Designing a simple 50 kVA, 50Hz three phase dry transformer with cooling channels?

General Information

Technical Specification

Creating Input

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

• Winding parameters per limb
• Core
• Environment
• Diverse

end 3 screens for selection and set up of material:

• wires
• steels
• cores.

Winding

In this power range you normally use a 3 phase EI core with grain oriented steel M111 (M6).

Due to the relative small window width of this core type there is no enough space for more than one cooling channel (15mm) between the primary and the secondary. In order to increase the cooling surface the primary and the secondary are created with 2 one-sector-windings with the cooling channels (15mm) between them located only outside of the core window.

Around the core leg is the tube (3mm). There are cooling channels (15mm) between the first primary winding and the tube located only outside of the core window. The primary and secondary windings have the same distance (15mm) to the yokes.

The transformer works in continuous operating mode on the sine wave input voltage and the sine wave output current.

The program has to use Cu flat wires with the layer insulation of 0.1mm (100 micron). In a case that where the program needs to use 2 or more parallel connected wires the program does not need to use the transposition.

The factor of the eddy current losses (RacRdc = Pcu_total/Pcu_dc) must not exeed 1.05.

Note that at this point in the design you can not prescribe the wire size. You can prescribe only the wire family which the program has to use in order to select the applicable wires for your application.

Core

On this input screen you can:

• select and manipulate a steel (M111<=>M6)
• set the operating induction (1.55T)and the frequency (50Hz)
• select the core assembly (EI)
• and choose the core selection.

Using the ollowing screen for the selection of the core family the program will pick up the first sufficiently large core for your application out of the selected core family LTCM_EI_3_L_1.USR Note that you can also select or create a new core within this screen.

Normally for this application use and M11, 0.35mm (M6, 14mil) grain oriented, not annealed after stamping, 3 phase EI lamination.

Environment

The cooling medium is air with an ambient temperature of 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

Diverse

The selected criterion of the design is the temperature rise of 100°K for insulation class F.

The oval space between the first winding and the tube (stomach), all gaps between the insulations & windings, and the varnish fill factor play a very important roll from the thermal point of view.

Output

The first step of the design is the selection of the core:

and the next step is the presentation of the output screen DIAGNOSIS: it is the summery of the most important calculated parameters of your transformer.

The following 3 screens will not be printed. 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.

Finally, here are 4 printed pages with all the results of the design

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 manually:

• Turns
• Wire size
• Material (Cu or Al)
• Number of 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 could not create full layer windings at the prescribed temperature rise of 100°K. In order to get the full layer windings you have to select the wire size and number in parallel connected wires manually.

The following Test Mode input shows the operation mode with:

• the input voltage on the overvoltage of 10%
• the decreased load resistance for 50%

• Setting the load resistance to 0 you can simulate the short circuit operation of the winding.
• Setting the load resistance to >1000 you can simulate the "no-load" operation of the winding.

• the duty cycle operation with 5 minutes on and 60 off