A
The rated ambient temperature is the highest temperature at which the transformer can be operated continuously under specified operating conditions.
A temporary overshoot of +10°C is generally permissible. Otherwise, the necessary cooling cannot be guaranteed.
SI base unit of electric current.
Unit: Ampère, defined as 1A = 1W/1V = 1C/1s.
The symbol is I.
Named after the French physicist and mathematician André-Marie Ampère (1775 – 1836).
It is also the unit of magnetic flux (magnetic voltage).
An autotransformer is a transformer where the input and output voltages are derived from a common winding, leading to smaller and more cost-effective sizes. In part, this results in significantly smaller and thus more cost-effective sizes.
ATTENTION: There is no galvanic isolation between input and output circuit!
Note:
It is the user’s responsibility to check whether a transformer without galvanic isolation may be used. Among other things, it must be considered whether the insulation level of a device connected to the output of the autotransformer is sufficient.
Example: Operation of a 115V unit on the European 230V mains: We would generally advise against using this unit with an autotransformer and recommend a transformer with separate windings.
An autotransformer, also known as a “Spartransformator” in German.
See also: rated poweer equivalent, throughput power
B
At BREMER Transformatoren GmbH, we name our mains frequency series based on the standardized core laminations used, adhering to the traditions of our industry. The core laminations are designated as EI, UI, or 3UI, reflecting their external shapes.
Single-phase transformers up to about 650VA are available in various EI series.
Single-phase transformers ranging from 150VA to about 20kVA belong to the UI series.
Single-phase transformers above 20 to about 100kVA are categorized as ETN series, with their cores based on an in-house standard.
Three-phase transformers from 60VA to approx. 25kVA are called 3UI type series.
Three-phase transformers from 25 to max 180kVA are the DTN type series, these cores also originate from their own in-house standard.
Mains frequency PCB transformers are EE and EI types (EE20 … EI66).
The so-called PCB flat transformers are called UI30 and UI39, as three-phase 3UI30 and 3UI39.
Product families of transformers, transformers and inductors for higher frequencies made of ferrite and other materials in all common forms from E to RM cores, we name as the core manufacturers do.
High-voltage and other special transformers have their own names.
You can find more information about core sheets in our flyer BREMER+GERTH_Kerne.pdf under Downloads.
C
Chokes for power applications are categorized into Alternating Current chokes (AC chokes) and chokes for Direct Current circuits (DC chokes).
AC chokes can be single or three-phase. Usually, these are choke coils with iron cores, with or without air gaps.
In the industry and universities, various terminologies are used to categorize chokes based on their application purposes:
Line Choke, Commutation Choke:
AC chokes for network fundamental frequency, mainly in a three-phase design, used to improve network current load (PFC) and to reduce the steepness of the flanks in the semiconductor valves of subsequent converters.
These chokes are primarily used in drive technology for a voltage drop uk = 4%, but uk = 2% is also common.
Motor Choke:
AC chokes, mainly three-phase, placed between the inverter output and the drive motor to shape the motor currents. These chokes extend the life of the motors and reduce noise. The required inductance values depend, among other things, on the length of the cables between the inverter and the motor.
Smoothing Chokes, DC-Link Chokes:
Direct current (DC) chokes, for example, for the DC link in converters between input rectifiers and output inverters. Also used for the excitation circuits of generators. The current dependency of this “smoothing element” must be considered in power supplies.
Compensation Chokes, Filter Chokes:
Filter chokes with matching capacitors, tuned to filter certain frequencies, or for (passive) compensation of reactive power loads.
Current-compensated Chokes:
These filter chokes have 4 (single-phase) or 6 connections (three-phase). They reduce asymmetric disturbances, influencing the useful signal only slightly.
Neutral Point Chokes:
Chokes for damping the currents and their steepness in the (via the choke) grounded neutral point in the case of earth faults in power supply networks (including railway power supplies).
Air Coils:
represent a special design. The advantage of their largely linear characteristic curve is offset by a large size and low inductance compared to iron core coils.
The essential characteristic of a choke coil is its inductance.
