting current can be increased after using an air gap opening, but the core volume can be decreased either. For the two-way operation of magnetic flux, the flux density ΔB is twice the maximum operating flux density Bm, that is ΔB=2Bm. In bidirectional operating mode, we should pay attention to the problem of transformer DC magnetic bias due to the inequality of volt-second areas of positive and negative excitation variation, which is caused by different reasons. A small air gap will be needed in the core, or a DC capacitor can also be added to the circuit design.   Magnetic properties of ferromagnetic materials Magnetic hysteresis loops of the core   (4) Coil parameters Coil parameters include: turns, conductor section (diameter), wire form, winding arrangement and insulation. The conductor section (diameter) depends on the current density of winding, using taking 2.5～4A/mm2. When doing some choosing of section conductor diameter don’t forget to take the skin effect into consideration and do regulations necessary after some temperature rise tests of the transformer.   General winding arrangements: the primary winding is close to the core and the secondary windings & feedback windings are gradually arranged outward. The following two winding arrangements are recommended:   1) If the voltage of the original windings is high (for example, 220V) and meanwhile that of the secondary windings is low, a more appropriate arrangement is the secondary winding being close to the core, and then goes the feedback winding, the original winding is arranged on the outermost ends, which is advantageous to the insulation arrangement of the original winding to the core;   2) If we want to increase the coupling between the primary and secondary windings, we can make half of the original windings be close to the core, then goes the feedback winding and secondary winding, and the other half of the original winding being the outermost ends, which is an arrangement advantageous to reduce the leakage inductance.   (5) Assembly structure The assembly structure of high-frequency power transformers are divided into horizontal and vertical types. If you'd like to select the planar core, sheet magnetic core and thin-film magnetic core, then a horizontal-type assembly would do you good.   (6) Temperature rise tests The temperature rise tests can be carried out by calculation and sample test. The temperature rise is lower than the allowable temperature rise above 15 degrees, the current density and the cross-section of the wire are appropriately increased. Appropriately increase the current density and decrease the cross-section of the wire, and do the exact opposite if temperature rise exceeds the allowable value, such as increasing the diameter or enlarging the core if necessary, to increase the area of coil heat dissipation.       3.2 Matters needing attention when doing the core material selection (1) Soft ferrite, due to its low price, good adaptability, and high performance at high frequency, has been widely used in switching power supply.   (2) Soft ferrite is commonly divided into two series: Mn-Zn ferrite and Ni-Zn ferrite. The Mn-Zn ferrite is composed of Fe2O3，MnCO3，ZnO and so on, which is widely used in all kinds of filters, inductors, transformers, and so on below 1MHz. The Ni-Zn ferrite is composed of Fe2O3，NiO，ZnO and so on, which is widely used in all kinds of adjustable inductor windings, anti-jamming magnetic beads, antenna matching devices, and so on above 1MHz.    (3) Mn-Zn ferrite is the most widely used core in switching power supply, and the selection of its material depends on its use. The core for the input filter part of the power supply is mostly high-conductivity magnetic core, and its material number mostly is R4K～R10K, that is, the ferrite core of relative permeability is about 4000~10000, but the main transformer and output filter are magnetic materials with high saturation flux density, where Bs is about 0.5T (5000GS).     3.3 Ferrite magnetic material requirements   Ferrite magnetic materials for switching power supply shall meet the following requirements:   (1) High saturation flux density Bs and low residual flux density Br   The residual flux density Bs has a certain influence on the transformer and winding results. Theoretically speaking, the number of turns of transformer windings can be reduced and the copper loss can be reduced because of the high Bs. In practical applications, there are different types of circuits of high-frequency converters in switching power supply.    