How should one design a low inrush current
three phase universal autotransformer
for 10kVA continuous output power as per IEC 61558 ?

Technical specification relevant only to design

Electrical data and diagram

Ambient and operating conditions:

Specification

• Autotransformer as per IEC 61558
• Insulation class E

Design criteria

IEC 61558
An autotransformer with non-inherently short-circuit protection as per IEC 61558 is equipped with a internal protection. Very often, we arrive at a combined protection solution consisting of a primary-side fuse (short circuit protection) and a thermal cut-out (overload protection). For this reason, short-circuit and overload is not design criteria. The criterion for design for purposes of IEC 61558 is only temperature q nominal at the overvoltage of 6%.

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

The criterion for design
The autotransformer has to be designed for the temperature rise <75°K at 40°C ambient temperature and insulation class E.
The criteria of design is the temperature rise: Criterion=2

Bobbin unit
An autotransformer is constructed exclusively with single-chamber bobbin units.

Induction and Fe-quality
Low Inrush-current transformers are operated at a low induction (1.2T-1.5T). In the output range up to approx. 200VA, cold-rolled Fe quality 5.3W/kg is used at 1.5T, 50Hz. Over and above that, we find cold-rolled Fe-quality 4.00W/kg at 1.5T, 50Hz and grain-oriented Fe-quality 1.11W/kg at 1.5T, 50Hz.

Inrush current
The peak value of the inrush current in the first period after switch-on at the zero-axis crossing point should not exceed a factor of 15 x the peak value for nominal input current. This can best be achieved if:

• The nominal induction is 1.2T-1.5T.
• The core is constructed with a defined gap of approx. 0.05-0.10 mm, so as to reduce residual magnetism (remanence).

Design procedure

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 functions key F1. There is extensive description for each input field.
   
   IF YOU WANT TO CREATE OFF-LINE AN INPUT FILE FOR DESIGN OF AN AUTOTRANSFORMER YOU CAN'T DELETE THE FIRST SECONDARY WITH YOUR OLD DEMO PROGRAM. YOU HAVE TO DO IT ON-LINE! IN ORDER TO DO IT, PLACE THE CURSOR ON ANY POSITION OF THE FIRST SECONDARY WINDING AND PRESS Ctrl+D .

3. 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 core family. The primary Circuit is set to 4 for the star connection of the windings. The Overvoltage = 1.06 means that the prescribed temperature rise of 75°K must not be exceeded at the 6% overvoltage of the primary voltage. Note that the induction is set lower in order to limit the inrush current. For this reason the core is constructed with a defined gap of 0.10 mm (Gap = 4 x 0.025mm= 0.1mm with Assembly = 2), so as to reduce residual magnetism (Remanence= 0.1 x 1.35T = .135T).

4. Save your input data file. In this specimen design calculation, we saved the input data in input data file CAL0012E.TK1. This input data file was supplied together with this document. Copy it into the directory in which your Rale demo program is installed.
5. Connect up to the Rale design server.
6. Load up your input data file.
7. Now select the three-phase core family and (optional) the core from which a suitable core is to be searched by the computer program.
   
   
   
8. Click on OK.
9. Start your design work. In the system for automatic selection of the core from your prescribed 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 the three pages:

10. Checking of the design data follows this.

• We now check the winding data and the filling factor (76<100%).
• The maximum temperature of the windings is 40°C+66°K = 106°C < 115°C.
• The inrush current factor is slightly less than 15 x: 14.4<15.
• The primary windings' wire size calculated by the program is not optimized for production. In the test program, only three-wire thickness should be employed: 2.65mm to 115V, 2.50mm between 115V and 230'V and 1.90mm between 230 and 400V.

This is followed by checking of the output voltage at the nominal input voltage of 230V: Uin = 1.

11. It is recommended at this position that you change the input voltage tap (Tap = 1…8) and test temperature rise and the output voltage. Note that the program starts designing by the tap with max. core power and selects the wires for the max. power losses in order to control prescribed temperature rise. The value of the output voltage can be designed as prescribed only for ONE input voltage.
12. 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.

Tips&Tricks

Three phase Autotransformer and non-symmetrical load

If have designed an autotransformer for the operation in accordance with the following figure A and your autotransformer works under the operation described in the figure B the use inherently a thermal protection. But if you don't like to use a thermal protection, then the following table for the declaration of the allowed non-symmetrical load of your autotransformer.

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