It must also be designed for the expected continuous and peak current. Instead of an inductance specification, a voltage drop at rated current can also be specified (see line chokes, uk…%). The rated frequency and the voltage to be considered for insulation technology are also required.
D
Transformer in which the windings and core are not in an insulating liquid.
All BREMER transformers are dry-type transformers.
E
F
SI unit of electrical capacitance
Unit: 1F = 1(A*s) / 1V = 1C / 1V (here C = Coulomb = electric charge)
Formula symbol: C
Michael Faraday: English physicist (1791 – 1867)
Charles Augustin de Coulomb: French physicist (1736 – 1806)
A fixed-mounted (stationary) transformer is designed to be securely mounted during operation as specified by the manufacturer. It is either permanently mounted or weighs more than 18kg and lacks carrying handles.
A movable transformer can be moved while connected to the power supply. Transformers in the form of “plug-in power supplies” are also considered movable transformers.
An appliance transformer is specifically designed to supply power to certain devices or systems and is intended for use with them.
An embedded transformer is an appliance transformer designed for installation in a device housing that provides protection against electrical shock.
Unless otherwise expressly agreed, BREMER transformers are always executed as stationary transformers or appliance transformers and/or embedded transformers.
The term “flat transformer” is commonly used for PCB transformers of the core type series UI30 and UI39.
The rated frequency is the frequency for which the transformer is designed.
For inquiries and orders without frequency information, we assume a frequency of 50-60Hz and a sinusoidal input voltage.
Note 1:
A transformer cannot transform frequency. The frequency of the output voltage is identical to the frequency of the input voltage.
Note 2:
Transformers explicitly ordered for a frequency greater than 50Hz lead to smaller core types than 50Hz transformers of comparable performance. However, they cannot be operated – not even for testing purposes – with 50Hz and rated input voltage.
Note 3:
A transformer labeled with a rated frequency of 50Hz can be operated with 60Hz without any concerns.
G
H
SI unit for electrical inductance
Unit: Henry 1H = 1Vs/1A
Formula symbol: L
Joseph Henry: American physicist (1797 – 1878)
SI unit for the frequency
Unit: 1Hz = 1(1/s) (repetitions / second)
Formula symbol: Hz
Heinrich Rudolf Hertz: German physicist (1857 – 1894)
I
Inductance is a measure of the storage capacity of magnetic energy. The inductance is the essential parameter of a choke coil.
Unit: Henry 1H = 1Vs/A
Note 1: Do not confuse with induction.
Note 2: In technical jargon, “AN INDUCTANCE” sometimes refers to an inductive component, i.e., a choke or choke coil.
Magnetic induction; formula symbol: B
Also called magnetic flux density or flux density.
Induction is one of the elementary internal parameters in the dimensioning of magnetic (inductive) components. Unit: Tesla 1T = 1Vs/m²
Note: Do not confuse with inductance!
The rated input voltage, also known as the supply voltage, is the voltage assigned to the transformer for the established operating conditions. In multiphase systems, this is referred to as the “line-to-line” voltage between the outer conductors.
See also: Multiple input voltages, multiphase systems
The low-voltage (network voltage) standard values in Europe are established in IEC60038, being 230V (single-phase) or 3x400V (linked) with a tolerance range of +/-10%. BREMER transformers are designed to accommodate these tolerances unless otherwise specified or correction taps are intended.
Note: In user specifications, an input voltage indication of, for example, 230V +/-10% is often found. Clarification is then required as to whether this is intended to describe the input voltage range or express a desire for correction taps.
Therefore, we ask for precise formulation, e.g.,
- Input Voltage: 230V Input Voltage Range +/-10% or
- Input Voltage: 230V with taps for +/-10%
Note: BREMER transformers with positive taps are designed for a maximum input voltage according to the desired positive value. An additional addition with an input voltage range (tolerance range) would require agreement.
Insulating materials in transformers are classified into thermal classes (insulation classes) according to IEC 60085 and IEC 60216. This classification considers the maximum allowable temperatures of the insulating materials, with temperatures being reduced by the so-called hot spot value according to EN61558-1:2019-12.
