Тороидальные трансформаторы идеально подходят для электрической изоляции в медицинском оборудовании, поскольку они компактны, при необходимости могут быть полностью герметизированы и имеют низкие поля рассеяния, что снижает вероятность возникновения электромагнитных помех.
Изоляция означает физическое и электрическое разделение двух частей цепи, которые могут взаимодействовать. Изоляция достигается за счет использования электромагнитного поля между двумя цепями. Чтобы изоляция была надежной для использования в медицинском оборудовании, должны выполняться два правила: высоко интегрированные изоляционные компоненты и безопасный изоляционный барьер. Например, изолятором может быть кусок пластика, зазор в печатной плате или воздушный зазор.
Три наиболее часто используемых метода изоляции — это оптопары (свет), transformers (магнитный поток) и емкостные соединители (электрическое поле).
Изоляция обеспечивает несколько преимуществ безопасности в медицинском оборудовании:
Она разрывает контуры заземления
Улучшает отклонение напряжения синфазного тока
Она позволяет двум частям цепи находиться на разных уровнях напряжения, что означает, что одна сторона может быть безопасной, в то время как другая находится на опасном уровне напряжения.В рамках данного обсуждения мы сосредоточимся на использовании трансформаторов в качестве метода электрической изоляции медицинского оборудования.
Трансформатор — это электрическое устройство, передающее энергию между двумя или более контурами посредством электромагнитной индукции. Обычно трансформаторы используются для повышения или понижения напряжения переменного тока в электроэнергетике. Это достигается путем пропускания переменного тока через первичную обмотку для создания магнитного потока в сердечнике трансформатора. Этот поток затем индуцирует напряжение во вторичной обмотке трансформатора. В зависимости от соотношения первичной и вторичной обмоток выходное напряжение трансформатора может быть увеличено или уменьшено.
Для большинства трансформаторов, предназначенных для использования в домах и офисах, используется один из двух типов трансформаторов: наборной сердечник Ш-образный (EI )или тороидальный сердечник.
Конструкция трансформатора Ш-образной конструкции
В конструкции E-I соответствующие компоненты «E» и «I» штампуются из листов тонкой зернистой электротехнической стали, которые затем складываются в стопку для создания сердечника. Первичная и вторичная обмотки наматываются на шпули. Несколько шпулей помещаются на шпиндели и вращаются для наложения обмоток.
Этот метод использования шпули позволяет автоматизировать процесс, что сокращает время изготовления, а также обеспечивает изоляцию между обмотками и сердечником. Ламинаты сердечника EI укладываются внутрь шпули для замыкания магнитопровода трансформатора.
Конструкция тороидального трансформатора
Тороидальный сердечник изготавливается из непрерывной полосы кремниевой стали, которая наматывается подобно тугой часовой пружине. Концы прихватываются небольшими точечными сварными швами, чтобы предотвратить разматывание намотанной стали. Сердечник изолируется эпоксидным покрытием или набором колпачков или несколькими витками изоляционной пленки.
Обмотки трансформатора накладываются непосредственно на сам сердечник. Для изоляции обмоток требуется дополнительная изоляция.
Поскольку обмотки должны быть намотаны через центральное отверстие сердечника. А сам сердечник является цельным, шпули не могут быть использованы в тороидальных трансформаторах. Это делает производство тороидальных трансформаторов более трудоемким.
Если тороидальные трансформаторы более трудоемки и не поддаются автоматизации, почему они используются? Ответ кроется в производительности.
Непрерывная полоса стали, используемая в сердечнике, позволяет трансформатору быть меньше, легче, эффективнее и тише, чем EI-ламинат. Эти качества очень желательны для медицинского электрооборудования (и многих других применений) и оправдывают дополнительные расходы.
Тороидальные трансформаторы идеально подходят для электрической изоляции медицинского оборудования, поскольку они компактны и при необходимости могут быть полностью герметизированы. Они также имеют низкий уровень паразитных полей и поэтому менее склонны вызывать излучаемые электромагнитные помехи
Конструкция изоляционного трансформатора медицинского класса
Isolation transformers медицинского класса предназначены для изоляции пациента и оператора от поражения электрическим током и защиты оборудования от скачков напряжения или неисправных компонентов.
Для обеспечения безопасности пациента в больницах все диагностическое или терапевтическое медицинское оборудование (медицинские электроприборы и немедицинские электроприборы в окружении пациента и зонах медицинского назначения) должно быть полностью изолировано от линии питания с помощью усиленной изоляции. Полную безопасность пациента/оператора обеспечивают изолирующие трансформаторы медицинского класса с очень низким током утечки (IEC 60601-1 медицинское электрооборудование).
Трансформаторы медицинского класса предназначены для изоляции пациента и оператора от поражения электрическим током и для защиты оборудования от скачков напряжения или неисправных компонентов.
Трансформаторы медицинского класса также строго соответствуют следующим требованиям:
Максимальные значения тока утечки на землю, пациента или корпус;
Variac looks like a sci-fi lab prop. But they have a useful purpose.
Variac is the common trade name for a variable autotransformer. If that doesn't explain much, let's take a look at what a conventional transformer is and how they are related.
I'm sure you've seen the transformer, and chances are it's in the room with you right now. The transformer provides electrical isolation (galvanic isolation) and impedance matching, and also, in our case, changes the voltage between the primary and secondary circuits. During operation, an alternating current of one voltage is supplied to the primary winding of the transformer, as a result of which, an alternating current of a different voltage appears in the secondary winding of the transformer.
Variable autotransformer windings
To understand how this happens, you need to know two basic concepts. First, the principle that follows from the fundamental relationship between electricity and magnetism: a current flowing through a wire creates a magnetic field in the area around the wire. It directly follows from this that a change in the current leads to a change in the magnetic field. Secondly, the principle discovered by Michael Faraday and named after him: a changing magnetic field in the presence of a circuit will induce an electric current in that circuit.
Using these two principles, we see that using an alternating current to create a changing magnetic field in the presence of another circuit will result in a current in that circuit. This is exactly what happens in a transformer. A typical transformer consists of coils of wire for the primary and secondary windings wound around a common iron core to maximize the overall magnetic flux and thus increase the efficiency of the transformer. The ratio of the voltage induced in the secondary windings to the voltage in the primary windings is proportional to the ratio of the number of turns in the two coils.
