Hot Transformer
In article ,
"Phil Allison" wrote:
"Fred McKenzie"
I have a couple of 20 Amp linear DC power supplies with hot transformers.
With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer.
** FYI there is a much simpler, low tech way to find the internal temp of a
transformer.
1. Measure the primary winding's resistance when at room temp.
2. Make the same measurement ( quick as you can) when the tranny's running
temp has stabilised.
3. Calculate the increase as a decimal factor and multiply that by 254 to
get the temp rise in degrees C.
Eg.
Room temp = 4 ohms
Hot temp = 5 ohms
Increase factor = 0.25
0.25 x 254 = 63.5 degrees C
Add on the actual room temp to get final result.
If the number is over 100C there is cause for concern.
Phil-
This is useful information, but my results did not agree with IR
measurements.
I think my problem was that it is hard to measure low values of
resistance accurately. For my "hot" transformer, cold resistance
measurement of the primary was around one Ohm on a Fluke digital meter.
However shorting the meter leads produced 0.4 Ohms.
After several hours, primary resistance measurement was 1.2 Ohms.
Subtracting 0.4 from both measurements, the increase factor was
(0.8-0.6)/0.6 = 1/3. Calculated rise was 84.7º C, plus original ambient
24.4º C gives 109.1º C.
The IR thermometer measured the top of the transformer laminations.
Initial temperature was 24.4º C. Final temperature was 53.5º C. I
could hold my hand on the transformer without screaming, so I am fairly
sure the temperature was nowhere near 109.1º C!
I think a 4-terminal resistance measurement would provide more realistic
readings.
Fred
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