Temperature characteristics and insulants


Design of the transformers
Our transformers are dimensioned in a way that doesn’t allow impermissible temperatures. The insulants have at lest class E conform to IEC 85, mostly class B and on requst even higher temperature classes. For the technical calculation the temperature is computed at 1,06 times the primary voltage in rated operation and in short-circuit case.


Acceptable heating in standard operation conform to EN 61558-1
The transformers may not reach improper temperatures when used as intended. The conformance is checked according to EN 61558-1 §14.2. 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 won’t be changed. The test will be repeated in no-load condition, if this is an infavourable situation.
Winding temperatures are determined via the resistance method. The value of temperature increase of a winding can be calculated with the following equation:

tui-formel

Here: x = 234,5 for copper and:

D Temperature increase over t2, so that the highest temperature is D + t2,
R1 Resistance at test begin with temperature t1,
R2 Resistance at test end, in preserved state,
t1 Ambient temperature at test begin,
t2 Ambient temperature at test end

At test  begin the windings need to 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 rated ambient temperature (25°C or ta).
Table 1: Highest temperature values at intended use

Components

Temperature

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

- Material with thermal class A1

- Material with thermal class E

- Material with thermal class B

- Material with thermal class F

- Material with thermal class H

 

100

115

120

140

165

The classification of materials is conform to IEC 60085 and IEC 60216; the values are adapted given the fact that during these tests the temperature is an average value, not hotspots.

 

Directly after the test the test item has to pass the voltage solidity, which is set in §18.3 EN 61558, though the test voltage is just connected between input and output current circuit. After the test the electric 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 to EN 61558-1
Table 2:
Peak values of temperature in short-circuit and overload conditions.

Classification of the insulation

A

E

B

F

H

 

max. temperature °C

unconditionally short-circuit-proof winding

150

165

175

190

210

Windings with protective devices:

- during the length of T, see table 3)

- after the first hour, peak value

- after the first hour, arithmetic average

 

200

175

150

 

215

190

165

 

225

200

175

 

240

215

190

 

260

235

210

Outer housing (which can be touched with norm probe)

Rubber insulation of the conductor

PVC-insulation of the conductor

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

105

85

85

105

Table 3: Values of T and k for fuses

Given rated current In of the protective fuses for gG
A

T
h

k

In < 4

4 < In < 16

16 < In < 63

63 < In < 160

160 < In < 200

1

1

1

2

3

2,1

1,9

1,6

1,6

1,6


Insulation declaration conform to IEC 85
On request you obtain an insulation declaration of each transformer. This includes information about the used insulation materials and their temperature class.


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