Designing a 12Vdc, 30kAdc Rectifier Transformer

General Information

Technical Specification

Creating Input

4 input screens are used to set the input parameters for the designing of a transformer:

• Winding parameters per limb
• Core
• Environment
• Other parameters

and 3 screens for selection and set up of material:

• wires
• steels
• cores.

Windings parameters per limb

The following rectifier circuit is often used for low voltage&high current output. For a good current distribution between 2 parallel connected rectifiers (with the drainage choke) the relationship Ucc_s1-s2/Ucc has to be bigger than 2; Ucc_s1-s2 is the short-circuit voltage between the secondary 1 and the secondary 2; Ucc is the short-circuit voltage of the transformer. For this condition the primary will be "sandwiched" between both secondary. The core cross section and the induction have to be set so that each secondary has only one turn. The form of the legs cross section have to be "round".

Note that the short-circuit voltage of a rectifier transformer is a complex issue reflecting:

• the rectifier protection in a short circuit operation mode of all secondary winding, a group of windings or of only one winding.
• the commutation operation mode of a group of windings
• the voltage drop of the dc-output voltage
• the current distribution between the parallel connected rectifiers

It has to be prescribed by the user of the transformer.

Primary

The primary is created in star connection. The sine wave input voltage (UA,UB,UC)is 230V (230V per winding).

There is no duty cycle operation mode.

The primary will be manufactured with Cu-foil with a layer insulation of 0.100mm. Note that there no big difference from an electrical or magnetic point of view (if the distance between the sectors is small) between the winding made by foil with one sector and the winding made by foil with more (2-8) parallel connected sectors. The first and the last sector will be overloaded by a higher eddy & circulated current losses and due to the thermal insulation to the other sectors they will normally be hotter.

The primary lies between the secondary windings and the core. In order to avoid using very large foil with it is created with 2 in series connected sectors All the surfaces of the primary are cooled via the cooling channels of 20mm . The space between the yoke and the primary windings is 20mm. With the eddy current losses factor (RacRdc) 1.15 shall be limited the number of the parallel connected foils per sector.

Secondary

The both secondary windings are created with 2 in series connected ONE ROUND TURN, BAR WOUND SECTORS.

The sine wave output voltage per sector is 10.9V.

The rms current through each sector (secondary) is 8774Arms. The set current harmonics are calculated for the worst case: Ucc= 0 and Ld = ∞:

Also, there is no duty cycle operation mode on the secondary.

With the eddy current losses factor (RacRdc) 1.1 and 1.25 the use of parallel connected bars per sector shall be avoided . Note that at this point of the design you cannot prescribe the wire or foil (bar) size. You can select only the wire or family or foil (bar) which the program has to use in order to select the suitable wires or foils (bar) for your application.

The secondary winding has only 20mm cooling channels.

The space between the yoke and the secondary windings is 20mm

Core

On this input screen you can:

• select and manipulate the selected steel M111, 035mm (M6, 14mil)
• set the operating induction (1.55T) and the frequency (50Hz)
• select the core assembly
• and prescribe the core selection.

The "round" core cross section was prescribed by the designer for easier winding of the high current foil (bar) windings: The value of the cross section and the induction were set in order to get only one turn per sector.

The window height was optimized for the low eddy current losses with a Cu-bar thickness between 5mm and 6mm. Normally you use for this application M111, 0.35mm (M6, 14mil), not annealed after stamping, grain oriented strips.

Environment

The cooling medium is air with the ambient temperature 40°C. The cooling surface of the core is increased by using 4 L-brackets on the core. The impregnation is practically "dry" because there is only 10% varnish (90% air) in the windings and in all the gaps between the insulations and the layers of the windings.

Other...

The selected criterion of the design is the temperature rise of 120°K for insulation class H. The oval space between the first winding and the tube (stomach), all gaps between the insulation, the windings and the varnish fill factor of them, play a very important roll from the thermal point of view.

Output

The first step is the presentation of the output screen DIAGNOSIS: it is the summary of the most important calculated parameters of your transformer.

Note that the program uses the numerical calculation of the magnetic fields and the temperature rises. Due to this technology the calculations of the eddy current losses, the steel losses, the short-circuit voltage, the circulating current and the transposition are very powerful.

The following picture shows the magnetic field outside the core window. The ampere-turns of 1., 5., 7... current harmonics in the primary and in the secondary are compensated. They produce axial leakage magnetic field. The ampere-turns of the 0.(dc-current), 2., 4. ,... current harmonics do not exist in the primary. They exist only in the sectors of the secondary, are compensated too and produce radial leakage magnetic field.

Finally here are 4 printed pages showing the design results

Input

Core

Windings

Nominal operating mode

Test Mode

If you are not satisfied with the solution made by the program you can switch into the Test Mode and change your transformer by hand:

• Turns
• Wire size
• Material (Cu or Al)
• Number parallel connected wires and their order in strand
• Cooling channels and insulations
• Margin
• Steel
• Technology parameter (impregnation, gaps,...)

and then you can set it under an operation mode changing:

• Input voltage
• Frequency
• Loads and their K-factors
• Duty cycle of each winding
• Ambient temperature
• Air flow

Note that the program will calculate (not select from a data base) the thickness of the foil (bar) for the prescribed temperature rise of 120°K. In order to get an available foil (bar) you have to set the thickness of the foil by hand.

NOTE

If you would like to modify this transformer in order to use it for 12Vdc, 15kAdc then you need only to change the foil&bar width (200mm instead 400mm) and reduce the height of the core window for 400mm.