In this more precise indication the work cycles and the powers
involved in individual periods must be shown, as prescribed by
the Norm EN61558-1 8.1, point p:
“transformers for temporary or intermittent supply must be
labelled with the nominal service time or the nominal service time
and the nominal rest time, unless the service time is limited by
the construction of the transformer or corresponds to the working
conditions specified in the corresponding second part”, and
“The indication of temporary function or intermittent function
must correspond to that in normal service. The indication of intermittent
function must be such that the nominal service time precedes the
nominal rest time, and both the indications must be separated by
an oblique stroke”.
It is easy to understand how important it is to define the duty
cycle correctly, in fact if the transformer were used continuously
(duty cycle = 100%), the dimensions of the core and the cross-section
of the copper and the materials used would have to support the
maximum power foreseen for an indefinite period. In the case of
non-continual use, however, (duty cycle = <100%) as in the supply
to an engine with continuous supply, for example an electric gate,
a high initial couple is required (to overcome the initial inertia),
that is a high current with a stable supply voltage in the first
few seconds, after which the power will be limited to the time
necessary for the complete opening or closing of the device, and
at the end will be on standby for a certain period. Thus the toroidal
transformer will be significantly overloaded in the initial phase,
with a tendency to overheat considerably. In the following phases
the transformer has to supply reduced power, and in this period
the temperature is reduced, keeping the maximum temperature under
Another fundamental aspect to take into consideration is the case
of a transformer with several secondary coils, where in addition
to the exits used intermittently there are other secondary coils
for electronic use or auxiliary checks with continuous use. In
this case the single powers and relative usage times must be evaluated
for the transformer.
Thanks to the correct application of the aspects described above,
TECA’s toroidal transformers for civil automation can be
overloaded significantly compared to the nominal power, with a
consequent reduction in the dimensions of the core and in the section
of the coils. This means that compact transformers can be produced
that are easily installed in very small spaces and have a very
low cost, thanks to the use of a smaller quantity of material.
The reduction in size and in material does not, of course, compromise
efficiency or safety in that the requirements of EN61558 are always
The charge curve
is another important parameter in the calculation
and sizing of a transformer for civil automation since, as we have
said, in the initial phase an engine in continuous supply must
furnish a considerable initial couple value with a stable voltage
at its ends even in the presence of absorption at very high currents.
In the diagram we can see the charge curve for a 100VA transformer
(dimensions: diameter 60mm, height 55mm, weight 650g, duty cycle
60%, secondary voltage 12V). Despite the reduced dimensions the
voltage at the ends of the secondary coil are more than sufficient
to guarantee a high initial couple value even with high currents
The values for the lowering of the secondary coil voltage in these
conditions are verified with the requirements of functionality
and safety imposed by the technical norms, that is, when we use
the term functionality we mean that the performance of the product
can always be guaranteed even in unfavourable conditions of overload.
In any case, the percentage difference between the load voltage
and the nominal voltage must not exceed 5% while the percentage
difference between the minimum voltage and the maximum load must
not exceed 15% (the limits for the nominal voltage/load are shown
by the red curves
while the limits of minimum voltage/load are
shown by the blue curve
Load curve of a 100VA transformer (about 650g) used non-continuously
(duty cycle 60%).
The current at 100VA is 8.44A, while at 50VA
the current is 4.22A.
Safety is guaranteed because the insulation of the coils is not
damaged in any way; this would compromise the dielectric safety,
also in the more critical situations of function (for example
overloads or overheating).
When TECA designs transformers for civil automation, it takes
all these factors into account and establishes from time to time
the best compromise between performance and dimensions. Correct
design is tested with production and checks, using suitable laboratory
tests on various samples.
Using preliminary electrical and thermic tests, the following
points are checked: the sections of the coils, the strength of
the soldering, the number of coils, the presence of hidden short
circuits, load curves, heating and overheating of the various
coils and of the supports. In conclusion the final definitive
test is that which pertains to the type of transformer. That
is, all the tests and checks listed in the reference norms: resistance
to cold and heat, test with zero load and with load, efficiency,
heating, overload etc.
This allows TECA to produce transformers which conform perfectly
to the client’s requirements as regards the size, shape,
insulation and the norm’s requirements.
TECA offers a complete service which not only helps and proposes
technical solutions for its clients, but also supplies accessories
and personalised cables; the latter are produced in an internal
department with computerised machinery and automatic apparatus
(for further information see the section “cables” in
the general catalogue.