How should one design a three-phaserectifier/safety transformer for 24Vdc, 80Adc supplywith RC load,
 as per IEC 38 and IEC 61558 ?

Technical specification relevant only to design

Electrical data and diagram

Ambient and operating conditions:

Specification

• Safety transformer as per IEC 61558
• Insulation class E

Design criteria

IEC 61558
A transformer with non-inherently short-circuit protection as per IEC 61558 is equipped with a safety. Normally, we try to provide effective protection both for the transformer and for the rectifier. Very often, we arrive at a combined protection solution consisting of three fuses matched to the rectifier and a thermal cutout. For these reasons, short-circuit and overload are not design criteria. The design criterion for purposes of IEC61558 is only temperature q nominal

Insulation class
Max. winding temperature in nominal operating mode = 115°C

Max. winding temperature in test mode = 215°C

Insulation class E is prescribed.

IEC 38
The above-prescribed limit values for voltages and for the ripple of a 24V dc supply are laid down by IEC 38. A rectifier/transformer with RC load, for which the minimum and maximum output voltages are prescribed, is designed in accordance with the regulation criterion (Criterion = 1).

Note 0:
Rectifier/transformers with RC load can be designed ONLY in accordance with the regulation criterion. For that reason, we have to select a regulation solution such that the prescribed limit values for output voltage and temperature of the windings do not exceed q nominal.

Regulation (voltage increase)
For purposes of designing the transformer in accordance with the regulation criterion (voltage increase), we have to input the value for the increase in the secondary voltages. This is calculated as follows, on the basis of the DC voltage information:
The transformer is calculated for an input voltage of 207Vac. In this context, the output voltage under load - in the hot state - must not fall short of 20.4Vdc. The maximum no-load voltage must not exceed the level of 28.8 at 243.8Vac input voltage.
At the input voltage of 207Vac, the no-load voltage must be below the values of 28.8*207/243.8=24.59Vdc. This corresponds to a DC voltage increase of 100*(24.5-20.4)/20.4 = <20%. The secondary voltage increase should be <10% in accordance with the following table.

 Note 1:
The first design calculation was performed with 10% regulation. At that level, the temperature of the windings was too high in operation at 243.8V. Only at 3.5% regulation who were satisfactory results obtained.

Output voltage ripple

Ripple= 100*(Udcmax-Udcmin)/(Udcmax+Udcmin)

The program calculates the magnitude of the required capacity on the part of the smoothing condensers for the prescribed DC ripple.
5% ripple is prescribed in accordance with IEC 38

Bobbin unit
Our calculation was performed with a single-chamber bobbin unit. With a double-chamber bobbin unit, the transformer is somewhat larger. For this purpose, for however, we can use smoothing condensers of three to five times smaller capacity, and thus achieve the same ripple. In order to find the optimum solution, we have to perform one calculation with a single-chamber bobbin unit and one calculation with a double-chamber bobbin unit.

Induction and Fe quality
The choice of induction is selected at a voltage below the input voltage of minus 10% between 1.2 and 1.4.
The rectifier/transformer is very often installed together with the rectifier in a case or in a cabinet. For that reason, the loss output of the transformer should be sized low and the grain-related Fe quality should be used: M6X, ORSI 111,…

Connection of transformer windings
The primary winding is connected in a star circuit. The secondary is used in "Delta", so that the current through the winding is less and the application of parallel-connected flat wires can be avoided.

Procedure for design

1. If you are not yet acquainted with Rale design software, please read the text "How should I design a small transformer?". Keep a copy of this text within convenient reach whenever performing design work.
2. Fill in the design input mask as follows. If you need any help, press function key F1. There is extensive description for each input field.

• The output voltage of 21Vdc at 80Adc is ensured at the under-voltage for the 207V input voltage. Temperature is designed simultaneous with the over-value of 243.8V for input voltage of 207V.

• Regulation = 3-5% <10% was selected such that the maximum temperature of the windings does not exceed the value of q nominal and the output voltage is within the limits prescribed by IEC 38.
• Rac/Rdc = 1.25 was increased such that the program uses no parallel-connected flat wires.
• Winding space = 0.8 should be used for automatic selection of the core (Selection=0) with thick wires.
• The Selection input field is set at 0. This means that the program should search on-line for a suitable core for this application, from your selected triple-phase core family.

3. Save your input data file. In this specimen design calculation, we saved the input data in input data file CAL0006E.TK1. This input data file was supplied together with this document. Copy it into the directory in which your Rale demo program is installed.
4. Connect up to the Rale design server.
5. Load up your input data file.
6. Now select the three-phase core family from which a suitable core is to be searched for your application. Ensure that the marked core is AUTO.

7. Click on OK.
8. Start your design work. In the system for automatic selection of the core from your prescribed three-phase core family, the program will offer you an adequately sized core for your application. Click on OK in order to accept the core.

On completion of the design work, the following design data will be available and can be printed on three pages:

9. This is followed by checking of the design data.

• Firstly, we check the DC output voltages in no-load mode for input voltage of 244.3Vac: 27.1Vdc < 28.8Vdc
• We then check the winding data and the filling factor (82.0<100%).
• The maximum temperature of the windings is 40°C+66.9°K = 106.9°C < 115°C.

This is followed by checking of the output voltage for an input voltage of 207V:
U in = 1.

 On the third page of the winding datasheet we should check the output voltages for an input voltage of 207V: 21Vdc > 20.4Vdc.

10. If the design data is not satisfactory, then there are two ways by which we can implement the desired correction:

• You can return to the input mask (function key F2), correct the input data and redesign the transformer.
• Or you can access the test program (function key F5), modify the designed transformer manually and redesign the transformer by that means.

11. On completion of the design work, you can print out the design data on-line, or save it on your local PC and print it out off-line. The output data file from this design example, CAL0006E.TK2, is supplied together with this document. Copy it into the directory in which your Rale demo program is installed.

Tips & Tricks

Temperature in nominal operating mode is too high

• Reduce your regulation and increase your induction.
• Employ a better iron quality
• Increase your cooling surface area. In the case of 3UI cores, the base angles are very effective for this purpose. You can reduce the windings' temperature rise by approximately 10%-15%.

Smoothing condenser

For the prescribed DC ripple of 5%, the program has calculated the value of 33000µF+-20%.

For the same output voltage ripple, a 2-chamber transformer requires only 6700µF!

Regulation

We selected regulation = 3.5%, so that the overtemperature would not be too high. In the case of a double-chamber transformer which has a higher scatter inductance, a higher degree of regulation is permissible. In our case, regulation = 5% is perfectly suitable.