For transformers, their operations can be divided into two categories:   1) Bipolar: The circuit topologies include half-bridge, full-bridge, push-pull, etc. In the primary winding of the transformer, the excitation current is equal and opposite in direction during the positive and negative half-cycles. Therefore, the magnetic flux changes in the magnetic core of the transformer are symmetrically moved up and down. The maximum variation range of B is  ΔB=2Bm, and the DC component of the magnetic core is basically canceling out.   2) Unipolar: The circuit topologies include single-ended forward, single-ended flyback, etc. The transformer primary winding adds a unidirectional square wave pulse voltage in one cycle (this is the case for single-ended flyback). The magnetic flux density varies from the maximum Bm to the residual flux density Br in the unidirectional-excitation transformer core. If we decrease the Br and increase the saturation flux density Bs, then the △B will be increased, and the turns and copper loss will also be reduced.   (2) Transformers or inductors are divided into three categories according to their topology:   1) An DC-filter inductor's magnetic core only works in one quadrant, the topologies of this operating state including Boost, Buck, buck/boost inductors, single end flyback converter transformer, forward and all push-pull converters and output filter inductors.   2) The core of the transformer in the forward converter also works in one quadrant, but the transformer needs to magnetic reset.   3) The core of the transformer with push-pull topology is in bidirectional alternating magnetization. These kinds of converters include push-pull, half-bridge and full-bridge converters, AC filter inductors, and so on.   (3) Low power loss at high frequency   The power loss of ferrite not only affects the power output efficiency but also leads to the heating of the magnetic core and waveform distortion.   The heating problem of the transformer is very common in practical applications, which is mainly caused by copper loss and core loss of the transformer. If the selected Bm is too low and the turns of winding are too many, it will cause the winding to heat up and transfer the heat to the core at the same time, and vice versa.   When selecting the ferrite material, we must make the power loss change with temperature characterized by a negative temperature coefficient. This is because if the core loss is the main heating, making the transformer temperature rise up, which then will lead to a further increase of core losses, thus it will form a vicious circle and eventually make the power tube, transformer, and other components burn down. Therefore, in the researches of power ferrite at home and abroad, we must solve the problem of negative temperature coefficient of magnetic material power loss itself, which is also a remarkable feature of magnetic materials having met the requirements for power supply applications, such as PC40 from Japanese company TDK and R2KB from China manufacturers and so on.   (4) A relatively moderate permeability   (5) How we choose the appropriate relative permeability?    Well, this depends on the switching frequency of your actual circuit, mostly 2000, meanwhile its applicable frequency must be below 300kHz, and sometimes can be a little higher, but the maximum will not be higher than 500 kHz.   (6) A relatively high Curie temperature   Curie temperature is the temperature at which a magnetic material loses its magnetic properties, generally above 200 ℃. However, the actual operating temperature of the transformer should not be higher than 80℃, at which the saturation flux density Bs will drop to 70% of that at the normal temperature when the temperature is above 100℃. That is, the saturation flux density of the core will drop more seriously when the operating temperature is too high. Furthermore, when the temperature is higher than 100℃, the power loss has been experiencing a positive temperature coefficient, which will lead to a vicious circle. For R2KB2 materials, the temperature corresponding to the allowable power consumption has reached 110℃ and the Curie temperature is up to 240℃, which meets the requirements of high-temperature use.     IV. Power Transformer Classification Power transformers are divided into three categories according to their topology: (1) Flyback transformers; (2) Forward transformers; (3) Push-pull transformers (full-bridge/half-bridge converters)   The appropriate topologies for various core structures are shown in the following table:   Core structureTypes of converter circuitFlybackForwardPush-pullE cores++0Planar E Cores-+0EFD Cores-++ETD Cores0++ER Cores0++U Cores+00RM Cores0+0EP Cores-+0P Cores-+0Ring Cores-++    "+"=fit; "0"=normal; "-"=unfit Summary of High frequency transformer core.XLS V. Principle and Method of Transformer Design   (1) There are two main ways to design transformer: Area Product (AP) Method AP: The product of core effective cross section Ae and Area of window Aw PT-The calculation power of the transformer Ae-Core effective cross section Aw- Area of window Ko-Core window utilization coefficient, typically 0.4 Kf-Waveform coefficient, usually square wave being 4 and sine wave being 4.44 Bw-The operating magnetic intensity of core FS-Switching frequency Kj-Current density coefficient, usually 395A/cm2 X-Core structure coefficient     (2) According to the area product (AP) method, the general steps of designing transformer are as follows: 1. Select the core material to calculate the apparent power of the transformer; 2. Determine the core cross section AP and select the core size according to AP value; 3. Calculation of the primary side inductance and the number of turns; 4. Calculation of the length of air gap; 5. Calculating the line diameter according to the current density and the secondary side RMS current. 