> ORIGINAL MESSAGE:
>
> On Wed, 9 Jun 2010 16:22:11 +0200, Andre Goldschagg
> <totiranger@gmail.com> wrote:
>
>>Is there a standard formula to use to calculate the KVA rating of a
>>transformer?
>
> REPLY:
>
> This may sound like I'm joking, but I'm not: When running
> continuously, if the transformer runs too hot to touch, it is
> overloaded. Heat is the enemy of the insulation and will deteriorate
> it over time, leading to failure.
>
> 73, Bill W6WRT
>
The formula is calulated by the area of the core but varies by the age and
design.
Formulas for the old tube rectifier day from the Radiotron Designer's
Handbook 4th Edition page 235:
core sized needed (for 60 Hz.)
cross-sectional area in inches = (V*A)1/2 / 5.58
So, solving for Volt*Amps, knowing core cross-sectional area:
(5.58 x core area)2 = Volt*Amps capacity
The above is not accurate for the latest hi silicon steel or tape wound
(C-Cores).
A rather broad rule of thumb is 10W per pound (core and wire only, not bolt
on end bells, etc) for the real old iron from the WW2 era to 30W/Lb for the
new ones. This is for CCS so a 28# SB-220 xfmr is a 500va CCS piece which
you can see is a good fit into the 30W/Lb end figuring on about a 40% duty
cycle SSB rating.
Carl
KM1H
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