What is a variac?
A transformer consisting of only one coil, which is common to the primary and secondary windings, is called an autotransformer. A variable autotransformer is known collectively as a variac, which I am reviewing here. The ratio of the primary and secondary windings is variable, which means that the ratio of the secondary voltage to the primary is variable.
Variable autotransformer slider
The inside of the variac is like a giant rheostat. There is one winding which is partially open to allow the movable ejector to make the electrical connection. The primary connection of the transformer is made to both ends of this winding. The secondary winding is connected to one end of the winding, called the common connection, and to a movable runner. When moving along the winding, the transformation ratio of the autotransformer changes.
Some people use variacs to gradually bring old electronic equipment back to life. They are also used in experiments and tests to simulate various voltage and network conditions. Electrical equipment rated for voltages other than 120 V or 240 V domestically supplied can be supplied from variable autotransformer.
Why grounding transformers are necessary for large wind farms with multiple turbines
When we think of wind farms, images of majestic towers with huge rotating blades crossing the horizon probably come to mind. Engineers are no exception, as their focus is on location, procurement, erection and connection of towers, turbines and blades. Many people don't know that grounding transformer is often overlooked in the design and installation of a wind farm, as evidenced by the fact that 90% earthing transformers for wind farms are purchased after the start of the installation of the main structure. However, those who neglect proper grounding planning do so at their own risk. In reality, millions of dollars in damages can be due to an earth fault, so grounding issues should be high on the list of concerns for anyone developing a wind farm.
Why are grounding transformers needed? In simple terms, a grounding transformer is used to provide grounding for either an ungrounded star or a delta-connected system. Grounding transformers are commonly used for:
-Providing the shortest path to earth with relatively low resistance, thereby maintaining the system neutral at or near ground potential. - Limitation of the magnitude of transient overvoltages during repeated earth fault. - Current source for earth fault. -If necessary, connect loads between phase and neutral. If a single earth fault occurs in an ungrounded or isolated system, there is no return path for the short-circuit current, so no current flows. The system will continue to operate, but the other two healthy lines will increase the square root of three, overvolting the insulation of the transformer and other related components in the 173% system. Metal Oxide Varistors (MOVs), solid-state devices used to suppress surges / surges (lightning arresters), are particularly susceptible to heat damage from leakage through blocks, even if the voltage rise is not enough to breakdown. The grounding transformer provides grounding to prevent this.
Grounding transformers are required for large wind farms with multiple turbines, where the substation transformer is often the only ground source for the distribution system. A grounding transformer located on the turbine column provides an earth path in case the tower becomes isolated from the system ground.
When a ground fault on a collector cable causes the substation circuit breaker for that cable to open, the wind turbine string becomes isolated from the ground source. Turbines do not always detect this fault or the fact that the column is insulated and not grounded. As a result, the generators continue to supply power to the collector cable, and the voltage between the good cables and ground rises well above the normal voltage value. As a result, the costs can be staggering.
According to one source at Iberdrola, the world leader in wind energy development, the loss of revenue for a chain of 10 turbines alone could exceed $ 10,000 a day. Taking into account dismantling and replacement, the cost of the equipment could approach an additional $ 40,000 for a transformer. A typical wind farm configuration is actually somewhat similar to a carriage wheel with ring, hub and spokes. The outer ring of the wheel is like a fence around a wind farm, and the hub in the center is where the collector is located, which connects to the grid. The spokes are the radial lines on which each wind turbine is located. Typically, each radial turbine column is connected to an earthing transformer as shown in fig. 1.
Correct design Grounding transformers usually have one of two configurations: a zig-zag winding (Zn) (with or without auxiliary winding) or a star-connected winding (Ynd) (with a delta-connected secondary winding that may or may not be used to supply auxiliary power) ). Both options are shown in Fig. 2.
The current trend in the design of wind farms is to connect the primary winding in a star with a secondary winding in a delta. In our experience, there are several reasons why 2-winding star-connected grounding transformers seem to be more popular than zigzag designs.
- Double winding transformers are considered more readily available for replacement or retrofitting. -Lack of understanding of zig-zag configuration means that engineers tend to work with more understandable diagrams. -The double-winding star-connected design allows secondary loading and dispensing, while the zig-zag design does not. Not all manufacturers provide zig-zag grounding options to potential customers, even those for whom such a configuration may be most appropriate. Zig-zag connection geometry is useful for limiting third harmonic circulation and can be used without delta winding or without the 4- or 5-bar core structure commonly used for this purpose in distribution and power transformers. Eliminating the need for a secondary winding can make this option less expensive and compact compared to a similar double winding earthing transformer. In addition, the use of a zig-zag transformer provides grounding with a smaller device than a double-winding Y-Delta transformer, which provides the same zero sequence impedance.
On the other hand, wye-connected earthing transformers require either a delta-connected secondary or a 4- or 5-pin core design to provide a return flow path for the unbalanced load associated with this primary connection. Since it is often desirable to supply auxiliary power from the secondary winding of the grounding transformer, this advantage may make it preferable to use a double winding grounding transformer instead of a zigzag connection. Both zigzag and double-winding earthing transformers can be designed with auxiliary power supply - this can be a star or delta load.
A solid earthed system using an earthing transformer offers many safety improvements over an ungrounded system. However, a single grounding transformer lacks the current-limiting capacity of a resistive grounding system. For this reason, neutral ground resistors are often used in conjunction with a ground transformer to limit the magnitude of the neutral ground fault current. Ohm values must be specified to ensure that the earth fault current flows sufficiently high to ensure reliable operation of the protective relaying equipment, but low enough to limit thermal damage.
Definition of a grounding transformer When choosing a grounding transformer for your wind farm, be sure to consider the following key parameters:
Primary voltage Is the system voltage to which the grounded winding is to be connected. Be sure to include the base impulse level of the transformer (BIL), which measures its ability to withstand lightning strikes. In some cases, the BIL will be driven by equipment considerations such as 150kV BIL ratings in 34.5kV wind farms due to the limitation on dead front connectors.