6. Determine whether the copper loss and iron loss meet the requirements (eg allowable loss and temperature rise)   Selecting the flyback topology, the basic parameters of the power supply are as follows: Input voltage: 175-264 VAC Output voltage: 21V Output current: 3A Output power P0=63W Frequency set at 60Khz Duty cycle set at 0.45 initially   1) Select the core material to determine the apparent power PT of the transformer   and select the PC40 material here considering the cost factor and check the PC40 data to get Bs=0.39T, Br=0.06T. In order to prevent the core from becoming saturated instantly, a certain margin is reserved. Let Bm= ΔBmax*0.6=0.198T, and pick up the 0.2T. For flyback topology, the transformer apparent power PT is:   2) Calculating AP values with Excel tables   Where, J is the current density, usually 395A/cm2, and Ku is the effective use coefficient of copper window, usually 0.2～0.4, now we set Ku as 0.4.    Based on the figure above, we select the core EE3528 due to its being greater than the calculated AP value, with the following parameters: Ae: 84.8mm2 AP：1.3398cm4 Wa：158mm2 AL：2600nH/H2 In order to adapt to the abrupt load current, the power supply is designed in critical mode and the critical current is: I0B=0.8×I0=2.4A     3) Calculation of the primary side inductance and the number of turns (A) Minimum input voltage Vimin=ViACmin*1.2=210V (B) Turns ratio n=[Vimin/(V0+Vf)]*[Dmax/(1-Dmax)] n=[210V/(21V+1V)*[0.45/(1-0.45)] n=7.8 (C) Peak secondary current ^IsB=2*IoB/(1-Dmax) ^IsB=2*2.4A/(1-0.45) ^IsB=8.72A (D) Secondary inductance Ls=(V0+Vf)*(1-Dmax)*[1/(Fs*1000)]/^IsB*1000000 Ls=(21V+1V)*(1-0.45)*[1/(60Khz*1000)]/8.72A*1000000 Ls=23.58Uh (E) Primary inductance Lp=n*n*Ls Lp=7.8*7.8*23.58uH Lp=1434uH   Primary and secondary peak currents (F) Calculation of peak secondary current in continuous mode ^Isp=Io/(1-Dmax)+(^IsB/2) ^Isp=3A/(1-0.45)+(8.72A/2) ^Isp=9.81A (G) Calculation of peak primary current in continuous mode ^Ipp=^Isp/n ^Ipp=9.8A/7.8 ^Ipp=1.257A (H) Calculating the turns of the primary and secondary auxiliary windings a) Number of turns in the primary side Np=Lp*^Ipp/(^B*Ae) Np=1434uH*1.257A/(0.2*84.8) Np=106.28T After rounding: Np=106T b) Number of turns in the secondary side Ns=Np/n Ns=106T/7.8 Ns=13.58T After rounding: Ns=14T c) Number of feedback turns Nv=(Vcc+Vf)/[(V0+Vf)/Ns] Nv=(14.5V+1V)/[(21V+1V)/14T] Nv=9.87T After rounding: Nv=10T   To avoid core saturation, an appropriate air gap is added to the magnetic loop, the calculation go as follows: The number of turns may need to be corrected by the air-gap flux edge effect.   4) There are two ways to calculate the wire diameters of the primary, secondary and auxiliary windings: Effective current of original side diameter: Iprms=Po/^n/Vimin Iprms=63W/0.8/210V Iprms=0.375A (A) Calculating the area of bare wire (B) Calculating the wire diameter (current density J to take 4A/mm2) Using two 0.18mm-diameter wires wound around or AWG #28 a single strand The secondary diameter: Use four wires with a diameter of 0.25mm (AWG #31) and wind around. Calculation of Skin Depth: The diameter of multi-strand parallel winding must be less than or equal to dwH, in single wire winding, however, if the diameter exceeds the dWH value,  the multi-strand wire winding should be taken into account.   5) Calculation of copper loss Pcu and iron loss Pfe (total transformer loss Ploss) (A) Calculating the loss of primary and secondary windings.  Where, MLT is the average turn length of magnetic core (B) Calculating the allowable total loss Ploss and allowable iron loss at efficiency η (C) According to the loss curve of iron core, the actual loss (iron loss per unit weight and actual iron loss) is obtained by: The Ploss is the loss of the whole circuit, including diode/MOSFET losses and other losses, the actual losses Pfe must be much smaller than the calculated one, so here is only for reference. (D) Calculating the loss per unit area by Φ=Ploss/As If the temperature rise caused by Φ is less than 25 degrees, then the design is wonderful.   