Safety 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.
Protection Class
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.
Secondary Winding
The winding of the output circuit of a transformer is referred to as the output winding or secondary winding.
IP rating
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.
Switching group
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
PCB Transformer
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.
Primary Winding
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
Ohm (Ω)
SI unit for electrical resistance
Unit: Ohm 1Ω = 1V/1A
Formula symbol: R
Georg Simon Ohm: German physicist (1789 - 1854)
Standards
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.
Mains Transformer
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.