The following insulation classes result:
Thermal Class A: 100°C
Thermal Class E: 115°C
Thermal Class B: 120°C
Thermal Class F: 140°C
Thermal Class H: 165°C
Transformers should not exceed the temperature values of these classes during intended use, considering also the ambient temperature.
The ingress protection rating, or IP rating, of electrical equipment such as transformers and chokes, is defined according to DIN EN 60529 (VDE 0470-1).
These ratings, supplemented by two numbers following the letters “IP”, provide information on protection against contact and foreign bodies (first number) as well as protection against water (second number). See EN / IEC 60529 for further information.
Common protection classes of enclosures are IP20, IP23, IP44, IP54 and IP65.
|
Number
|
Protection against solid foreign objects… |
Protection
against water… |
|
0
|
not protected | not protected |
|
1
|
… greater than 50mm / …hand | drop vertical |
|
2
|
… greater than 12.5mm / …finger | drops inclined up to 15 |
|
3
|
… greater than 2.5mm / …tools | spraying water up to 60° |
|
4
|
… greater than 1mm / … wires | splashing water any direction |
|
5
|
dust protected | jet with nozzle |
|
6
|
dust-tight | strong jet with nozzle |
|
7
|
— | temporary immersion |
|
8
|
— | permanent immersion |
Note:
Some transformer manufacturers label transformers with IP20 even if they have no housing and no cover. They argue that – provided the terminals are protected against accidental contact – the windings are protected by insulating tape or plastic covers. BREMER-Transformatoren cannot endorse this view. We designate a transformer without a cover as IP00. For protection class IP20, we either use ready-purchased sheet steel or plastic enclosures or we manufacture at least one cover made of aluminium or perforated sheet steel or plastic.
An isolation transformer is a transformer with a protective separation between the input and output windings. Isolating transformers for general applications, single-phase up to 25kVA and multi-phase up to 40kVA, are usually designed according to EN61558-2-4. If agreed between customer and supplier, this standard can also be applied to isolating transformers without limitation of the output power.
In contrast to the mains transformer (according to EN61558-2-1), the isolating transformer has so-called “double or reinforced” insulation in addition to the basic insulation. (Some types may also have a combination of the basic insulation with a shield winding between the input and output winding in its place).
Note:
A “transformer with separate windings” (ugs.) is a mains transformer and has basic insulation.
See also: Mains transformer
J
K
L
Leakage transformers are designed to have reduced coupling between the input and output circuit windings and increased leakage field, which is normally undesirable on fixed-coupled transformers. Operating curves with high open-circuit voltages and defined current-voltage values at load can be realised.
Leakage transformers can also be designed as absolutely short-circuit-proof transformers for higher power ratings, but they represent a very complex special design.
Note:
Typical applications for leakage transformers are gas discharge lighting (“neon lights”) with high ignition voltages and subsequent limitation of the current in the gas path which now represents a “short circuit”.
For some converter circuits, transformers with increased short-circuit voltages are required, which, depending on the level of the short-circuit voltage, can also already be referred to as leakage transformers.
By combining an AC choke and a fixed-coupled transformer, circuit characteristics comparable to a leakage transformer can be produced.
Because of the complex engineering work involved, leakage transformers are commercially unfavourable for single units and very small series.
M
A mains transformer for general applications is a transformer where the input winding and the output winding are separated by basic insulation at a minimum. Reinforced or double insulation between the input and output circuits is not required.
Mains transformers of 1(5) kVA, single- (three)-phase, are generally executed according to EN61558-2-1. This standard can also be applied to isolation transformers with higher output powers if agreed upon between buyer and seller.
Transformers can be designed for multiple input voltages by means of taps and / or multiple input windings.
However, it should be noted that the additional winding space typically leads to larger core sizes compared to a transformer of similar power designed for a single input voltage. An exception is when the input winding is divided into two equal parts, allowing the user to optionally connect them in series or parallel, requiring only a little additional winding space for insulation.