6) Calculating the BW The working flux density BW should be below Bs-Br within the design specifications, that is Bw<Bs-Br, to avoid saturation of the core.   FAQ   1. What is the use of transformer? Transformers are employed for widely varying purposes; e.g., to reduce the voltage of conventional power circuits to operate low-voltage devices, such as doorbells and toy electric trains, and to raise the voltage from electric generators so that electric power can be transmitted over long distances.   2. What are the 3 types of transformers? There are three primary types of voltage transformers (VT): electromagnetic, capacitor, and optical.   3. What is the basic principle of transformer? A transformer consists of two electrically isolated coils and operates on Faraday's principal of “mutual induction”, in which an EMF is induced in the transformers secondary coil by the magnetic flux generated by the voltages and currents flowing in the primary coil winding.   4. Does a transformer convert AC to DC? A transformer is built to transfer the energy from one circuit into another circuit by way of magnetic coupling. ... An alternating current creates a magnetic flux in the core on its way through the first winding, inducing the voltage in the others. It can convert high and low voltages, it cannot convert AC to DC.   5. What are the main parts of transformer? There are three basic parts of a transformer: a. an iron core which serves as a magnetic conductor, b. a primary winding or coil of wire and. c. a secondary winding or coil of wire.   6. What are the classification of transformer? Depending upon the type of construction used, the transformers are classified into two categories viz.: (i) Core type, and (ii) Shell type. Depending upon the type of service, in the field of power system, they are classified as: (i) Power transformers, and (ii) Distribution transformers.   7. Can a transformer work on DC? As mentioned before, transformers do not allow DC input to flow through. This is known as DC isolation. This is because a change in current cannot be generated by DC; meaning that there is no changing magnetic field to induce a voltage across the secondary component.   8. How do you convert a transformer? This conversion is made by winding two separate conductors around a common iron core. Applying an alternating voltage to the primary conductor produces current which sets up a magnetic field around itself. This is known as mutual inductance.   9. What are two components of no load current in transformer? The no-load current of a transformer consists of two components: The Magnetization Current iM is the current required to produce the flux in the transformer core. The Core-loss Current ih+e is the current required to make up for hysteresis and eddy current losses.   10. Which type of transformer core is most efficient? SHELL CORE. The most popular and efficient transformer core is the SHELL CORE, as illustrated in figure (4). As shown, each layer of the core consists of E- and I-shaped sections of metal. These sections are butted together to form the laminations.   You May Also Like: Analysis of Calculation Theory for Transformer Temperature Rise Some suggestions about protecting transformers Learn Some Basic Knowledge about Capacitor Voltage Transformer      

  This article will be divided into three parts--individual transformer , parallel transformer and redundancy requirements for bulk power transformers.     Catalog   I. Individual Transformer II. Parallel Transformer III. Redundancy Requirements for Bulk Power Transformers FAQ   I. Individual Transformer   Just see the following picture, it includes the protection for banks where fuses are used on the primary. Overall differential protection may be applied by using CTs in the transformer primary bushings for larger or important banks. The common connection is shown with delta on the source (primary) side and wye-grounded on the secondary side. Other possible connections: delta–delta, wye–wye, or primary-wye– secondary-delta. Figure 1 – Transformer protection without primary-side circuit breaker   About the analysis of power circuit, you can see the article: Analysis of Switching Power Supply Principle   Note: Secondary circuits should have 51 and 51N relays. Therefore, transformer secondary breaker and relays may be omitted unless another source connects to the secondary bus. 51N relay can be omitted with 51G available.   For transformer banks with primary breakers, the protection is summarized in Figure 2.   Relay 51G provides backup protection for secondary bus and feeder faults and must be time-coordinated, with other ground relays protecting the various feeder circuits on the secondary bus. Similarly, phase relays 51 must be coordinated with the phase relays on the feeders. The common connection is shown with delta on the source (primary) side and wye-grounded on the secondary side. Other possible connections: delta–delta, wye–wye, primary-wye– secondary-delta, three-winding, or autotransformer.   