Rated kilovolt-amperes (kVA)... Since an earthing transformer is usually a short-term device, it is smaller and less expensive than a continuous-duty transformer of the same kVA rating. For this reason, earthing transformers are often not calculated in terms of kVA, but in terms of DC and short-time current ratings. Regardless of how you judge it, the grounding transformer must have a rated direct current of the primary phase without exceeding its temperature limit. This load includes core magnetizing current, capacitive charging current for cables, and any auxiliary load, if applicable. The higher this value, the larger and more expensive the transformer. Typical DC currents range from 5 A to several hundred. Be sure to include additional download requirements.
Continuous neutral current - The continuous neutral current is defined as three times the phase current or, in other words, the zero sequence current. This is usually considered zero if the system is balanced. However, for design purposes of an earthing transformer, it is the value that is expected to flow in the neutral circuit without interrupting the protective circuits (resulting in zero current) or earth leakage current, which is not a symmetrical function. ... Again, this value is needed to calculate the thermal power of the grounding transformer.
Damage current and duration - This value is needed to calculate the transient heating resulting from a fault in the system and should be determined based on the engineering study of the system. Typical values range from a few hundred amperes to several thousand amperes, with durations expressed in seconds rather than cycles. For example, typically 400A for 10 seconds. The duration of the damage is a critical parameter for the transformer designer. Where protection schemes use an earthing transformer for trip functions, a relatively short time (5 to 10 seconds) is indicated. On the other hand, if an earthing transformer is used in an earth fault signaling circuit, a constant or extended duration of the earth fault current will be required.
Impedance - impedance can be expressed as a percentage or ohms per phase. In any case, it should be selected so that the phase voltages in good condition during an earth fault are within the permissible temporary overvoltage of the transformer and associated equipment such as arresters and terminal connectors. Values that can range from 2.5% to nearly 10% must be provided by the system designer.
Primary winding connection - be sure to indicate the type of primary connection: zigzag or grounded. Before making a decision, consider the factors discussed earlier regarding the situations for which a particular configuration might be most appropriate.
Secondary connection - specify secondary voltage and connection, if applicable. In addition, be sure to consider the size of the auxiliary load connected for Zn or star connected primary windings.
If there is an option to use a two-winding transformer without a secondary load, determine if the delta winding can be "buried" (ie not led out), or only one insulator should be led out for tank grounding or testing.
Important features and options In addition to the design features discussed, there are a number of other considerations or considerations that you should consider when designing a wind farm grounding transformer.
Inform your supplier if you need a pedestal-mount transformer with built-in tamper-evident or substation design. Consider whether the grounding transformer will be placed outdoors or indoors. Even outdoor units require special attention when placed next to other structures. Select the appropriate fluid type for your application. Options include mineral oil, silicone, and natural ester fluids. Consider connection options and choose the best one for the site. Options range from dull front, live front and blade terminals. The location of the terminals can be under the cover or on the side wall, open or closed. The temperature rise is assumed to be 65 ° C - adjust the design if necessary. Consider site height or any special environmental concerns. Special paint as required. Neutral Grounding Resistors - The rated voltage NGR should be equal to the line voltage of the grounding transformer to earth. The rated current and duration must match the rating of the earthing transformer. Remember to set the rated current high enough to exceed the cable charging current and the magnetizing current of the grounding transformer.
Eliminating power quality issues can help your business save money by optimizing energy use and protecting equipment from future damage.
The stability of the electrical system can be described as "power quality". The first step to assessing the health of an electrical system is collecting data from equipment, infrastructure, and a service panel. This is measured in three-phase electrical systems with instruments that account for multiple variables.
Poor power quality manifests itself as follows:
- Dips and spikes - voltage is lower or higher than expected. -Harmonics - Frequency effects caused by either the power supply or equipment operating in the system. -No symmetry - the influence of voltage or current fluctuations on each of the electrical phases. -Flicker - effects caused by repetitive switching of electrical loads, for example, arc furnaces or other processes.
Causes: Harmonics are currents or voltages with frequencies that are multiples of the mains frequency, which is 50 Hz. If the first fundamental frequency is 50 Hz, then the second is 100 Hz, and the third is 150 Hz. Here are some examples of problems that can be associated with harmonics:
Flashing lights Is a common symptom of a power quality problem. Equipment with rapid load current or voltage fluctuations is a potential source of flicker. These include, for example, large start-up motors, equipment with cycloconverters (such as rolling mill drives and mine hoists), and machines that use static frequency converters such as AC motors and arc furnaces. Overheated transformers and tripped circuit breakers can cause harmonic problems, which occur when non-linear loads that draw current in sharp pulses rather than smoothly (sinusoidally) force harmonic currents back to other parts of the power system. The state of harmonics in a system can be expressed in different ways. The first is total harmonic distortion or THD. THD is the sum of all harmonic effects; this is usually measured up to 50 times the power system fundamental frequency (50 Hz), at 2.5 kHz, or, according to some guidelines, up to 40 times (2.0 kHz). This THD value in terms of power quality is most often applied to voltage. The manual states that the influence of voltage harmonics should be less than 8% in relation to the fundamental. Values higher than those indicated by 8% are for further study.
The first level of research will be to determine the percentage of each individual harmonic, 2nd, 3rd, 4th, 5th - up to the 50th. This is displayed either in real time on the meter or in a diagram from the logged and loaded data - this is visualized as a “harmonic spectrum”.
The approximate spectrum of harmonics shown in Fig. 1 represents a very typical scenario. Voltage THD is in the middle range and is about 3.5% per phase. Note that the largest harmonics are at the 5th and 3rd respectively, and decay very quickly after the 7th harmonic. These harmonics generated by switching power suppliesused in electronic equipment such as computers, monitors, televisions and LED lighting. To a certain extent, these are harmonics that OEMs allow inside their devices. This equipment contains electronic filters that prevent the generation of higher harmonics. Prevention or mitigation is achieved by adding simple networks of passive components such as resistors, capacitors, and inductors. By incorporating this simple security feature into the product, the manufacturer can supply products that meet the required EMC standard.This graph displays the percentage of each individual harmonic. This is known as the harmonic spectrum. Fig. 1
If we look at the harmonics of the current, we see a completely different picture. In fig. 2 shows an amazing level of distortion - up to 40%. This is interesting, but not that important. First, in this case, the current will be low compared to what current is used in the circuit. We describe these two values as IL (load current) and ISC (short circuit current). When ISC is significantly higher than IL, THD for current is not important. The reason for this is that a large difference in these currents is unlikely to affect voltage harmonics. This concept is enshrined in IEEE 519 (Recommended Practices and Requirements for Controlling Harmonics in Electric Power Systems).
