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How do we design a 1.5VA inherently short-circuit proof safety transformer as per IEC 61558?

Technical specification applicable only to design

Electrical data and diagram

Environment and operating conditions:

Specification

• Safety transformer as per IEC 61558
• Essential to be inherently short-circuit-proof
• Insulation class E
• Potted in case
• Core and bobbin unit EI 30/10.5

Design criteria

IEC 61558

A transformer which has to be inherently short-circuit-proof as per IEC 61558 is constructed without protection. The procedure for testing is prescribed as follows in paragraphs 14.2, 15.1 and 15.3:After testing, as laid down in paragraph 14.2, with nominal load resistances and 6% over-voltage, all secondary windings are short-circuited and operated at 1.06 times the input voltage until permanent operating temperature is reached. In this context, it is important that the temperature of the windings and of the cases should not exceed the values laid down in the following table.

Insulation class

In this performance range, the bobbin, case and potting compound are used almost exclusively with insulation class E and B. Wire insulation and insulation foils from insulation class F are very often used.

• Let’s choose insulation class E.
• Max. winding temperature in nominal operating mode = 115°C
• Max.winding temperature in short-circuit = 165° C
• Max.case temperature = 105° C

Important note: the program will design the mean case temperature!

Ambient temperature

Normally, the transformer is operated in an environment whose temperature is between 25° C and 80° C. Very often, an ambient temperature of 40° C and 70° C will be encountered.

Case

For thermal reasons, transformers which absolutely have to be short-circuit-proof, and whose design performance is higher than 1.5VA, 50Hz are potted in a case either in vacuum or without vacuum. For potting without vacuum, the windings will remain "dry", which exerts practically no influence on the thermal characteristics at this transformer design size.

The main advantages of a potted transformer are as follows:

• The transformer is cooled better.
• You can use a bobbin without large wall thicknesses, without large leakage paths and also with high winding space.
• On large production runs, there is no problem in using the core insert with regard to mechanical robustness and humming of the core plates.

Let’s design a transformer potted without vacuum.

Thermal resistance of potting compounds

The best and most expensive compound with regard to thermal criteria has a specific thermal conductivity of 0.8W/m/°K. In practice, recourse is had mostly to a potting compound with conductivity of 0.4/W/m/°K.
Let’s choose the potting compound with thermal conductivity of 0.4W/m/°K.

Bobbin

In this performance range, recourse is had almost exclusively to double chamber bobbins. With regard to design criteria, only the bobbin dimensions are relevant. The bobbin with additional insulation or large leakage paths has a small winding space and smaller cooling surface area.
Normally, the primary side is fed with the voltages of 110V, 230V, 400V,... At an output of 1.5VA, the wire thickness of the primary is undesirably thin and hence too expensive, and too difficult to process, combined with an unacceptably high scrapping rate. The solution to this problem can partially be achieved by means of asymmetrical distribution of the winding space between the primary and the secondary, by giving the primary more winding space.
Let’s choose a double chamber bobbin from a group of bobbins with a large winding space and with asymmetrical chamber distribution.

Impregnation

Under this topic, Let’s examine the possibility of just potting the coil or the window of the core, or injecting under pressure. In this method, we can save on the quantity of potting compound with the same voltage resistance as a potted transformer, and we don’t need a case.
Our transformer is potted without vacuum in a case. The winding remains "dry".

Steel quality

In nominal operating mode, at full core losses, the over-temperature of the transformers, at a design output of 2VA, is approx 20°K to 30°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 over-temperature is extremely relevant, iron losses are practically negligible. For that reason, we opt for the cheapest cold-rolled core quality. And furthermore, the cheapest cold-rolled core quality has the highest saturation induction!
Let’s choose the cold-rolled core quality of 8W/kg at 1.5T, 50Hz, 0.5mm or 0.65mm thick.

Induction and regulation

The choice of induction in nominal operating mode and regulation (voltage increase in %) should be performed such that the no-load temperature at 6% over-voltage does not exceed the maximum permitted temperature of 115°C for the insulation class. In this context, the no-load induction is normally 1.6T- 1.7T. In order not to exceed this no-load induction, the induction should be entered in accordance with the following recommendation, in nominal operating mode:

Bnom/Bo = (1+0.5*dUr/100)/(1+dUr/100)/(1+dUp/100)

where

• dUp = Overvoltage of primary voltage as %
• dUr = Regulation or voltage increase = 100*(Uo-Unom)/Unom in %
• Bo = No-load induction in T (1.6T-1.7T)
• U0 = No-load output voltage in V
• Unom = Nominal voltage in V

Normally, we start designing with the following input data:

• dUp% = 6%
• B0 = 1.7T
• dUr = 100% (For maximum output power in the power range <2VA)
• Bnom = 0.75*1.7T = 1.2T

Permitted tolerance of output voltage

The output voltage of a transformer which absolutely must be short-circuit-protected is tested in the hot 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.
Let’s design the calculation for an output voltage which is approximately 10% less.