Note:
A sometimes-observed mistake is that users, in transformers with two input windings intended for optional series or parallel connection, connect only one input winding in applications with a lower network voltage. This leads to overheating and destruction of the primary winding at rated load and should be strictly avoided. This also affects the fusing. (This error may not be immediately noticed at partial load or no-load operation!)
N
The no-load power and no-load current of a transformer refer to its behavior at rated input voltage and frequency when no load is connected. The no-load power is the absorbed active power, while the no-load current is the apparent input current in this state. Due to core sheet tolerances, these values can vary significantly.
The losses of a transformer mainly consist of iron losses and copper losses. To a first approximation, no-load current and no-load power are a measure of iron losses. The copper losses in turn can be estimated by a short circuit test.
If a transformer is expected to be operated mainly at no-load or under small partial load, it is possible and sensible to optimise it with regard to no-load losses.
Note:
The standards for core materials allow for considerable tolerances. The quality of the core materials and thus the idle currents can vary accordingly. Transformer manufacturers and suppliers unfortunately have limited influence on this and limited alternatives on the global market.
The no-load output voltage of an unloaded transformer connected to the rated input voltage and rated frequency is always higher than the rated output voltage (voltage under load). In very small transformers, it can be up to twice the rated voltage. The permissible maximum values are specified in EN 61558 in various “Parts 2-..” and are given there as a percentage of the ratio between no-load voltage and output voltage under load.
Important: Always specify a maximum no-load output voltage when using components whose voltage tolerance could be affected. In such cases, it may be advisable to choose a slightly larger transformer type.
Note: The no-load voltage is especially important when the protective low voltage (max. 50V at no-load) or other voltage limits (e.g., 1100V limit) could be reached.
Hint: The rated values, to which the output voltage tolerance and the no-load deviation refer, apply to the ambient temperature for which the transformer is specified.
O
SI unit for electrical resistance
Unit: Ohm 1Ω = 1V/1A
Formula symbol: R
Georg Simon Ohm: German physicist (1789 – 1854)
In continuous operation (CO), a transformer is operated indefinitely.
In short-time operation (STO), a (cold) transformer is operated for a certain period, allowing it to return to ambient temperature before the next cycle.
In intermittent operation (IO), a transformer is operated in a series of defined identical cycles.
It is practical for the user to specify their varying loads and the respective subsequent breaks for the “worst-case scenario” in minutes or hours. The indication of STO and IO in percent is often misleading.
BREMER transformers are always dimensioned for years of continuous operation unless otherwise specified.
The rated output current refers to the current in the output winding at the rated input voltage and frequency for which the transformer was designed and manufactured. When loaded with the rated load impedance, this current is set.
Comment 1:
For the thermal dimensioning of transformers, the effective value of the output alternating current must always be considered. We understand all electrical quantities provided to us to be sinusoidal effective values, unless they are expressly described otherwise.
Comment 2:
Unless otherwise ordered, the rated output current is always assumed to be continuous operation.
Comment 3:
Electronics engineers often give us the DC values behind an input rectifier. We will be happy to calculate the corresponding AC values for you, but then we also need the details of the rectifier circuit and smoothing. The mixed indication of AC voltage and DC current is not useful.
The rated output voltage is the voltage for which the transformer is designed and manufactured. his is established when the transformer is connected to the rated input voltage at the rated frequency and loaded with the rated output current and power factor.
In the case of multi-phase systems, the so-called “chained” voltage between the outer conductors is referred to.
Note:
The no-load output voltage of an unloaded or partially loaded transformer is always higher than the rated output voltage.
See also: Output voltage tolerances
Under rated load, according to EN61558-1, the output voltage may deviate a maximum of 5% from its rated value, 10% on transformers that are inherently short-circuit proof, usually <y 2VA, plus an additional 5% on transformers with rectifiers. For more details - for example, on transformers with multiple output windings - refer to EN611558-1 (2019-12).
Note:
The rated values to which the output voltage tolerance and the no-load deviation refer apply to the ambient temperature ta for which the transformer is rated.
P
Small transformers whose winding connections are led to solder pins for circuit boards (THT assembly) are also referred to as PCB transformers or print transformers. They are mainly produced for single-phase assemblies with capacities up to about 30VA and in encapsulated versions.
