http://atltransformers.uk/komen-nochtans/

ATL design and deploy leading magnetics to the rail network and its principle contractors. We offer the highest specifications on the market while maintaining competitive pricing against alternative legacy technology. Our magnetics are approved for use by Network rail, Crossrail, London underground and are widely used to support Designers, Route asset manager, maintainers & Installers. As the preffered solution in rail ATL’s New generation of magnetics, eco-rail® is raising the bar on quality and performance, delivering unprecedented levels of weight/size reduction, ergonomics, Carbon reduction and safety.

# Transformer Frequency

### Effect of frequency

Transformer universal EMF equation

If the flux in the core is purely sinusoidal, the relationship for either winding between its rmsvoltageErms of the winding , and the supply frequency f, number of turns N, core cross-sectional area a and peak magnetic flux densityB is given by the universal EMF equation:

$E_text{rms} = {frac {2 pi f N a B_text{peak}} {sqrt{2}}} ! approx 4.44 f N a B$

If the flux does not contain even harmonics the following equation can be used for half-cycle average voltage Eavg of any wave shape:

$E_text{avg}= 4 f N a B_text{peak} !$

The time-derivative term in Faraday’s Law shows that the flux in the core is the integral with respect to time of the applied voltage. Hypothetically an ideal transformer would work with direct-current excitation, with the core flux increasing linearly with time. In practice, the flux would rise to the point where magnetic saturation of the core occurs, causing a huge increase in the magnetizing current and overheating the transformer. All practical transformers must therefore operate with alternating (or pulsed) current.

The EMF of a transformer at a given flux density increases with frequency. By operating at higher frequencies, transformers can be physically more compact because a given core is able to transfer more power without reaching saturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss and conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies which reduce core and winding weight. Conversely, frequencies used for some railway electrification systems were much lower (e. g. 16.7 Hz and 25 Hz) than normal utility frequencies (50 – 60 Hz) for historical reasons concerned mainly with the limitations of early electric traction motors. As such, the transformers used to step down the high over-head line voltages (e.g. 15 kV) are much heavier for the same power rating than those designed only for the higher frequencies.

Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current; at lower frequency, the magnetizing current will increase. Operation of a transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers may need to be equipped with “volts per hertz” over-excitation relays to protect the transformer from overvoltage at higher than rated frequency.

Knowledge of natural frequencies of transformer windings is of importance for the determination of the transient response of the windings to impulse and switching surge voltages.

##### Transformer Safety Standards

Low voltage Low power Transformer Standards EN61558-1

The old VDE 0550 and VDE 0551 transformer standards do not exist anymore. The previous EN 60742 (VDE 0551) transformer standards is outdated now also.
The up-to-date standards for transformers are now BS EN 61558 IEC 60076 (VDE 0570). Part 1 of the standard explains general requirements and tests. Part 2 lists special transformer types like safety isolating transformers (part 2-6) or SMPS transformers (part2-17) a separate standard, which still has a connection to part 1, for general requirements.

For larger power and voltage ranges IEC 60076 is consulted. Consisting of many parts this standard covers an array of transformer products from Dry type, Oil filled & Cast resin transformers.

UL5085 is typically adopted for UL approved transformers.

##### Transformer Temperature & Insulation

ATL Transformer Design

Our transformers are designed in a way that does not allow impermissible temperatures. The insulating materials have at least class E conforming to IEC 85, mostly class F (155 deg C) & class H (180 deg C) and on request even higher temperature classes. For the technical calculation, the temperature is calculated at 1,06 times the primary voltage in rated operation and in short-circuit case.

##### Transformer Screening

Definitions of ATL screens

Safety isolating earth screen

A metallic material (typically copper foil) between the primary and secondary windings offering isolation to ground between the two windings sometimes referred to as: Separation of dangerous active components by use of a conductive shield, which is located between the two parts and is connected to an external earth terminal.

##### Transformer Voltages

Low voltage

Low voltages are rated voltages that are not higher than 1000 V AC.

High voltage

High voltage is defined as a voltage, that is higher than a low voltage of 1000 V. The maximum we are able to produce at the moment is at 50 kV AC.

The AC high voltage output has the following special characteristics: One terminal is connected to earth (minus pole, green/yellow connector) and the other terminal is connected to the high voltage side (plus pole or “hot side”).

Current is calculated with the power I = P/U and is usually given in [mA].

The frequency is at 50 Hz sinusoidal. For special fields of application, frequencies can be higher. We already built transformers with 20 kV and 1000 Hz.

The insulation test primary – secondary is done with 120% of the high voltage value..

##### Line Reactors

ATL Transformers Line Reactors conform to EN 61558, EN 5008-1 and -2, EN 50082-1 and -2

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