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Transformer Temperature & Insulation


ATL Transformer Design

Our transformers are designed in a way that does not allow impermissible temperatures. The insulating materials have at least class E conforming to IEC 85, mostly class F (155 deg C) & class H (180 deg C) and on request even higher temperature classes. For the technical calculation, the temperature is calculated at 1,06 times the primary voltage in rated operation and in short-circuit case.


Valid temp rise in standard operation conforming generally to BS EN61558

The transformers may not reach improper temperatures when used as intended. The conformance is checked according to BS EN 61558 .The transformers and chokes are connected to the rated input voltage and then they are stressed with an impedance, which would give the rated power at rated output voltage and at alternating currents with the rated power factor. Subsequently the input voltage is increased for 6%. After this voltage increase, the current circuit will not be changed. The test is repeated under no-load conditions.
Temperatures of windings are calculated with the resistance method. The value of temperature rise of a transformer winding can be calculated with the following equation:

Here: x = 234,5 for copper and:

D   temperature rise over t2, the highest temperature is D+t2
R1 resistance at test begin with temperature 1
R2 resistance at test end, in a presevered state
t1  ambient temperature at test begin
t2  ambient temperature at test end

At the start of the test the windings need the be at the same temperature as the ambient temperature.

For transformers with more than one input- or output winding or transformers that have a tapped input or output winding, the values with the highest temperature are considered. During the test, the temperature may not exceed the values given in table 1, if the transformer is operated with a rated ambient temperature (25°C or ta).


Table 1:Highest temperature values at intended use



Windings (coils and metal sheets in connection to them), if the insulation system is made of:

- Material with thermal class A

- Material with thermal class E

- Material with thermal class B

- Material with thermal class F

- Material with thermal class H






The classification of materials is conform to IEC 60058 and IEC 60216. The values are adapted given the fact, that during these tests, the temperatures are average values, not hotspots.


Immediately after the test, the test item has to pass the voltage solidity, which is set in § 18.3 EN61558, though the test voltage is just connected between input and output current circuit. After the test, the electrical terminals may not be detached, the air and creepage distance may not be smaller than the values set in § 26 EN 61558-1; potting compound may not have phased out and overload protection may not have reacted. With extensive technical calculation (done with a computer software) of each transformer, we make sure, that the values in table 1 are not exceeded. A higher ambient temperature lowers the possible power of a transformer at given size.


Max. warming at short-circuit and overload conform generally to BS EN61558

Table 2: Peak values of temperature in short-circuit and overload conditions

Classification of the insulation







max. temperature °C

absolutely short-circuit proof windings






Windings with protection device:

-  during the length of T, see table 33)
-  after the first hour, peak value
-  after the first hour, arithmetic average






Outer housings (which can be touched with norm probe)

Rubber insulation of the conductors

PVC-insulation of the conductors

Surface (e.g. every area of the surface of a plywood board, which is covered by the transformer)







Table 3: Values of T and k for fuses

given rated current In of the protective fuses for gG in A




 In < 4

     4 < In < 16

  16 < In< 63

   63 < In < 160

160 < In< 200













Insulation declaration conform to IEC 85

On request, you obtain an insulation declaration for each transformer. This includes information about the used insulation materials and their temperature class.


Energy losses
An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those rated for electricity distribution usually perform better than 98%.

Experimental transformers using superconducting windings achieve efficiencies of 99.85%. The increase in efficiency from about 98 to 99.85% can save considerable energy, and hence money, in a large heavily-loaded transformer; the trade-off is in the additional initial and running cost of the superconducting design.

Losses in transformers (excluding associated circuitry) vary with load current, and may be expressed as "no-load" or "full-load" loss. Winding resistance dominates load losses, whereas hysteresis and eddy currents losses contribute to over 99% of the no-load loss. The no-load loss can be significant, so that even an idle transformer constitutes a drain on the electrical supply and a running cost; designing transformers for lower loss requires a larger core, good-quality silicon steel, or even amorphous steel, for the core, and thicker wire, increasing initial cost, so that there is a trade-off between initial cost and running cost. (Also see energy efficient transformer).

Transformer losses are divided into losses in the windings, termed copper loss, and those in the magnetic circuit, termed iron loss. Losses in the transformer arise from:

Winding resistance
Current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses.

Hysteresis losses
Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected.

Eddy currents
Ferromagnetic materials are also good conductors, and a core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness. Eddy current losses can be reduced by making the core of a stack of plates electrically insulated from each other, rather than a solid block; all transformers operating at low frequencies use laminated or similar cores.

Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound commonly associated with transformers, and can cause losses due to frictional heating. Leakage inductance is by itself largely lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer's support structure will give rise to eddy currents and be converted to heat. There are also radiative losses due to the oscillating magnetic field, but these are usually small.
In addition to magnetostriction, the alternating magnetic field causes fluctuating forces between the primary and secondary windings. These incite vibrations within nearby metalwork, adding to the buzzing noise, and consuming a  small amount of power.