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Frequency - Temperature characteristics of transformer winding

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makkena
#1
Jul25-14, 01:14 PM
P: 5
Dear Sir,
The rating of a single phase transformer is 230V / 230V (1:1), 5A, 50 hz, 1200W.
When this transformer is operated at 230V, 5A, and 50 hz the transformer winding saturated temperature may be some value. Now when the same transformer is operated at 230V, 5A and 40 hz (reduced frequency), what will happen to winding temperature? will temperature decrease or increase? what can be the reasons for temperature change?
is there any relation between frequency and winding temperature of transformer?

Thank you
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jim hardy
#2
Jul25-14, 01:52 PM
Sci Advisor
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jim hardy's Avatar
P: 3,758
when the same transformer is operated at 230V, 5A and 40 hz (reduced frequency),....
The transformer will probably burn up.
Its core will overheat from too much magnetic flux
and its windings will overheat from the high current required to create that flux.

Go back to your very basics:

Since voltage is derivative of flux,
flux must be integral of voltage

∫sin(ωt) = -(1/ω) X cos(ωt) + C.... and since it's AC we'll ignore the constant of integration C.

ω is in the denominator
so as ω gets smaller the result gets bigger
and that's why for a constant voltage, as frequency goes down flux goes up .

look up "volts per hertz";

see
http://hyperphysics.phy-astr.gsu.edu...ic/transf.html

and
http://www.allaboutcircuits.com/vol_1/chpt_14/4.html

and here's a brief summary:


A transformer is designed to operate at or below a maximum
magnetic flux density in the transformer core. Above this design
limit the eddy currents in the core and nearby conductive
components cause overheating which within a very short time
may cause severe damage. The magnetic flux in the core is
proportional to the voltage applied to the winding divided by
the impedance of the winding. The flux in the core increases
with either increasing voltage or decreasing frequency. During
startup or shutdown of generator-connected transformers, or
following a load rejection, the transformer may experience an
excessive ratio of volts to hertz, that is, become overexcited.
When a transformer core is overexcited, the core is operating
in a non-linear magnetic region, and creates harmonic
components in the exciting current. A significant amount of
current at the 5th harmonic is characteristic of overexcitation.
https://www.gedigitalenergy.com/smar...7/article5.pdf

old jim


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