On 06/ 9/10 03:22 PM, Andre Goldschagg wrote:
> Hi,
>
> Is there a standard formula to use to calculate the KVA rating of a
> transformer? I know what the line feed to the transformer is as well as the
> voltage and current that is needed from the transformer feed.
>
> Regards
> Andre ZS5UA
I'm not sure I understand exactly what you know here.
Are you saying.
Line feed = input to primary.
Load feed = output from secondary?
Why do you want to know the KVA rating?
It looks like you are implying you know the kVA of the input and the kVA of the
output. That tells you nothing about the kVA rating of the transformer. You
could be using a 1000 kVA transformer to supply a micro Watt.
The rating is rather subjective, and depends on application. An industrial
grade
transformer would have a certain rating at a certain ambient temperature. Push
it to intermittent service, or increase the cooling, and it would have higher
ratings. Use it in a medical device (assuming it was suitable), and it would
have a lower rating than industrial use, due to the need to ensure high
reliability.
All transformers will decrease the output voltage as the output load is
increased. Big ones much less than small ones, so that does not tell you
anything very useful.
From a practical point of view, what will probably be your best indicator of
whether a transformer is overloaded or not is the temperature. For small
transformers, that would be quite easy to tell by hand. For larger
transformers,
less so.
One could probably infer the average temperature by measuring the DC resistance
at room temperature and after its been in use, taking into account the
temperature coefficient of conductivity. As a rough guide, I'd be tempted to
1) Measure the DC resistance of similar size at room termperature.
2) Load the transformer to its full ratings for a time sufficient for the
temperature to stabilise.
3) Remove mains input and measure DC resitance.
4) Calculate the ratio of the two resistances.
5) Measure the DC resistance of your unknown transformer at room temerature.
6) Load your unknown transformer up to a load that is significant, but which
you
know will not overheat it for such a period as the temerature will stabilise.
7) If the ratio of resistances exceeds say 80% of what you got with the known
transformer, you are probably running it too close to maximum, given you don't
know the specification.
Really, in the absence of a data sheet, temperature rise is liekly to be your
best indicator.
I'm not an expert on transformers, so what I say may be completely wrong!
Dave
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