Re: Calculating the VA rating of transformers?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "caudlet" <thom@t...> wrote:

>
> --- 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.

Excelent post. I've been waiting for something like this for years.

I do have a question.

Most of my transformers have the center section as a rectangle that
is roughly 2:1 height by width, and then the thickness is dependant
on how many laminations have been stacked.

The measurements have been square area, not cubic so I am wondering
which measuremetns to use ? Thickness of laminations and one face
side ? both face sides and not the thickness ? Lam thickness and
average of face ?

Dave