Designing inherently short-circuit-proof, potted safety transformers with IEC 61558
A transformer which is inherently short-circuit-proof as per IEC 61558 is not equipped with
any protection. The procedure for designing and testing these transformers is set out in the chapter 14.2, 15.1 and 15.3 on the standard.
1. First the transformer is connected to the rated supply voltage and loaded with an impedance which would give rated output current at rated output voltage, and then the supply voltage is increased by 6%. Following this increase in voltage, no change is made in the circuit until the permanent-operating temperature is reached. In this context, the temperature of the windings must not exceed the value of θnom (see the table below).
2. Immediately after this test all secondary windings are short-circuited and operated at 1.06 times the input voltage until the permanent operating temperature is reached. In this context, it is important that the temperature of the windings should not exceed the values θcc laid down in the following table.
Normally short-circuit operation governs the design of an inherently short-circuit proof
transformer. The prescribed temperature is realised in the short-circuit operation by the
delimitation of the short-circuit current with a very high short-circuit voltage from 25 to 50%.
Additionally, the cooling of these transformers is increased by potting with a thermally effective compound into the case. It should also be mentioned that all these transformers (for safety reasons with reference to voltage strength), must be potted.
At the lower end of the output performance up to approx. 5VA (Pisc), the short-circuit current is limited exclusively by the ohm resistance of the windings. The temperature of these transformers during nominal operation is under the max. temperature of the insulation class θnom and the short-circuit temperature is just under θcc. During the power Pisc both the rating temperature and the short-circuit temperature lay marginally below θnom & θcc. Above the power Pisc, the inductive resistance of the windings must be activated for assistance.
In these output ranges, the bobbins, the case and the potting compound are employed
exclusively together with insulation class E and B. Wire insulation and insulation foils are very often employed in insulation class F.
Normally, the transformer is operated in an environment where the temperature is between
40°C and 70°C.
Case and Chassis
For safety reasons, inherently short-circuit proof safety transformers are almost exclusively
potted in a vacuum within a case (Fig.1) and are intended exclusively for the printed board.
In this output power range, recourse is made almost exclusively to a double-section bobbin.
From the viewpoint of design, only the dimensions of the bobbin are important. A bobbin with increased insulation or large leakage paths has a smaller winding space and a smaller cooling surface area.
In nominal operating mode, at full core losses, the temperature rise of the transformers is
approx. 30°K to 50°K. The relationship between the copper and iron losses is normally
between 5 and 10. In short-circuit mode, in which the magnitude of the temperature rise is
extremely relevant, iron losses are practically negligible. For that reason, the optimal steel is the cheapest cold-rolled steel 8.0 W/kg (@ 1.5T and 50Hz). And furthermore, the cheapest cold-rolled steel has the highest saturation induction!
Up to the power Pisc, the optimal regulation of an inherently short-circuit proof transformer
amounts to 100%. This rule applies to the transformer with which the no-load operation
induction is situated within the linear area of the magnetizing curve. Below the power output
of 2-3VA the no-load operation induction is situated in nonlinear area of the magnetizing curve between 1.6 and 1.7T and the actual regulation amounts to less than 100%.
For an inherently short-circuit proof transformer induction selection plays an important role
and has a very difficult function, this is taken over automatically by the program. It depends
particularly on the core construction, output power, the insulation class and the ambient
temperature, and is normally situated between 0.5T and 1.4T.
Permitted tolerance of output voltage and output current
The output voltage of a transformer, which must be inherently short-circuit proof, is tested in
the hot and cold state with the nominal primary voltage and the nominal load resistance. In this context, it must not deviate by more than +-10% from the nominal value.