Figure 2 – Transformer protection with primary-side circuit breaker   "52S may be omitted in some applications requiring 151G to coordinate with and trip the secondary circuit devices if used." II. Parallel Transformer   The protection for transformer banks where the secondaries are connected together by a bus tie breaker is summarized in the following picture(a,b,c).   The arrangement shown is typical for large- or critical-load substations, especially for industrial plants. The loads are supplied from separate buses that are connected together by a bus tiebreaker (52T) that may be operated either normally closed (NC) or normally open (NO).   Figure 3a – Single line diagram of transformer and secondary bus protection for a typical double-source supply with secondary tie and breaker   If you operated NO, the protection of the first picture and second is applicable. If operated with 52T NC, the protection of the first picture and second is applicable with the secondary side modified. Figure 3b – Secondary protection with high-side fuses With the bus tiebreaker closed, there is a possibility for the interchange of power between the two sources. Here, current flows from one source through its transformer, the secondary buses, and back through the other transformer to the second source. Generally, this is neither desirable nor permitted.   "To prevent this operation, directional time–overcurrent relays (67, 67N) are applied to each transformer."   Figure 3c – Secondary protection with high-side breaker The single-line connections are shown in Figure 3b and Figure 3c, with complete three-line connections in the following figure.   Note: They operate only for fault current that flows into the transformer and trip the secondary breaker (52–1 or 52–2). This is also important in removing a secondary fault source for faults in the transformer bank. The phase relays (67) can be set on a low of the minimum tap.   Load current certainly flows through the relay, but normally not in the operating direction. The low tap continuous rating must not be exceeded by increasing the maximum load current. The 67-time setting must coordinate with the protection on the transformer primary. When used, the ground relay can be set on minimum setting and time, because coordination is not necessary.   Figure 4 – Three-line connections for reverse-phase and partial differential backup protection The inverse-time–overcurrent relays (51, 51N) provide bus protection and backup protection for the feeder circuits. These relays trip both 52–1 (or 52–2) and 52T. This is a partial differential connection and these units must be time-coordinated with the protection on the several feeders that are connected to the bus.   "Only two-phase relays are required, but the third relay (shown optionally in Figure 4) provides additional redundancy. When a ground differential is used, as illustrated in Figure 3c, 67N and 51N are omitted."   Ground-fault backup is provided by 51G, 151G, and 251G inverse-time overcurrent relays (Figure 3abc). Relay 251G provides bus ground-fault protection and backup for the feeder circuit ground relays. It must be time- coordinated with these. It trips the bus tie 52T, as the fault could be either on the bus or on the associated feeders.   If the fault continues to exist with the bus tie open, relay 151G trips breaker 52–1 (or 52–2). Thus, 151G must coordinate with 251G. If the fault persists, it is between the secondary breaker, in the transformer winding, or in the grounding impedance.   Relay 51G set to coordinate with 151G is the last resort. It trips the high-side or primary breaker to remove the transformer from the service.   III. Redundancy Requirements for Bulk Power Transformers   When transformers are connected to bulk power systems, redundancy requirements for related protection need to be addressed. To provide the required redundancy, two separate differential schemes may be applied.   "Redundancy for transformer faults may also be obtained by a differential scheme and sudden pressure."   In such an application, the sudden pressure protection needs to be supplied with additional protection for faults on the transformer bushings and leads, as sudden pressure devices will not respond to faults in these areas. Redundant schemes for disconnecting the transformer from the system when a high-side breaker is not applied can be obtained by using various combinations of the methods. Take an example, two separate transfer trip systems may be applied although they are expensive. A cheaper alternative is to combine a transfer trip scheme and a faulty switch. It may be possible to delay closing the fault switch for a few cycles to allow time for the transfer trip scheme, provided it is operational, to de-energize the failed transformer before the closing of the fault switch. This would spare the power system from being subject to a solid fault when the fault switch closes, whenever the transfer trip scheme works properly.   