Re: Calculating the VA rating of transformers?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, JanRwl@A... wrote:

> SOMEwhere I read a comment to another member that the core in

question was

> 2.3 x 2.3". This is, then, 5.29 sq.in. core-area. In my chart,

this would be

> about 750 VA; maybe, yes, 800 VA, if it may run hotter than

ideal. So, if

> the output is a single secondary of 46 volts, the current might be

up to

> 750/46=16 amps. However, check the wire-diameter with a caliper.

For 16 amp, you

> must have at least #8 wire. #9 if well-ventillated.
>
>
> [Non-text portions of this message have been removed]

The core area in a standard EI transformer is the core section (cross
sectional area) that passes through the center of the core (not the
size of the transformer overall). The formula to calculate the power
density of the core is:

A = 1.1 * Sqrrt of P

Where A = Cross section Area in CM
P = power in watts.

Given we have a known area of 5.842^2 CM (34.128 sqr CM) we can
calculate the approximate power it can handle.

Simplify the equation into:

31.02^2 = P

P = 962 Watts

That's more than the chart shows but there are other factors. The
actual design of a transformer to deliver a given power is dependent
on the permibility of the core in Gauss, the wire winding 'window'
and the size of the primary and secondary wire. The operating
frequency is a big factor as well.

I would not expect to see a 60 hz 750VA transformer that was much
smaller than about a 6" cube and weighing over 25 lbs. (I used to use
the number of 25Watts/lb as a rough estimator for surplus 60 HZ power
transformers)

Best rough indicator of the power handling ability of a transformer
is to measure the secondary AC and then measure the wire diameter of
the secondary. Designers try to use the smallest wire for the rated
current since bigger wire means a bigger bobbin which means a bigger
core which means more dollars. In other words designers won't use
bigger wire than they have to so you can consider that the max
consistant current. There are design charts for suggested wire cross
secional area per amp. Multiply the secondary volts times the amps
from the chart and get watts.

Toroids operate with a different set of numbers. The secondary wire
size is still a good indicator but you can use slightly smaller wire
since the secondary is spread out and the heat density is less.