Last Update 5/03/2007
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TECA S.p.a.
TECA S.p.a.

Via 4 Piccoli Martiri, 3
25040 Cividate Camuno
(BS) Italy
P.Iva 00691310981

TEL. ++39 (0)364 342001
FAX ++39 (0)364 344676
Email: info@teca.net


 

 



Civil automation requires small, compact transformers which can be set into small spaces, able to support continuous supply with high starting powers and overloads for limited periods of time.TECA’s toroidal transformers which have been developed specifically for this sector, thanks to a careful calculation and application of the duty cycle, are ideal for devices which require high initial couple values, for example, engines with continuous supply.

The duty cycle is the ratio (expressed as a percentage or in seconds) between the time in which the transformer functions with a maximum load and the time in which it is switched off or when it supplies limited power, as shown in the graph below.


Standard
Standard transformers
Boxed
"Boxed" transformers
Carenati
"Carenati"transformers



The duty cycle expressed as a percentage is given by the following ratio:



The duty cycle expressed in seconds can also be described indicating the times ON or OFF:

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 later:
“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 control.
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 fully respected.

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 (blue curve).
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.



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