In this screenshot, you can see a high level of current distortion. But does it really matter? Fig. 2
Effects In industrial environments, harmonic distortion is most often caused by electrical equipment in service. Modern industrial plants contain many pieces of equipment that can contribute to overall distortion - a few obvious examples include frequency converters and inverter-driven motors. These drives take AC voltage and current, convert them to DC, and then generate a variable frequency output so the motors can be controlled more accurately. When current is supplied to the inverter, it is not perceived as a pure sine wave and irregularly accepts current to charge the components that are at the input of the inverter. This irregular consumption distorts the current and therefore the voltage. These inverters can be used to drive motors that are part of an industrial process such as pumping cooling or heating water, liquid materials, moving conveyors or cooling fans. Other types of electronic controls are also part of the process, and each one creates some distortion. When all of this equipment is connected to the same network, distortion in general increases.
Equipment manufacturers are responsible for ensuring that their equipment does not generate unacceptable levels of distortion, and power quality standards are designed to prevent this. But in some cases, when the user creates a unique combination of higher than expected distortion in the electrical network, close to the bandwidth, the distortion can become severe and affect other parts of the equipment. For example, older transformers were not always designed with harmonics in mind. Although some time has passed since the advent of industrial power electronics. From the beginning, most of the loads on the site were linear (where current and voltage are directly proportional - a simple resistive load). If the harmonics are high, distortion can cause overheating of old transformers, and there are two problems associated with this. First, the heat generated is the waste of electricity. Secondly, damage to the transformer is possible, sometimes even catastrophic.
There are two possible solutions.
-Reduce harmonics by installing filters. -Replace the transformer with a high K-factor transformer that can handle the distortion. Of the two solutions, both have advantages and financial costs.
Installing filters can be very efficient economically and technically, depending on the source of the harmonic distortion. Finding the specific source (s) requires a study of the harmonics of the equipment connected to the system. It is best to start with the largest electronic drives - consider which equipment draws the most current, such as large drives or powerful UPS systems, to figure out which one has the highest THD. Collect as much harmonic data as possible over several days to see how THD changes and identify worst-case scenarios. This data can then be passed on to the filter supplier, who can advise on suitable solutions for each load. Only one or two pieces of equipment can be the cause of the problem. In the worst case, you will need a larger system, but again, the vendor can advise on a suitable solution.
Replacing transformers is more difficult. Harmonic studies are still required to detect the K-factor, the heating effect due to harmonics. The K factor is derived from harmonics using the IEEE recommended method; however, the right tool will calculate this for you. K-factor transformers are more expensive than standard transformers and downtime due to the installation of a new transformer can be serious and result in significant downtime. However, in some cases, this may be the only viable solution.
Again here, but measurements matter. Knowing the health of your system is essential to maintaining your equipment in order to get the most out of it and maintain a reasonable power consumption.
Power quality studies should be considered routine maintenance. By taking semi-regular measurements, you can detect any changes that may occur so you can find potential problems and fix them early. The period between surveys depends on the opinions of users, but they need to take into account their expectations regarding the reliability of the system - the higher the expectations, the more regular the survey is. This can be monthly, quarterly, semi-annually, or, if you feel you are truly in control, annually.
Performing research on a regular basis will not be such a difficult task if you organize the process correctly.
Choose your measurement location wisely. Locate critical points in the system where equipment can cause problems and where equipment can be more sensitive.
Install in the same place every time.
Listen to the operators of the equipment for hints about their experience with what is happening at their level - they have the best information.
Watch trends and make comparisons for easy correlation.
Save historical data.
Measure and record over several days to see the rhythm of the equipment.
By collecting this research data, you can monitor your electrical system, manage it efficiently, and maintain the longevity of your electrical equipment.
A typical method for making a toroidal transformer involves creating a steel core, insulating the core, winding a magnetic wire around the core to create a primary winding, insulating the primary winding, winding a magnetic wire over the insulation to create a secondary winding, and insulating the secondary winding. To fasten the transformer, either a mounting washer and a bolt are used, or the center of the transformer is poured with epoxy with a bolt hole. Operations are almost always performed in this sequence.
Design and manufacture of segmented core covers
Segmented core covers support the primary and secondary windings in alternating sectors to reduce leakage current. A plurality of modular electrical insulating segments are typically snapped or otherwise joined together to form annular or semi-annular core covers to cover or partially cover the transformer's annular toroidal core. Segments or modules are typically made from Zytel®, FR50, Rynite® FR530, or Zytel® E103HSL.
The core cover modules isolate the copper windings from the core across the entire winding range and provide double layer insulation between adjacent windings, significantly reducing leakage current compared to conventional toroidal transformers. They also provide direct cooling of the transformer core with ambient or forced air without intermediate insulation. The core cap can also be assembled from component modules on a finished wound toroidal core.
The segments of each module include a pair of spaced apart, usually electrically insulated walls, as well as a panel extension that separates the windings. The walls are positioned at a predetermined angle relative to each other, typically 30 degrees, 45 degrees, 60 degrees, etc., so that each modular segment spans an arc of approximately 30 degrees, 45 degrees, 60 degrees, etc. The walls include engaging, usually male and female, connecting portions, so that adjacent segments can repeatedly engage with each other, with a sufficient number of connected segments forming the cap of the annular core.
The number of segments required to complete the core cover is predetermined and is usually a function of the predetermined angle between the walls; for example, if the angle is 45 degrees, you would need to connect eight segments together to define the shape of the ring. If the angle is 60 degrees, it only takes six segments to get the ring shape. Although core covers are typically made from identical modules, they may alternatively include combinations of core cover modules spanning different arcs, for example, four core cover modules spanning 45 degrees each and six core cover units spanning 30 degrees each.
While modules of the same size and shape are generally more convenient, there is little or no limitation on the combinations of sizes and shapes of core cover modules that can be combined to provide the desired core cover having the desired properties and characteristics.
When connected (interlocking), relatively flat and smooth sides (surfaces) are formed, and the barriers are located opposite each other. Barriers define the parameters that limit alternating wire windings, usually alternating primary and secondary windings.