We manufacture and sell safety transformers for printed circuit boards in our GERTH Transformatorenbau GmbH as standard products and customer-specific variants derived from them.
BREMER Transformatoren develops and manufactures ladder-type transformers in special designs, for example with several different secondary windings. Also ferrite core transformers for printed circuit boards.
The input circuit of a transformer is intended for connection to the supply circuit. The winding of the input circuit is called the input winding or primary winding.
See also: Input voltage, multiple input voltages
Electrical equipment must have protection against electric shock in the event of a fault. In this respect, they are classified in protection classes in DIN EN 61140:2016-11 (VDE 0140-1). Usually, transformers from our production are intended for installation in devices or systems. Therefore, they do not have a protection class themselves, but are prepared for devices of protection class I or II.
- Transformers prepared for protection class I devices have a protective earth connection. This is connected to the touchable conductive parts and is intended for the fixed wiring of the installation. In addition, these transformers have at least basic insulation.
- Transformers prepared for protection class II devices do not have a protective earth connection. Double or reinforced insulation serves as an additional safety precaution here.
- Protection class III refers to equipment in which protection against electric shock is based on the safety extra-low voltage (SELV) or protective extra-low voltage (PELV) supply and in which no voltages higher than SELV are generated.
Q
R
Rated power is the product of the rated output voltage and the rated output current.
In three-phase systems, it is calculated as the product of √3 times the rated output voltage times the rated output current, where the voltage is the line voltage.
If the transformer has more than one simultaneously loaded output winding, the rated power is the sum of the individual products.
If only one output winding is loaded, the product with the highest value is taken as the rated power. The same applies to taps. In three-phase systems, the calculation is done analogously.
Compare: Rated power equivalent
Compare: Throughput power
The term “Typenleistung,” or Rated Power Equivalent, doesn’t refer to a direct electrical size. It is used as a guideline to estimate the space requirements and weight of transformers, representing a common equivalent between a transformer core type and electrical power under simplified conditions (e.g., 50Hz, ta=40°C).
The actual output power that can be achieved with a particular core type depends on various factors, such as the ambient temperature, maximum permitted component temperature, core material, operating mode, cooling situation, number of output windings, number of primary voltages, maximum open-circuit voltages, insulation requirements (type of transformer) and others.
Note:
Users should note that the desire for multiple input voltages often leads to a larger transformer type compared to a transformer of identical output power designed for only one input voltage. A “universal transformer” is thus often not economical. Likewise, designing the transformer for a higher than realistically expected ambient temperature leads to larger types.
See also: Rated power, Throughput power
S
In principle, the study of n-phase systems is an interesting academic field. In our day-to-day practice – the dimensioning and production of transformers – we only deal with 1- or 3-phase transformers or voltage systems. 3-phase is also referred to as three-phase current.
Remarks:
A so-called core transformer, also called KT or UI transformer, i.e. a transformer with two coils, is also a 1-phase transformer. The primary windings of this transformer are connected either in series or in parallel. The term 2-phase transformer is not in use.
Similarly, a transformer connected between two outer conductors of a 3-phase supply network is a 1-phase transformer. (In the usual public low-voltage grid, this is then, for example, a 1-phase transformer, primary voltage 400V).
It is often desired to distribute a 1-phase load evenly over the 3 phases of a network. This is not possible with a single transformer.
A safety transformer is specifically designed to supply safety extra-low voltage (SELV) or protective extra-low voltage (PELV) circuits.
ypically, safety transformers up to 10/16kVA single/three-phase are executed according to EN61558-2-6, with higher power transformers also possible by agreement between the buyer and manufacturer. These transformers are designed so that the sum of all output AC voltages at no-load is a maximum of 50V.
The winding of the output circuit of a transformer is referred to as the output winding or secondary winding.
Short-circuit proof according to the definition of DIN EN IEC 61558-1:2019-12 is a transformer in which the temperature, even in the event of an overload or short-circuit, does not exceed the values permitted for this transformer and which continues to meet the requirements of the above standard after the overload or short-circuit has been removed*.
