Transformers

Principles of Operation of the Safety Isolating Transformer
A transformer transfers electrical energy from one circuit to another through the medium of a magnetic field. The transformer may step up the voltage, step it down or deliver energy at the same voltage.

The safety isolating transformer is one of the most widely used of all transformers. It is designed to achieve an accurate voltage ratio within a specific load range.

There is no direct connection between the windings. They are only connected by the intangible lines of magnetic flux in the core. In some types of circuits the transformers may have 1:1 ratio.

Long Life
Because of the long hours of use, low voltage transformers have to be designed and built to a high specification, which involves the use of quality heat-resistant materials. Class H insulation materials ensure long service life, which, according to EN60742, allows a maximum winding temperature rise of 140°C at 6% over voltage.

At an actual winding temperature of 130°C, 10 years life can be expected. The graph above shows the theoretical service life of a transformer against winding temperature. Every 10°C over the winding temperature of 130°C, halves the transformer life.

The manufacturing technique used by Hella is to vacuum impregnate the transformer with an unsaturated polyester resin. Typical characteristics are thermal class ‘H’ (180°C IEC600). Vacuum impregnation improves quality and longevity because the resin is drawn into the heart of the core and coil. This maximises heat transfer and also ensures silent operation.

Safety Extra Low Voltage (SELV) Lighting Recommendations

Voltage Regulation
The secondary voltage of a transformer changes with the load, therefore, for multi-lamp applications, a transformer with good voltage regulation must be used. This is because unloading of the transformer occurs as lamps burn out, eg: there may be 4 x 50W lamps wired to a 200VA transformer and if one or two lamps fail, the transformer regulation may not be able to compensate for the decrease in load. This will subject the remaining lamps to an over supply of secondary voltage. Therefore, it is recommended that failed lamps always be replaced.

On a one lamp/one transformer installation, the regulation of the transformer is not important as long as the lamp wattage matches the full load of the transformer. These small transformers generally have higher impedance and regulation characteristics.It follows that the higher impedance limits the in-rush current on a cold lamp.

The lower in-rush current (or “soft start”) helps lamp life. Filament evaporation eventually creates a weak spot on the filament. This weak spot is effected by in-rush current. As a result, the one lamp/one transformer system is beneficial for ferro-magnetic transformers, but does not apply for correctly rated electronic transformers.

High current on the secondary side
Another important consideration in Extra Low Voltage systems is the impact of high currents. Circuits must be designed and cables sized to carry the higher currents. A 12V lamp can draw 20 times more current than a 240V lamp of the same wattage.

For maximum safety, optimum voltage at the lamp and consequently lamp performance, it is preferable to use one (1) ferro-magnetic transformer per lamp.

On multi-lamp systems, each lamp should be cabled back to the transformer independently, particularly as the relatively smaller cables used would be more practical and easier to terminate at both the light fitting and the transformer. In these installations it is necessary to estimate voltage drop to each lamp and use the appropriate sized cable according to each cable run. If each light fitting in a multi-lamp system individually wired back to the transformer, then the possibility of using cable lugs to a common stud connection should be considered.

Safety Extra Low Voltage (SELV) Lighting Recommendations (continued)

Protection and Approval

It is mandatory under the wiring rules that transformers for remote mounting are housed in a suitable enclosure and, as a prescribed item, must be approved by the Statutory Authority to EN 60742.

The approval number must be clearly marked on the enclosure. Also, under the wiring rules, the transformer must be installed in a position where it is easily accessible for inspection and maintenance.

The transformer enclosure and wiring to the light fitting must have adequate ventilation. If the wiring and the transformer enclosure are covered with insulation material then substantial derating and overheating may occur.

Electronic Safety Isolating Transformers for Low Voltage Lamps

Immunity
A transient mains voltage surge may occur when inductive loads (e.g. motors or conventional chokes) are switched off. TE electronic transformers are protected against voltage surges and spurious pulses of any polarity by random phase position superimposed on the supply voltage, in accordance with EN 61547.

Emission
The limits for emission as per EN 61000-3-2 are maintained by high-quality filter circuits.

Electromagnetic Compatibility
Harmonics from non-linear loads cause distortions in the mains supply. The special circuit design fulfils the requirements of EN 61000-3-2.

RFI-Protection
The sophisticated filtering system of electronic transformers TE ensures compliance with EN 55015 and VDE 0875-151

Harmonics
The requirements of the EMS Directive 89/336/EEC are fulfilled by observance of the standards for emission and immunity.

Disposal of Old Lamps and Control Gear

Hella products comply with the RoHS regulations passed by the EC on electronic waste

Disposal of Incandescent Lamps
Incandescent lamps consist of glass and metal. They do not contain any materials that will harm the environment so they can be simply thrown away with the household refuse. They should not be placed in containers for recycled glass, however, because the glass used for these lamps is not the same as the glass used for bottles.

Tungsten-Halogen lamps contain very small quantities of halogens and Halogen-hydrogen compounds, but the amounts are insignificant (only a few millionths of a gram). Even several lamps together do not present any risk to people or the environment. The lamps can therefore be thrown away with household waste.

Disposal of Discharge Lamps
Small quantities of mercury can be found in most HID fluorescent and compact fluorescent lamps. These lamps also contain small quantities of recyclable phosphors so they should not be placed in household refuse or recycled glass containers but sent for proper recycling at community recycling centres. Low-pressure sodium lamps and sodium Xenon lamps can be disposed of without the need for any special action.

Disposal of Control Gear
Hella ECGs, ballasts, ignitors, capacitors and other electronic units do not contain any material that will harm the environment but they do contain recyclable electronic components so they should be disposed of as electronic waste.

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