The segments include one or more baffles or walls positioned to extend partially or completely through the top of the panel to further define the parameters between which the wire windings are directed. One or more baffles are usually equidistantly spaced between the walls and / or each other, respectively. The spacers are typically oriented to extend radially outward from the center of the core and / or the annular space formed by the connected segments; in other words, each respective division usually lies within the radius of the annular space, although the division walls can have other convenient shapes and contours as desired.
The segments further include a cover panel with an outer diameter D and / or a panel with an inner core diameter D, which extend downwardly so as to at least partially cover the outer D and inner D, respectively, of the core toroidal ring opposite the core cover panels of partially or fully formed annular space. These panels can be flat to cover a core ring having flat sides of the outer and inner diameters, or curved to follow the core ring having rounded or curved inner and outer diameters.
The walls are truncated and do not fit through the panels. In some of them, the lower walls are located opposite the panel from the corresponding wall. The bottom wall can also include mating connectors for joint connection. Some of the segments contain ribs located on the top of the panels to create an air gap between the wire turns and the top of the ring. The air gap makes it easier to air-cool the windings by allowing air to circulate between the windings and the top of the cover.
Segmented core lid winding tool
The winding tool is used to facilitate winding of the single-bobbin lid core. The winding tool is usually a flat ring with a raised rim or flange protruding from the outside diameter. The ring usually has a slot, which gives it a C-shape. The ring is sized to fit the segment and the slot is sized to allow the wire to pass per segment. The winding tool also typically includes an elongated arcuate anchor wire having multiple partial slots and one or more anchoring holes for connecting the anchor wire to one or more segments during the wire winding process.
During operation, several segments can be connected to each other to form a ring. The ring includes a top cap portion of the ring core formed by panels of individual segments. In most cases, the ring also includes (typically) equally spaced radial protrusions formed by mutually engaging connectors extending outwardly from the ring. Each radial protrusion is usually part of an elongated wall located on the upper side of the ring and extending radially inwardly partially or completely through the upper surface. Some of the walls terminate in radial ridges extending inward from the ring. These radial ridges are usually formed by joining the two bottom walls, although they can be formed separately.
The ring may also include an annular core, an outer diameter cap and / or an inner diameter cap of the annular core, each cap being generally perpendicular to a portion of the upper core cap and extending downward.
Corresponding lids typically consist of adjacent lid panels when the segments are joined to form a ring.
Typically, a pair of cap rings are comprised of interconnected segments and are placed on opposite sides of a toroidal core with outwardly aligned protrusions. An even number of segments are connected to form each ring. The wire is wound continuously around alternating segments to define the primary windings, N turns per segment. Typically, all windings can be made from a single bobbin or shuttle in one continuous bobbin winding operation, with the wire guided from one segment to the next through a groove or gap between two opposing core covers. The wire is usually cut or cut to insulate the primary windings from the secondary windings, and then the wound core can be wrapped with insulation as in conventional winding of a toroidal transformer. Some windings may use a tool to facilitate winding of the core. Coils wound in this way retain the advantages of toroidal transformers, but are lighter, smaller, more efficient and quieter than dialed EI cores. Cores wound in this way exhibit lower interwinding leakage current compared to standard toroidal transformer cores.
Typically, the primary windings occupy the odd numbered segments starting from the winding of the first segment, and the secondary windings occupy the even numbered segments. Each ring can contain multiple segments such as six, nine, or twelve, and the core can be wound with primary, secondary and tertiary (not shown) windings as described above to form a three-phase transformer. Alternatively, the ring can contain segments of various configurations.
An insulating material, such as a strip of MYLAR, can be positioned to cover the portion of the core that is exposed by the gap, or the core can be partially or completely wrapped in insulating material prior to installing covers on it. In other designs, the walls are spaced and oriented relative to each other to form an annular space, but are not physically connected to each other. All leads are double insulated / braided and secured with cable ties.
Summary: the benefits of a segmented closed core.
If it is necessary to design the segmented core cap to meet the safety requirements for leakage and clearances, the manufacturing time is reduced because:
-No need for grounding and interwinding insulation, as well as external winding. -The primary and secondary windings can be wound on the same machine, which reduces maintenance time. - Provided the cover has a mounting hole, there is no need to fill the center of the transformer with epoxy. -Can be designed with a segmented core cover made up of repeating sections that "snap together" then the tooling and assembly costs of the covers will be even less. the tool cost for the smaller injection molding part is less than the tool cost for the larger part.
-Assembling "snap-fit" parts requires less skill than other methods of insulating conductors.
It is possible to design a segment transformer with a core cover that allows air flow around the core and windings, resulting in less temperature rise because:
-There is a direct path for heat to escape from the bare core to the environment. -There is no interwinding insulation and an outer sheath that retains heat. -All windings have a direct path to transfer heat from them to the environment. If it is possible to design a segment core cover with mounting holes, then the weight of the transformer will be less because:
No center epoxy required
No mounting washer required
Segment transformer with core cover and standard toroidal transformer - comparison.
Segment cap transformers provide significant reductions in leakage current and heat dissipation compared to standard toroidal transformers. Heat transfer in a segment transformer with a base is comparatively better, since the cover design provides all the necessary insulation. During the experiment, it was found that a simple isolation transformer with a simple circuit (1: 1 transformation ratio) covered with segment caps has heating-cooling advantages over a conventional 13 ~ 17 ° C transformer.
Standard toroidal design (1500VA)
Segmented coated core design (1500VA)
Open circuit voltage 240 V
Idle current 240 V
Core loss 240 V
Idle current 264 V
Core loss 264 V
Maximum leakage current 264 V
Leakage (high potential) 5 kV, 50 Hz, 2 sec.
DC resistance in primary winding 28 ° C
Secondary DC Resistance 28 ° C
Thermal equilibrium power output
Thermal equilibrium power input
111.5 ° C
98.6 ° C
29.4 ° C
30 ° C
82.1 ° C
68.6 ° C
Ø200 × 90mm
Ø200 × 90 mm
Standard toroidal design
Segmented core design
Temperature rise comparison
Temperature rise (° C)
Standard toroidal design
Segmented core design
Comparison of working hours
Standard toroidal design
Segmented core design
The segmented core cover design of the transformer provides better heat dissipation, so they can be rated for increased power for the same volume, which is a major advantage. Thus, they are relatively smaller and lighter in weight compared to standard transformers for the same power levels. Other advantages are lower leakage current, lower manufacturing cost, and economical mounting design.