A distinction is made between
- not short-circuit proof transformer,
- absolutely short-circuit proof transformer and
- conditionally short-circuit proof transformer.
A non-short-circuit proof transformer is intended to be protected against overload, short-circuit and otherwise unacceptably high temperatures by a suitable protective device fitted by the user which is not part of the transformer.
With an absolutely short-circuit-proof transformer, compliance with specified limit values for currents and temperatures is ensured by the design. For example, very small transformers up to approx. 2…3VA have such high-impedance windings that these transformers are absolutely short-circuit-proof.
A conditionally short-circuit proof transformer is equipped with a protective device. This interrupts or reduces the current in the input or output circuit in the event of an overload or short circuit. If the protective device is resettable, the original function of the transformer is restored after removing the overload, cooling down the transformer, and resetting the protective device (see resettable temperature limiters).
Addition: Small transformers – especially small print transformers – in a conditionally short-circuit-proof design are often equipped with temperature fuses (“thermal fuse”) permanently installed in the transformer. After an overload or short circuit, the transformer must be replaced.
If a BREMER transformer is not explicitly defined as short-circuit proof, it is a non-short-circuit proof transformer. In this case, it is intended that the transformer is protected against overload, short circuit and other inadmissible temperatures by suitable protective devices fitted by the user.
Remarks:
* This does not mean that all types of short-circuit proof transformers are still functional. Among other things, transformers equipped with a non-resettable and non-replaceable protective device must be replaced after it has tripped (thermal fuse).
The short-circuit voltage refers to the necessary voltage that must be applied to a transformer’s input winding to achieve the rated input current when the output winding is short-circuited, and the windings are at ambient temperature. It is usually expressed as a percentage of the rated input voltage and represented by the symbol uk.
A low short-circuit voltage indicates that the transformer is designed to have low copper losses at the rated current. However, in some applications, such as with certain converters in drive technology, a higher short-circuit voltage is necessary. In these cases, it might be beneficial to connect a line choke upstream of the transformer.
Small transformers up to a power of 40kVA are dealt with in the standard DIN EN IEC 61558-1, at the same time VDE0570 Part 1 and IEC 61558-1. It is currently available in the 12/2019 issue. Transition period for issue 07/2006: 21.06.2022
Part 1 of this standard describes general requirements and tests. It is complemented by a larger and growing number of “Parts 2”. These describe the requirements for transformers for a wide range of applications. An overview of these parts 2 can be found at www.vde.com or www.vde-verlag.de. In addition, transformers are also specified in some national and international equipment standards.
Transformers and chokes of higher power ratings are described in the VDE 0532 / EN 60076 family of standards. However, in consultation with the purchaser, the above-mentioned EN 61558 for transformers with rated powers > 25/40kVA (single-phase/multi-phase) can also be used. BREMER Transormatoren GmbH generally proceeds in accordance with this option. Some high-voltage and insulating transformers of the BREMER manufacturing programme are not described appropriately by any standard. For these special transformers, agreements between the user and the manufacturer apply, especially with regard to the test procedures and test levels to be used.
Do not hesitate to ask us about the EN61558 family of standards.
The switching group is essential in three-phase transformers, providing information about the type of circuit and phase position, as well as the possible types of load of a three-phase transformer.
The code letters are:
- Y, y for the star connection,
- D, d for the delta connection,
- Z, z for the zigzag circuit,
- I, i for the open circuit of the windings.
- N, n indicates whether a star point is led out as an outer connection.
- a indicates an autotransformer, (which, as a three-phase transformer, is always designed in a star connection).
The upper case letter is generally used for the upper voltage winding, the lower case letter for the lower voltage winding.
The figure at the end indicates the multiple of 30°el. the voltage pointer of the output voltage lags the pointer of the input voltage in a counterclockwise direction.
If the orderer does not specify otherwise, BREMER three-phase transformers are manufactured in switching group YNyn0.
The selection criterion for the switching group is, among other things, whether the system feeding the input side has a star point and whether the load is a symmetrical load or whether single-phase loads are also to be fed. In the case of single-phase (asymmetrical) loads, this is also referred to as secondary-side star point loading.
