Transformer shortcut

To be able to categorise the types for admittance like VDE, we use shortcuts, which you can download here as PDF.

Definition of small transformers

A transformer with a power less than 3 kVA, a primary voltage up to 1000 V and a secondary voltage smaller than 100000 V counts as small transformer.

Conventional transformers

For conventional transformers, swaged transformer metal sheets are pushed into the previously winded coils. Depending on method, this happens mutually or synchronistically. These transformers are also known as bobbin- or cover transformers.

Toroidal transformers

The circular and isolated iron core is wound directly with copper wire by using special machines.

Primary winding

This winding is meant to be connected to the mains, it is marked with a label “PRI”.

Secondary winding

This winding is meant to be connected to the installation, machine or another electronic components, it is marked with “SEC”.

Screening or screen winding

A metal insert between two windings reduces the transmission of disturbances. You have to differ between a static and a magnetic screen.

Safety screen

Separation of dangerous active components by using an inserted screen, which is connected to a PE.

Static screen

The screen as static screen serves the attenuation of mains disturbances and voltage peaks in the high frequency area (EMV-transformer). The static screen has a green/yellow connector. To control the function, the screen can have a connecter at the input and at the output. One of the connectors is cut off after the test.
A bigger attenuation can be achieved by two static screens. One screen is connected to the PE and is green/yellow. The second screen has secondary potential and the connector is black.
Usually, copper foil is used for a screen. For EMV purposes, high permeable foil is better though, as the attenuation is bigger here at high frequencies. With high permeable foil, we get attenuation rises up to 20dB at relatively low frequencies already, compared to Cu-foil. The connector is in touch with the core.

Magnetic screening

The magnetic screening lowers the outer magnetic stray field of the transformer. The screening is done by a coating with high permeable material. This material, e.g. mu-metal, can be built as housing around the transformer, or simply be mounted as metal sheet. The magnetic screen usually has no connector.

Both screening methods increase the size of the transformer or decrease the power at the same type size.

Acoustic requirements

Inductive components are constructed, to not generate a lot of noise.
Certain noises are unavoidable though. The magnetostriction (Length-width change) of the material generates a hum noise. By mounting on a chassis sheet, which works like a loudspeaker membrane, acoustic noises can be amplified.
If there are no sinusoidal voltages or a steep dU/dt, we get induction rises, which can lead to a saturation in the material, and this is audible. Here chokes can help, which generate a sinusoidal current.
It is possible to construct the transformer for low hum noise, but this worsens the efficiency. Alternatively rubber-bonded-metal dampers are usable - similar to suspension in cars - the vibrations are damped and thus, the housing can’t amplify noises.
To avoid magnetic interspersion of disturbances, transformers can be screened.

Transformer with separated winding

This transformer has a galvanic separation between primary and secondary, which has to redeem an easy insulation test.

Auto transformer

This is a transformer with a connection between primary and secondary winding.

High voltage transformers

These are transformers with secondary voltages from 1000 V up to 100000 V.

Safety transformers

A safety transformer has a max. voltage of 50 V at the output and needs to pass an insulation test between primary and secondary.

Isolation transformer

The insulation transformer has a voltage of 51 V up to 10000 V at the output and needs to pass an insulation test between primary and secondary.

PFC

Power factor correction
An active or passive power factor compensation is a special filter, which is used mainly for power supplies of a certain power, e.g. computer power supplies, to minimise the amount of disturbing harmonics and to get the power factor as close to 1 as possible.

Protective class I

Transformers in this protective class have a PE connector

Protective class II

Transformers in this protective class have an increased insulation and cover, thus they don’t need a PE.

Open transformer

Meant for mounting

Enclosed transformer

Transformer in a housing

Absolutely short-circuit proof

The output may be in a short-circuit permanently.

Partly short-circuit proof

A transformer with built in overload protection

Not short-circuit proof

Here the user has to deal with the overload protection

Rated power

This is the power, which can be obtained permanently from the transformer.

No-load voltage

This is the secondary voltage of a transformer in no-load

Small voltage

Voltages below 50 V

Low voltage

Voltages between 51 V and 1000 V

High voltage

Voltages above 1000 V

Overload protector

This is a switch, that will open, if there is too much current

Temperature switch

This is a temperature-sensitive switch, which turns off the transformer at too high temperatures.

Operation temperature

This is the maximum temperature, which the transformer reaches at full load (ambient temperature + temperature rise = operation temperature)

PTC

Reacts on too high currents and too high temperature and switches the transformer off

European standards

EN  61558-1: safety of transformers (VDE  0570 Part 1). General requirements and tests

EN  61558-2-4: Special requirements to isolation transformers (VDE 0570 Part 2-4) for general use

EN  61558-2-6: Special requirements to safety transformers (VDE 0570 Part 2-6) for general use

EN  61558-2-17: Special requirements to transformers (VDE 0570 Part 2-17) for switch mode power supplies

CE

This is the European conformity sign

CB

Test on the basis of IEC-standards

CTI

CTI stands for Comparative Tracking Index – comparable figure of the creep distance. Transformers can fail, if creep distances for current occur due to dirt or humidity. CTI is the maximum voltage, where 50 drops of water, won’t evoke a creep distance on the insulation material. The test is set in IEC 112.

Boundaries in electric insulation systems

  

“Boundaries are often the weakness of electrical insulation, but they can be evoked on purpose to influence materials and insulation systems. They determine the operation and ageing characteristics of electrical insulation. Due to construction and function, there are a lot of boundaries in insulation materials. Next to the visible outer boundaries, there are also inner boundaries.”
Prof. Kindersberger, TU Munich

International compilation of main voltages and frequencies

International, country specified mains voltages can be told to you on request. The leaflet "Electric Current Abroad" of the U.S. Department of Commerce does not only show voltages, but also plug types. This leaflet can be obtained at ZVEI in Germany, Frankfurt. Here as download:
www.ita.doc.gov/media/Publications/pdf/current2002FINAL.pdf

For further basic information we recommend the programme of the TUIlmenau "Transformator"

http://getsoft.net/trafo/index2.html

Beginning with the electro magnetic basic laws (magneto motive force and induction laws) and the electrical net equations of primary to secondary, the electrical equivalent circuit diagram for a one-phase transformer is derived step by step. No-load and short-circuit test demonstrate, how to determine all values of the equivalent circuit diagram. Real technical transformers show current ideal characteristics. Which parameters, diagrams and equations are needed to describe the characteristics is given. The focus is on the no-load characteristics by considering non-linear characteristic lines of the core material.

The chapter “tasks” give parameters of technical transformers. On an example, these parameters are used to calculate the elements of the complete equivalent circuit diagram.