When a high-side breaker is applied and it fails to operate, breaker failure protection is required to enable isolation of a faulted transformer. The breaker failure scheme may require the application of a fault switch, transfer trip scheme, or a second interrupting device if other local breakers are not available to isolate the transformer.   FAQ     1. What is the use of transformer? Transformers are employed for widely varying purposes; e.g., to reduce the voltage of conventional power circuits to operate low-voltage devices, such as doorbells and toy electric trains, and to raise the voltage from electric generators so that electric power can be transmitted over long distances.   2. What are the 3 types of transformers? There are three primary types of voltage transformers (VT): electromagnetic, capacitor, and optical.   3. What is the basic principle of transformer? A transformer consists of two electrically isolated coils and operates on Faraday's principal of “mutual induction”, in which an EMF is induced in the transformers secondary coil by the magnetic flux generated by the voltages and currents flowing in the primary coil winding.   4. Does a transformer convert AC to DC? A transformer is built to transfer the energy from one circuit into another circuit by way of magnetic coupling. ... An alternating current creates a magnetic flux in the core on its way through the first winding, inducing the voltage in the others. It can convert high and low voltages, it cannot convert AC to DC.   5. What are the main parts of transformer? There are three basic parts of a transformer: a. an iron core which serves as a magnetic conductor, b. a primary winding or coil of wire and. c. a secondary winding or coil of wire.   6. What are the classification of transformer? Depending upon the type of construction used, the transformers are classified into two categories viz.: (i) Core type, and (ii) Shell type. Depending upon the type of service, in the field of power system, they are classified as: (i) Power transformers, and (ii) Distribution transformers.   7. Can a transformer work on DC? As mentioned before, transformers do not allow DC input to flow through. This is known as DC isolation. This is because a change in current cannot be generated by DC; meaning that there is no changing magnetic field to induce a voltage across the secondary component.   8. How do you convert a transformer? This conversion is made by winding two separate conductors around a common iron core. Applying an alternating voltage to the primary conductor produces current which sets up a magnetic field around itself. This is known as mutual inductance.   9. What are two components of no load current in transformer? The no-load current of a transformer consists of two components: The Magnetization Current iM is the current required to produce the flux in the transformer core. The Core-loss Current ih+e is the current required to make up for hysteresis and eddy current losses.   10. Which type of transformer core is most efficient? SHELL CORE. The most popular and efficient transformer core is the SHELL CORE, as illustrated in figure (4). As shown, each layer of the core consists of E- and I-shaped sections of metal. These sections are butted together to form the laminations.   11. What is the power factor of transformer? The power factor of a distribution transformer is between (0.75 to 0.80) when secondary is connected to u.p.f loads.   12. Why do we need Transformers? Transformers help improve safety and efficiency of power systems by raising and lowering voltage levels as and when needed. They are used in a wide range of residential and industrial applications, primarily and perhaps most importantly in the distribution and regulation of power across long distances.   13. What is the difference between a step up transformer and a step down transformer? A transformer that increases the voltage from primary to secondary (more secondary winding turns than primary winding turns) is called a step-up transformer. Conversely, a transformer designed to do just the opposite is called a step-down transformer.   14. Are transformers dangerous? There is no established evidence that the exposure to magnetic fields from powerlines, substations, transformers or other electrical sources, regardless of the proximity, causes any health effects.   15. Why transformer rating is in kVA not in kW? Copper losses (I²R) depends on current which passing through transformer winding while Iron losses or core losses or Insulation losses depends on Voltage. ... That's why the transformer rating may be expressed in VA or kVA, not in W or kW.