From: coatham@es.co.nz
Sent: Monday, 23 August 1999 21:03
To: hjleak@egroups.com
Subject: [hjleak] Temperature Rise in Power Transformers

Here is the gospel: 

To avoid deterioration of the insulating materials, it is necessary to
limit the working temperature to 105 deg C for Class A insualtion which
includes paper,cotton, silk, varnish & wire enamel. The temp rise in
the winding as measured by the change in resistance method, will be
about 10 deg C lower than the maximum(hot spot) temperature. Thus with
an ambient temperature of 40 deg C
 plus a margin of 10 deg C for the difference between measured and hot
spot temperature, it will be seen that the maximum permissible rise is
55 deg C, as measured by the change in resistance.
It is common practice to allow 10deg C margin for change in line
voltage, frequency, or operation in situations with restricted
ventilation. Thus 45 deg C is generally accepted as the maximum
permitted rise above ambient when measured by the resistance change.
The temp difference between winding and core varies between 10 & 20 deg
C according to the distribution of losses. This means that even with an
ambient temperature of 25 deg C the core temperature may be 60 deg C.
This will feel quite hot to the touch, although the internal
temperature may be well under the permitted maximum. Measurement of the
core temperature may be made with a spirit thermometer if good thermal
contact is maintained between the core and the thermometer bulb.
The winding temperature can be calculated by measuring the cold
resistance, Ro, at an observed temperature of Ti. After a heat run at
full load the hot resistance R is measured and the ambient temperature
T2 is again measured.
Taking the temperature coefficient of resistivity(a) of copper as
0.00393, the temperature rise T is found from the formula:
             T = R - Ro /Roa
To correct for any changes in ambient temperature during the heat run
it is necessary to subtract the difference T2 - Ti from T to find the
actual rise. An example will illustrate this:
The primary resistance of a transformer when cold was 30 ohm at an
ambient temperature of 20 deg C. After an 8 hour full load run, the
resistance was 36 ohms, while the ambient temperature was then 18 deg
C. It is required to find the winding temperature rise.

T = 36 - 30 / 30 x .00393   =  6 / 30 x .00393  =  51 deg C

Now T2 - Ti = (18 - 20) deg C = -2 deg C

Winding temperature rise = [51 -(-2)]  =  53 deg C


Reference - Radiotron Designers Handbook 4th Edition page 237


So there you are folks - the gospel according to Langford-Smith..







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