Procedure for design

1. If you are not yet acquainted with Rale Design software, please read the text: "How do I design a small transformer?". You should keep a copy of this text within your reach whenever performing design operations.
2. Start the demo program and fill out the input mask as follows. If you need any help, press function key F1. There is extensive description for each box.

3. Select the core and bobbin with the corresponding case. If the data set for the core and bobbin which you chose has no case, then you have to create the case for yourself.

4. To enter the case, press Copy so as to copy the desired core onto the clipboard.

5. Click on Case, in order to see whether the data set for the copied core has a case.

You can see that the data set for the selected core also has a case.

6. Click on OK, in order to quit the clipboard.
7. Save your input file. In this design example, the input data has been stored as input file CAL0002E.TK1. This input file has also been supplied with this document. Copy it into the directory in which the RALE Demo Program has been installed.
8. Connect up to the Rale Design Server.
9. Load up your input file and start designing.

After the end of the design work, the following design data will be available:

10. This is followed by checking the design data.

• Firstly we check the max winding temperature in nominal operating mode = ambient temperature + dTprim in nominal operating mode = 40+50.4 =90.4 < 115°C
  
• The max winding temperature in short circuit is:
  Ambient temperature+ dTprim in short circuit = 40 + 120 = 160<165°C
• The average case temperature in short circuit is:
  Ambient temperature + dT case in short circuit = 40 + 94 = 134>105°C
  This means that this transformer has to be fitted in a unit such that it cannot be contacted during operation.
• We now check the winding data.
• We finally check the output data: nominal (11.37V>10.8) and no-load output voltage
  (22.2V < 24V)

11. If the design data is not satisfactory, then we have two ways for implementing the desired correction:

• We either return to the input mask (Function key F2), correct the input data and re-design the transformer, or
• We move to the test program (Function key F5), modify the designed transformer manually and convert the transformer by that means.

12. On completion of design, we can print out the design data on-line or store it on the local PC and print it out off-line. The output data file from this design example, CAL0002E.TK2, is supplied together with this document. Copy it into the directory where your Rale Demo Program is installed.

Tips&Tricks

The transformer is too full

This is often what happens if you select your own core. For example, suppose that you want to use this core for 2VA output.

• Increase regulation to the desired value.
• Increase your induction. Check that the idling temperature does not exceed the permitted level for the insulation category.

The transformer is relatively empty

This is what often happens if you select the core yourself. Suppose, for example, that you want to use this core for 0.5VA output.

• Select a smaller core
  or
• Reduce regulation.
  or
• Reduce induction.

The chambers of the bobbin are asymmetrically filled, e.g. 70%:90%.

Change the wire thicknesses on the test program manually:

• Choose the next higher wire thickness in the chamber which is less full.
• Choose the next lower wire thickness in the chamber which is more full.
• Correct the number of windings of the secondary in order to arrive at your desired output voltage.

Nominal operating data

This transformer was designed for 6% over-voltage. In order to arrive at the nominal data for this transformer design operation, test the designed transformer with the nominal input voltage in the test program (Uin=1).

The temperature in short circuit is too high

• Increase regulation to the desired level.
• Increase the cooling surface area. Select a larger case. The colour of the case should be dark.

Temperature in nominal operating mode is too high

• Reduce regulation and increase induction.
• Use a better core quality
• Increase your cooling surface area. Select a larger case. The colour of the case should be dark.

Is your no-load current too high?

The no-load current is not a criterion for design. The no-load temperature must not exceed the permitted limit.

The primary wire is too thin

• Reduce regulation
• Reduce induction.
• Select the bobbin with asymmetrical chambers.
• Select a core with a large winding space.

Potting: yes or no?

In relay technology, coils are very often externally protected with a "potting compound". We try to use this technology with small transformers as well, by injecting the window of the core together with the bobbin and the windings, under pressure. This technology is supported by your Rale Design Program in the box Impregnation = 4.
A second possibility is to select the bobbin with the required voltage resistance and the required leakage paths. This technology is supported by your Rale Design Program in the box Impregnation = 5, 6 and 7.

PEI Cores

PEI cores are EI zero production waste cores with reinforced yoke and reverse yoke EI. For that reason, a slightly higher induction can be used with a PEI core.

The procedure for design of a transformer with a PEI core is as follows:

• Select an EI zero production waste core
• Copy the selected core onto the clipboard.
• Change the I/O position and F/A positions of the copied core as follows:

Optimal Procedure

Note that a full automatical design of a inherently short-circuit proof transformer is not possible, because there is no input for the temperature rise in the short-circuit operation (I'm going to open it in the next version). Therefore you need the following design steps:

• In order to get big leakage reactance, select a core with big height and small width of the window.
• Select double-section bobbin: Bobbin = 2.
• Select the Regulation = 100%, the Temperature Rise in accordance with the insulation class and the Criterion = 0
• If the the short-circuit temperature rise is too high and the transformer is not "full", than reduce the induction (0.2-1.0T) and redesign the transformer.
• If the the short-circuit temperature rise is too high and the transformer is too "full", than select a bigger core.

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