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

John L Marshall john.l.marshall at gte.net
Mon Dec 24 20:45:37 CET 2001


Good techniques. I think that current goes up by cube (exponent 3) when
saturation is reached.

Oh, yes! There are transformers that have an intentional gap in the core
between the primary and secondary windings. These transformers, by design,
saturate and use that feature to work as a voltage regulator. Unless you
have all of the specifications and the correct capacitor, do not use
constant voltage or ferro-resonant transformers.

John

----- Original Message -----
From: harry bissell <paia2720 at yahoo.com>
To: John L Marshall <john.l.marshall at gte.net>;
<synth-diy at dropmix.xs4all.nl>; Glen <mclilith at ezwv.com>
Sent: Monday, December 24, 2001 11:25 AM
Subject: Re: [sdiy] How to predict a tansformer's current capability?


> You should be able to detect core saturation by
> watching the current very carefully... as saturation
> approaches the current increases very quickly... a
> knee
> similar to diode conduction (but going the other
> way...
> bigger!)
>
> Another way is to use a "telephone pickup coil" to
> listen to the hum (flux leakage) around the
> transformer. When the core saturates, the magnetic
> field will grow in the air surrounding the core.
>
> This will also identify transformers that by design
> or mistake, have substantial gaps in the core (usually
> a bad thing...)
>
> H^) harry
>
>
>
> --- John L Marshall <john.l.marshall at gte.net> wrote:
> > 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
> > >
> >
> >
> >
>
>
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