Download - Industrial and Bearing Supplies

136 Transformers
TECHNICAL
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 multilamp
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.