Designing inherently short-circuit-proof,
Important conditions and parameters are isolated for the calculation of inherently short-circuit proof transformers in accordance with IEC61558. In addition, the complete winding data 9, for inherently short-circuit proof transformers are provided.
Normally the primary coil is placed in the larger section of a double-section bobbin. This technical innovation is the transfer of the primary coil into the small chamber, with which the core power of the cores EI30/5 and EI30/10.5 can be increased by 15%.
The inherently short-circuit proof transformers above 5VA are normally implemented with the magnetic shunt. An alternative is to apply the "long" EI cores with a double-section bobbin and keeping the operation induction lower than 1T.
Thirty years ago, designers performed the calculations for
transformers on their pocket calculators. Designers had to pencil all the input
and output fields into a form and then feed them into the calculator. Today,
they can forget the pencil, but they need to enter the figures into spread-sheet
programs such as Excel and Lotus 123.
Once the first affordable 8-bit computer became available in 1978, professionals could begin to develop programs for designing transformers and inductors. This development work moved in two directions:
First, companies developed their own computer programs to meet their own specific requirements. These usually used algorithms and experience that were already available. Once an acceptable level had been reached to meet the company’s needs, both in terms of technical capability and ease of use, further development ceased.
Secondly, small companies began to develop professional computer programs which were sold or leased to the manufacturers of transformers and inductors.
With the aid of continuous input from the various manufacturers, they were able to develop universal, powerful, easy-to-use tools for use throughout the industry.
Designing with the Rale Design System
The Rale Design system automatically calculates designs for transformers and inductors. Consequently, its database incorporates all the necessary materials including cores, bobbins, wires, steels, etc. in both metric and USA units. This database is totally user-expandable. To use the programs, Designers need only a basic knowledge of transformers or inductors and their operation mode. Designers do not need to use any complicated formulas; they only need to follow two simple phases:
The user normally loads a template-input file from the library and only needs to fill in the input mask with the global parameters (voltage, current, temperature rise, regulation, etc.) and run the program.
Once the program has finished the design work, the user can switch to Test Mode and make manual changes to the parameters of the designed transformer (turns, wire sizes, steel, ...), and run the program in order to redesign it. During this stage the user can also test his design, changing the input voltage, frequency, load, duty cycle, etc.
Design criteria of an inherently short-circuit-proof transformer
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 paragraphs 14.2, 15.1 and 15.3:
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 qnom
and the short-circuit temperature is just under qcc.
During the power Pisc both the rating temperature and the
short-circuit temperature lay marginally below qnom
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 order to have good thermal contact with the print (Fig. 1), the transformer needs to be "fully" potted so that the air gap between the compound and the print is maximum 0.2 –0.5 mm.
During construction (Fig. 3) only the coil under pressure is sprayed with polyester. Production costs for this construction are lower than in the case of a potted housing. It must be mentioned, however, that from a thermal standpoint, it is less effective, and is not suitable for use with very thin wires and the ready core.
Thermal resistance of the potting compound
The potting compound which is best from a thermal viewpoint, but is also the most expensive, has a specific thermal conductivity of 0.8W/m/°K. In practice, we usually operate with a potting compound whose thermal conductivity is 0.40-0.55/W/m/°K.
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.
Normally the double-section bobbin has two unequal chambers. The primary coil is wound exclusively into the larger chamber, so that you can use thicker or cheaper wire. Below the output power of 2VA, you should wind the primary coil into the smaller chamber(!), which boosts the core power of the core to 15%. Above the power output of 2VA, the chamber distribution should be equal.
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.
Procedure for design
The inherently short-circuit-proof is made by using only laminations EI30, EI38, EI38-long and EI48-long. The decision regarding the choice of core sizes is made mainly in relation to output power, ambient temperature, and the insulation class. Thanks to this relatively small number of variants, for 10 output powers in insulation class B and ambient temperatures of 40 ° C , it was possible to record approximately 20 input files to serve as an aid to making entries into the Rale Design Systems library.
The calculation example shown below explains the calculation procedure in brief.
Technical specification relevant only to design
Environment and operating conditions:
Safety transformer as per IEC 61558
The above-stated parameters are stored mainly in the ISCP_075VA_6V_T40.TK1 input file.
This is followed by a check of the calculated data at the input voltage 230V.
In test mode the transformer can be checked in the same way as on a test rig, and if necessary be altered manually.
In Fig 8 the transformer designed above is tested in nominal operation at the input voltage 243V (U input = 1.06).
This is followed by a check of the calculated data.
Technical specification common for all designs
All non-short-circuit proof transformers in the table below were calculated under the same conditions:
Input voltage : 230Vac +6%,-10%
Steel : M800-50 ( 8.0W/kg @1.5T and 50Hz or better
Ambient temperature : 40°C
The parameters of the designs are core size, output power, max. regulation in the nominal operation or the temperature (max. 120°K) in the nominal operating mode and the temperature in the short-circuit operating mode (max 175°K).
The calculated output voltage and calculated output current are around 8% less than their defined nominal values. In the case of the application of the potted compound under 0.55W/m/°K,the short-circuit temperature can exceed the admissible temperature.In this case the ambient temperature should be set lower than 40°C.