For example, below is a comparison of a 1500 VA segment transformer (1800 VA extended power) versus our standard medical 1800 VA standard toroidal transformer.
Comparison of tests
Standard toroidal design
Segmented core design
Open circuit voltage 240 V
Idle current 240 V
Leakage current 264 V
Leakage at 5 kV, 50 Hz, 2 sec
DC resistance in primary winding 28 ° C
DC resistance in the secondary winding 28 ° C
120 ° C
121.5 ° C
30 ° C
30 ° C
90 ° C
91.5 ° C
Ø210 × 100 mm
Ø200 × 90 mm
Comparison by weight
Standard toroidal design
Segmented core design
Comparison of working hours
Standard toroidal design
Segmented core design
While toroidal transformer designs are generally quite advanced, this technical analysis shows that there is still room for innovation and efficiency gains through the use of segmented core cover technology. We hope this work will be useful to medical device manufacturers, magnetic materials developers and anyone else who may be interested.
The rectifier circuit used in most electronic power supplies is a capacitive filtered single phase bridge rectifier, usually followed by a linear voltage regulator. The schematic of this rectifier is shown below: (Fig. 1):
Most of our transformers are used in rectifier circuits (Fig. 1), so we decided to devote this article to rectifier transformers and give some practical advice to power supply designers.
"The alternating current supplied to the rectifier is always equal to the direct current drawn from the rectifier, when the leakage currents in the diodes are negligible."
This is true if we compare the average currents (Im) on the AC and DC side of the rectifier. But alternating current is always measured as rms current (Irms) and direct current is always measured as average current (Im). The original statement is not correct if we compare Irms on the AC side with Im on the DC side of a rectifier.
The RMS current Irms is always greater than the average current Im due to the peak AC waveform. If we divide Irms by Im, we get the peak current value, which is called the form factor. (F = Irms / Im). The sharper the peaks, the higher the F value.
The heating effect of electric current in wiring, resistors, and transformer windings is proportional to the square of the rms current. The heating effect of the alternating current in the rectifier circuit is accordingly proportional to IS2 = (F x Im) 2 = (F x IL) 2, or the square of the DC current times the form factor F squared. The temperature rise in a given rectifier transformer is thus highly dependent on the form factor (F) value, and the required rectifier transformer size cannot be determined until the actual form factor value is known.
In a rectifier of the type shown in Figure 1, F is anywhere between 1.11 and 5.0, depending on the relative impedances before and after the diode bridge. Once these impedances are known, F (and UC) using graphical methods. But at this point, the power supply designer usually has a transformer prototype in his hand, so UC and IS can be determined quickly by bench tests. (Be careful when measuring IS with a true rms current meter. Most AC meters measure Im, but are calibrated in IRMS, assuming F = 1.11, which is true only for a sinusoid).
The following describes an accurate and simple method for determining Form Factor (F) from an oscilloscope using graphs.
Suppose we observe the current and voltage waveforms in different parts of the circuit shown in Figure 1 on a cathode ray tube oscilloscope so that we can compare the waveforms before and after the diode bridge. Diagrams I-III show waveforms for different capacitance values (C), assuming a transformer with negligible series inductance, for example toroidal transformer.
С = 0 Without regulator
C - worker (Ur/ Uc<10%) With regulator
The desired effect of the capacitor is to smooth the DC voltage, but at the same time it causes the AC current to flow in short pulses, which means a higher F and a higher RMS current in the transformer. The "angle of conductance" (α) of the rectifier can be measured directly from the waveform - just remember that the full half cycle is 180 °.
It is clear that the form factor (F) should depend on the conduction angle (α). We have calculated the exact relationship between F and α for toroidal transformers and the result is shown here in this graph. By measuring the conduction angle (α) on an oscilloscope, a very accurate form factor (F) value can be plotted on the graph. Variations in the DC load will change the conduction angle, and corresponding changes in the form factor can be easily identified.
The diagramming table provides additional information that can assist in evaluating power supply design options. In the comments to the diagrams, we determined the coefficient η = UDC / úo, which relates the DC voltage to the peak no-load voltage of the secondary winding of the transformer. The flattening of the ac voltage waveform peaks is caused by the voltage drop in the total impedance ahead of the diode bridge, so it is reasonable to assume that η should vary with the conduction angle (α). We also calculated this ratio for toroidal transformers and the result is shown on the graph sheet as a dashed curve.
The graph can be used to determine the rectifier DC load regulation. DC load regulation δUDC / UDC = (1-η) x 100%. Remember that the voltage drop across the diodes is included in the value of UDC... Each voltage drop across the diode can be considered constant and equal to 1 V at all loads. Accordingly, net load regulation is slightly worse than 1-η, especially for low DC voltages.
It is important to note that better voltage conversion efficiency (measured by η) can only be obtained through a higher form factor, and conversely, a lower form factor can only be obtained through weaker DC load regulation.
The size of the transformer feeding the rectifier is proportional to the product of the open circuit voltage (U0) and current capacity (IS), which we call Po. The dotted line on the graph sheet represents the smallest Po value required for any value of α (any corresponding value of F or η) for a given DC power. (Po / PDC = F / η√2).
The transformer has a minimum dimension (Po) of about 1.52 x PDC (total DC power, including diode losses) for α = 75º, where η = 0.8 and F = 1.7. Unfortunately, it is not possible to always stay at a minimum, partly because better DC regulation is often required than the 20%, and partly because load regulation of transformers is highly dependent on the size of the transformer. DC load regulation and transformer load regulation are not proportional, but they tend to increase and decrease together, so very small transformers tend to operate above optimal values, and very large transformers operate at less than optimal α values.
Rectifier transformer design to meet specific U requirementsC, UL, DC regulation, temperature rise, etc., requires accurate data for both form factor (F) and rectification efficiency (η). But F and η, in turn, are determined by the data of a not yet designed transformer, so the power supply designer falls into a trap. One way out is to take an old transformer, change it, and pray for the prototype to work.
Another way out is to let Elsta engineers start designing the transformer. Our application engineers have solid experience in the design of transformers and power supplies, and they have tools at their disposal to calculate and optimize transformers, so they can design not only the transformer that will work, but also the most economical transformer that will work efficiently.
