RE: [CAD_CAM_EDM_DRO] current rating of transformers

Hi,

I'll try and explain the current rating the way I understand it
using mostly non-electrical engineering terms.

The power rating of a transformer is based on the core size and
material. Core size is measured using the cross sectional area
among other things but we'll ignore that for the moment. Take a
look in any transformer catalog and you'll see that the VA
rating of the transformer regardless of output or input voltages
remains the same across transformer core sizes.

The reason for this is quite simple. If you try to increase the
current for a given transformer past a certain point the core
material becomes saturated with the magnetic field and the
'impedance' to the input line voltage drops to a very low value,
allowing an inrush of current which usually burns out the
windings.

Back to the input impedance for a moment. The input/output wire
size, the number of turns etc, are all set up to provide an
impedance to the power line that prevents the transformer from
overheating. Let's say for example, the transformer at 60Hz
and 240VAC draws 1A. That means the impedance (not resistance)
is 240 Ohms and the transformer is a 240VA unit.

Now put that transformer on a European grid at 50Hz. At this
point the impedance of the transformer will be lower because the
frequency is lower hence the transformer will draw more current
and as a result run warmer exceeding the input VA rating. In
fact, reduce the frequency to 0 (DC) and 120V in will toast the
primary in very short order.

The impedance is dependant on the wire size, number of windings
on both the primary and the secondary and the load on the
secondary. Short the secondary and the impute impedance changes
again (lower) and once more you get overheating on the primary
and secondary or even core saturation.

I believe that's why wall warts fail by toasting their small
input fuse inside the primary windings when the output is
shorted. I have a few 12V Halogen lamp transformers where the
lamp suddenly decreased in resistance as the filament failed.
Blew the input fuse making the transformer useless. A
temperature resetable fuse would have opened until the bulb was
replaced and then stayed operational.

When we look at center-tapped verses full wave systems we find
that in fact once again the VA rating determines the output
power of the unit and determines how hot the transformer gets.
The primary and core are sized for the VA rating. The secondary
wire size for the current rating. The ratio between the primary
and secondary determine the output voltage.

A center tapped transformer is really a transformer with two
secondaries each rated at a particular amperage and voltage. If
each winding can deliver 1A then in parallel you get two amps at
the rated voltage. If the windings are put in series then you
get 1A but at twice the voltage. The difference is the center
tap connecting to the two windings is buried inside the core
somewhere rather than bringing out two wires that are then
connected externally.

You see the same thing on transformers destined for 120/240VAC.
The input side windings are put in parallel for 120VAC or in
series for 240VAC. In series, the input impedance of each
winding is added just like resistors so if each winding draws 1A
at 120VAC then they'll still draw 1A at 240VAC for 240VA. For
the 120VAC circuit, they'll be put in parallel and each draw 1A
at 120VAC. That's a total of 2A at 120VAC for both windings and
again 240VA.

Hope that clears things up.

John Dammeyer





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