[sdiy] How to predict a tansformer's current capability?

[John L Marshall]

Transformers are quite efficient. Volt-Amperes out = Volt-Amperes in -
losses (copper + eddy current + hysteresis). (A V-A rating must be used
rather than Watts because the copper wire must deliver current even if the
load is completely reactive.)

Actually it is better to look at the current drawn in the secondary as being
reflected in the primary but by the inverse ratio of the windings.

Small power transformers use 3 to 4 turns per volt at 50 to 60 Hertz.

The iron core is a magnetic coupler. The cross-sectional area of the iron
core will determine the capacity of the transformer. This approximately
works: Area in inches squared = square root of V-A divided by 5.58.

Another crude way to measure the V-A capacity is to weigh the transformer,
very roughly 15 V-A per pound.

One secondary winding on multi-winding transformers may deliver full V-A
rating if the copper is big enough. Conservatively use 800 circular mils per
Ampere. Less conservatively use 600 circular mils per Ampere.

Voltage drop is not a good way to measure the V-A capacity of a transformer.

The biggest danger with transformers is core saturation. When this occures
the only this limiting current is the copper resistance. Smoke.

John


----- Original Message -----
From: Glen <mclilith at ezwv.com>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, December 23, 2001 10:37 PM
Subject: [sdiy] How to predict a tansformer's current capability?


> I have some assorted used power transformers that have been removed from
> various types of equipment. I'd like to use some of these for projects,
but
> how do I predict what the maximum safe output current would be for a
> transformer that I have little or no information about? I can determine
the
> primary and secondary windings with no problem. I can hook them up and
> determine the output voltages with no load, but how do I determine the
> maximum load the transformer should be subjected to? I'm more interested
in
> a "continuous use" specification, instead of a peak current rating, but a
> peak current rating might be useful for some things as well.
>
> The transformers are a varied assortment. Some have multiple secondaries,
> perhaps a few have multiple primaries. Some transformers have "step-up"
> secondaries, but most are the "step-down" style.
>
> Do I just power one of them up, while increasing the load on the
secondary,
> and monitor the secondary's output voltage? (Perhaps I would stop
> increasing the load, when the output voltage has dropped by some
calculated
> "safe" amount, whatever that amount might be, and declare the amount of
> current flowing at that moment to be the maximum rating.)
>
>
> Thanks for any assistance,
> Glen Berry
>