Preferred switching groups for transformers:
- Yy0 – The secondary star point may only be loaded with the full load current if the mains feeding it on the input side has a star point conductor and this is firmly connected to the transformer star point. Otherwise, the star point of the output winding can only be loaded with approx. 10%.
- Dy5 – Secondary star point fully loadable
Other common switching groups for transformers:
- Dd0 No star points present
- Yd5 No secondary star point present
- Yz5 Secondary star point fully loadable
The core type power ratings of transformers with Z circuits are smaller than those of comparable transformers with Y or D circuits, so they are used less frequently.
Star points that have been led out (N or n indicator) cause additional costs compared to a winding without star points that have been led out. It therefore is advisable to specify in orders whether the star points must be accessible.
Examples of switching groups:
Example 1: YNyn0yn0
Star-star connection with 2 secondary windings and star points led out
- A transformer with such a vector group could, for example, be mounted in a machine tool and have a three-phase motor connected to one secondary winding and a B6 three-phase rectifier downstream of the other. The secondary windings can be loaded with approx. 10% of the rated output power in single phase for small control powers and lighting.
Example 2: Dyn5
Delta-star circuit with star point led out on the secondary side and phase angle 150°el. between input and output
- For example, three single-phase loads (e.g. three heating elements), which may not be switched on at the same time, could be operated on such a transformer. The transformer can be loaded with the rated current on each phase – regardless of whether the other phases are loaded.
Example 3: YNa0
Three-phase autotransformer
T
Tappings refer to the windings of transformers where subsets of the total number of windings can be “tapped”.
For input windings, this allows for multiple input voltages and input voltage ranges.
For output windings, unless otherwise agreed and labeled, all tappings are maximally loadable with the load current of the highest voltage level. Tappings may only be loaded alternatively unless otherwise agreed.
Transformers must be suitably protected against impermissibly high temperatures in the event of overload or short circuit. A number of protective devices (fuses, magnetic and thermo-magnetic switches, overload releases, temperature limiters, etc.) are possible and suitable depending on the application.
A temperature limiter cannot be adjusted by the user and, if a transformer is not used as specified, limits its temperature by opening the circuit or reducing the current.
A self-resetting temperature limiter (“temperature monitor”) restores the current flow as soon as the transformer has cooled down. A temperature limiter that does not reset automatically also requires manual intervention.
A “thermal fuse” or “temperature fuse” must be replaced after it has responded once. Usually, this involves replacing the entire transformer.
Small transformers – especially small print transformers – in a conditionally short-circuit-proof design are often equipped with thermal fuses permanently built into the transformer. After an overload or short circuit, the transformer must be replaced.
Alternatively, automatically resetting bimetal thermal switches (“temperature monitors”) can be installed, but these require a minimum size for space reasons and a certain unit value for commercial reasons.
Temperature monitors, for example bimetal normally open or normally closed contacts, can be installed in the windings and their contacts routed to terminals. After installing the transformers in a system or equipment, the user has an easy-to-handle signal for control or warning purposes or for emergency shutdown.
See also: Short-circuit strength
SI unit for magnetic flux density
Unit: Tesla 1T = 1Vs/1m²
Formula symbol: B
Nikola Tesla: Croatian-American physicist (1856 – 1943)
Some transformer manufacturers use the term throughput power in autotransformers to indicate the output power. This is intended to highlight the difference from the “type” power or “core” power of the transformer.
Static electromagnetic machine with two* or more windings. Transformers are generally used to transmit electrical energy by converting one alternating voltage into another alternating voltage of the same frequency.
*Exception autotransformer
U
V
SI base unit of electrical voltage
Unit: Ampère 1V = 1W/1A
Formula symbol: U
Alessandro Giuseppe Antonio Anastasio Volta: Italian physicist (1745 – 1827)
W
SI unit of power
Unit: Watt 1W = 1V*1A = 1J/s (At the transformer we indicate the apparent power Volt-Ampere VA)
Formula symbol: P
James Watt: Scottish engineer (1736 – 1819)