Transformer technologies are striding forward by leaps and bounds. At the same time, as transformers become more economical, cheaper, more technologically advanced in manufacturing and installation, the problem of compatibility of a transformer with a modern network comes to the fore. With a network that is overflowing with various electronic power and impulse devices. Devices of unknown quality and origin. Devices, where it is not clear how, implemented a filter system. As a result, a step forward in development turns into two steps back. Therefore, it is necessary to understand new problems associated with new high-tech loads; and understanding how they can affect the operation of the transformer.
Evaluating fundamental faults and examining harmonics provides practical experience to understand why harmonics destroy transformers. New technologies for the production and operation of transformers include understanding new problems associated with new types of loads; and understanding how these types of loads can affect the life of a transformer. Examples of new loads that are very widely used in recent years: variable speed drives and / or electronic ballasts; assembly of equipment for data processing in offices, industrial facilities and institutions. The design and installation of this equipment must take into account the interference, parasitic currents and harmonics that they "send" into the network. Considerable research has been carried out and many publications have been published concerning the effects of harmonics on transformers. Although harmonics can be associated with overloading neutral conductors and cause false tripping of circuit breakers, here we will focus on the effect of harmonics on transformers.
Harmonics Some loads cause a disproportionate change in current versus voltage during each half cycle. These loads are classified as non-linear loads, and the current and voltage have a non-sinusoidal waveform containing distortion. As a result, the 50 Hz waveform has many additional frequencies that are superimposed on it, creating many frequencies within the 50 Hz sine wave. Several frequencies are harmonics of the fundamental frequency. Examples of non-linear loads are uninterruptible power supplies (UPS), variable speed drives, battery chargers, electronic ballasts, frequency converters, and switched mode power supplies (commonly used in computers and other data processing equipment).
When non-linear currents flow through the electrical system of the facility and distribution lines, additional voltage distortions occur due to the impedance associated with the electrical network. Thus, when electrical power is generated, distributed and used, voltage and current waveform distortions occur.
Equipment designed to operate at a fundamental frequency of 50 Hz is susceptible to unsatisfactory performance and sometimes malfunction when exposed to voltages and currents that contain significant harmonic frequency elements. Very often, the operation of electrical equipment may seem normal, but under a certain combination of conditions, the effect of harmonics increases, which leads to disastrous results. As discussed earlier, changes to the system that will lead to potential failure due to harmonics may include the installation of frequency converters (thyristor and power transistors IGBT), electronic ballasts, capacitors for improving the power factor, arc furnaces, powerful electric motors.
Eddy currents Applying non-sinusoidal excitation voltages to transformers increases iron losses in the transformer's magnetic circuit in much the same way as in a motor. A more serious effect of harmonic loads on transformers is associated with an increase in eddy current losses in the windings.
Eddy currents are circulating currents in conductors caused by the oscillatory effect of a stray magnetic field on the conductors. The eddy current concentration is higher at the ends of the transformer windings due to the displacement effect of the magnetic leakage fields at the ends of the coil. Eddy current losses increase as the square of the current in the conductor and the square of its frequency. The increase in eddy current losses of the transformer due to harmonics has a significant effect on the operating temperature of the transformer. Transformers used to supply non-linear loads must be sized based on the percentage of harmonics in the load current and the rated eddy current losses of the windings. The resulting increase in eddy currents increases the operating temperature of the steel core, which in turn degrades the insulation performance between the layers of the core. This results in a significant increase in I²R losses above the design limits of the transformer and overheating of the winding insulation. The result of all this is the closure of the windings to the iron of the core.
As indicated above, eddy current losses increase as the square of the current in the conductor and the square of its frequency; hence, with higher and higher harmonics, this heating increases even more. When we consider a transformer, we consider that the high and low voltage windings are wound around a solid iron core; but if we look more closely, we see that the "hard" iron core consists of a stack of thin steel plates, usually silicon steel, and these thin plates are insulated from each other with an insulating coating that is applied to both sides. The purpose of the insulation of the plates (batch) is to limit their heating by eddy currents during normal operation. Even without taking into account the increased heating at higher harmonic frequencies, the heating losses that occur in the iron of the core, due to the deterioration of the insulation, lead to premature failure of the transformer.
For example, we have an eddy current value of 10 A. The insulating layer, as a result of improper operation of the transformer, between the lamellas of the core is damaged and damaged to such an extent that all sections of the core are in contact with each other. As a result, the total heating effect increases from 5 W to 50 W, or 900% - an increase in heating. This increased heating, which does not include the additional effect of increased frequency associated with harmonics, will overheat the insulation of the primary and / or secondary windings, causing damage to the internal transformer winding.
Outcomes Many end users are familiar with surge protection and therefore use surge protectors and / or surge arresters, but many do not fully account for the high harmonic currents of new or added loads. The first step in designing or modifying an existing power distribution system to connect new loads is to simulate the electrical system to accommodate the harmonics of those loads. It is also highly recommended that you measure the actual current harmonics that exist and, after connecting new loads, measure the actual current harmonics again. If changes are made to an existing power distribution system, you can carefully add new loads and then immediately measure the harmonics resulting from these new loads. We hope that the transformer supplying these new loads will not fail a few days after these loads are connected. Once the harmonic levels are determined, you must disconnect them from the transformer and determine your next steps. The solution can be different, for example, install harmonic filtering or connect the transformers in such a way as to suppress harmonics, or replace the transformer for harmonics. Another option to support long term diagnostics is to install a permanent harmonic meter. If you have a transformer failure and have not changed the new load types discussed above, then harmonic analysis should be part of the root cause analysis. As an example, the IEEE 519-1992 standard states that the total harmonic distortion of a voltage waveform provided by an electrical or electronic device cannot exceed 3% of an ideal sine wave. To ensure that harmonics are not generated by this device, a measurement must be made on site. This location could be, for example, the point where the utility and facility wiring meets (usually at the meter). If the voltage distortion exceeds 3%, the utility must provide some form of harmonic mitigation to correct the problem.
1). In a certain loaded transformer, the secondary voltage is one-fourth of the primary voltage, the secondary current in this case: a) a quarter of the primary current b) four times the primary current c) equal to the primary current d) one fourth of the primary current or equal to the primary current
2). Which of the list is variable loss? a) loss of eddy currents b) loss of hysteresis c) loss in copper in the shunt field d) losses in copper winding
3). The noise generated by vibrations of the steel lamellas of the core, caused by magnetic forces, is called a) magnetostriction b) whistle c) hum d) zoom
4). The primary winding of a power transformer should always be: a) open b) short-circuited c) switchable d) united
5). Eddy current loss depends on: a) frequencies b) flux density c) thickness d) all of the above
6). The hysteresis loss will depend on: a) f b) f² c) f³ d) f ^ 1.6
7). Total core loss is also called ———? a) loss of eddy currents b) hysteresis loss c) magnetic losses d) loss of copper
eight). The maximum efficiency of the transformer takes place when copper loss _________ iron loss? a) more than b) less than c) equal d) any of the above
nine). The main function of the transformer is to change
a) power level b) power factor c) voltage level d) frequency
ten). The flux in the transformer core depends mainly on: a) supply voltage b) supply voltage and frequency c) supply voltage, frequency and load d) supply voltage and load
Insurance company AXA has found an increase in accidents among larger electric vehicle models.
Electric vehicles have a surprise for insurance companies: they are associated with more accidents than cars with internal combustion engines. The reality is not as disturbing as it may seem, but the insurance company AXA Group (France), based on its internal service statistics, claims that some characteristics of battery-powered vehicles may surprise their owners unpleasantly. At least until they get used to the new driving style.
The problem is that battery-powered cars accelerate much faster than gasoline or diesel cars, because the electric motor delivers full torque as soon as the driver touches the gas pedal. Maximum acceleration is available immediately, and this circumstance, in turn, can lead to more road accidents. The Swiss branch of the insurance company came to this conclusion after analyzing the files of its clients, and conducting a crash test to confirm the data.
In its report, AXA indicates that large and luxury EVs and SUVs have 40% higher access rates than comparable ICE-equipped models. In the lower segments, the numbers are nevertheless similar.
“In addition to the classic driving lessons, special knowledge of different types of vehicles is gaining in importance. In particular with regard to electric vehicles, motorists must first get used to the different braking and accelerating methods, ”says AXA accident researcher Bettina Zand.
Additional problem The use of innovative technologies in cars has another negative side, which the insurance company mentions in its report: "99 out of 100 drivers of electric vehicles, whose models are equipped with an autopilot, say they also use it, more often on the road and over long distances."
Although the word "autopilot" used in the text is a driving aid in modern cars (adaptive cruise control, signal recognition, lane keep assist ...). It is more of a semi-automatic pilot. And this is one of the risks: to believe that the car is capable of driving on its own. “More automation” also means a greater risk of drivers relying too heavily on technology. There are several accidents that have been caused by the driver's over-reliance on the system, ”says the AXA study.
Following the responses from respondents, the report concludes that EV drivers tend to be more interested in innovation, know more about assistance systems, and use them more often, for good and bad. All currently available systems must be constantly monitored and double-monitored for a fool. Although these systems act as support, drivers should not rely on them too much to jeopardize their own safety and the safety of others, ”confirms Bettina Sand.
Fire risk Accidents are equally dangerous in an electric vehicle and a car with an internal combustion engine. They pass the same crash tests and are equipped with the same safety features, and if a very strong collision occurs, the high-voltage electrical unit shuts down to avoid a possible short circuit that could cause a fire. In fact, AXA data indicates that electric vehicles do not burn out more often than other vehicles. However, it is true that a burning battery ignites and burns very quickly, and this fire is also very difficult to extinguish due to the chemical components and reactions in the battery.
Electrical transformers are known to make humming and buzzing sounds that can be unpleasant and disturbing. The reason for this noise is the magnetostriction process that takes place inside the transformer, in which the magnetic steel sheet changes its size and shape when magnetized. Since magnetostriction is an important process in the operation of a transformer, noise cannot be completely eliminated. But yes, if desired, it can be reduced to a certain extent.
The important thing that we must understand is that magnetostriction is the expansion and contraction of laminated cores. Both toroidal and W-shaped, although in toroidal, of course, these processes are much less, which create noise during magnetization. Thus, the noise of the transformer comes from the core. It is clear that no transformer can operate silently, and the noise level is determined by the design of the transformer itself. Any adjustments made to the design can increase or decrease the noise level. However, the main task of reducing the noise level must be addressed in the design of the transformer.
Mounting the transformer on a solid surface
It is always recommended to install transformers on heavy, dense surfaces such as concrete walls and floors, rather than on plywood surfaces or thin curtain walls, which will only amplify the humming noise, making it unbearable. Typically, the mounting surfaces should be ten times heavier than the transformer being installed.
Avoid installing transformers in corners, corridors and stairwells
When little space is left between the transformer and the adjacent wall, the sound is amplified. It works the same way as the echo of our voice amplifies our own sound. Thus, there must be sufficient free space around the transformer. This is why places such as corners of rooms, corridors and stairwells should be avoided unless we want to increase the acoustic effect of a noisy transformer. Also, placing the transformer under the ceiling is also not a good idea!
The transformer mountings must always be tightly tightened
All bolts and screws in the cover and top of the transformer must always be tightened. Any loose parts will vibrate when the transformer is on. This noise is combined with the annoying noise produced by the transformer.
Use of sound absorbing materials
The use of noise-absorbing materials such as oil barriers or rubber and soft materials can help absorb sound, which reduces noise and prevents it from spreading. This is a way of absorbing sound, but it does not mean that the noise from the transformer will disappear. This will only lower the noise level. But here the issues of effective cooling of the transformer immediately arise, which require a solution at the design stage.
The following guidelines will help you ensure an acceptable installation.
-Maximum noise level of the transformer should be compared with the predicted environment of its location. The transformer must be relocated if its noise level is higher than allowed at that location.
-When installing transformers in places such as office buildings or hotels, it is necessary to plan so that at least a small technical room remains between the transformer and the "areas for people".
-Mounting surface for the transformer should not resonate and increase noise. The mounting base (eg concrete floor) must weigh at least 10 times more than the transformer.
- Angular placement should be avoided as sound will be reflected into the room.
-The product should not be installed on thin walls such as plywood or curtain walls as they amplify sound (speaker effect).
-The manufacturer's installation instructions must be followed to use any vibration suppression devices included in the transformer design.