transformer info

[new guy]

I have a transformer in my garage. It was used with 480v input and put out 135v. The rating on the side says 200va. If I hook it up to 120v household power (primary side) what voltage should I expect and what amount of amperage will be available on the secondary output?

[Crowbar]

Turns ratio= 4:1
ohm's law for the rest...

30v @ 6.6 Amps

[dacflyer]

if you ran the transformer backwards,,, 135v on the secondary,,shouldn't the primary still be 480v still? but at lower current ?

[Dean Huster]

Of course -- the 200V-A still applies. For the most part (i.e., within reason) the only thing that defines the primary is which winding is connected to the mains. Don't expect to connect a 6.3V/10A filament winding to the mains and expect to get 2200 volts out of the original primary without tripping breakers or internally arcing the windings of the thing. There are limits.

You're always OK using a step-down transformer at a lower primary voltage as the OP is suggesting. You have to be careful when you reverse the winding functions.

One of the most common primary/seconday reversals is connecting the two secondaries of identical filament transformers to each other and using the original primaries as the new primary and secondary, creating an isolation transformer, handy for working on old line-powered AC/DC radios. Much cheaper than buying an actual isolation transformer if you already have an identical pair of filament transformers.

Dean

[Robert Reed]

You have got good info here and basically is true. But one thing will change with your setup and that is magnetizing currents (affects efficiency) and winding impedances (affects regulation), but if you dont try and push it to close to its theoretical limits, you should be okay.

[rshayes]

The transformer rating is usually determined by the ability of the transformer to dissipate the heat generated by the losses in the transformer. These losses are made up of two components, usually called copper loss and core loss. In an optimum transformer, these losses are approximately equal to each other.

The core loss is the heat generated by eddy currents and hysteresis loss in the iron core. These losses increase as the flux density in the transformer increases. The copper loss is the loss due to current flowing through the resistance of the windings.

Applying a lower voltage to the windings will reduce the maximum flux density, which in turn reduces the core loss. This is the good news.

The bad news is that higher current through the windings increases the copper loss approximately as the square of the current.

The volt-ampere rating is the product of the output voltage times the output current. The maximum output current under the original operating conditions woud be 200 volt-amps divided by 135 volts, whicn gives a secondary current of 1.48 amps. Reducing the voltage level would reduce the core loss, but even if the core loss were reduced to zero, the allowable copper loss could only be increased by 100 percent to bring the total loss back to the original level. This would occur with a current indrease of 40 percent.

The output voltage of the transformer would be 33.75 volts when operated with 120 volts. The allowable output current would be between 1.48 amperes and some figure less than 2.1 amps, depending on how much the core loss dropped with the reduced primary voltage.

[jollyrgr]

This is actually a 3.5:1 turns ratio (480/135 = 3.555).

Converting 120 back up is 120*3.555 = 427V

The 200VA is the Volts*Amps and is straight forward. This is the MAXIMUM the transformer will support. Thus at the max on the HIGH side you could draw 0.47A used as a step up transformer. On the LOW side you would have 200VA/120V = 1.6A.

There are other things to consider such as losses in the magnetic core, resistance, etc.

[MrAl]

Hi there,

I got results similar to everyone else, with a turns ratio of 3.5555 approx,
except i got more output voltage combinations.

With a four terminal device there are many other ways to connect
it into a circuit. Looking at various ways to connect a 4 terminal
transformer up to a 120vac source, there are 10 possible output
voltages depending on how you wire the primary and how
you wire the secondary, and where you apply the input and
where you take the output from. Only two ways provide isolation,
but the other ways do provide additional output voltage possibilities.

Here is the list...

(note only two cases here are isolating)

120vac input across one winding only:

120v into 480 side, 33.75v out, isolating
120v into 135 side, 426.7v out, isolating

120v into 480 side, series aiding secondary, 153.75v out
120v into 480 side, series opposing secondary, 86.25v out

120v into 135 side, series aiding secondary, 460.45v out
120v into 135 side, series opposing secondary, 392.95v out



120vac input across two windings in series:

120v across series aiding primary, output across 135 side, 26.34v out
120v across series aiding primary, output across 480 side, 93.66v out

120v across series opposing primary, output across 135 side, 46.92v out approx
120v across series opposing primary, output across 480 side, 165.84v out approx


Did i miss any or miscalculate any output voltages? See if you can
find any more combinations and feel free to check my results of
those shown.

[rshayes]

Operation of a winding at higher voltage than its original rating but at the same frequency will be limited by core losses and core saturation. If the transformer is rated for 50/60 Hz, then the operating voltage can be raised by 20 percent if operation is limited to 60 Hz. Past this point, things get pretty risky. Depending on the core material, saturation can increase the magnetizing current very fast for an increase of a few volts across the winding. Operating at a lower voltage will reduce the core loss and may allow some increase in the copper loss before the transformer overheats.

The current which can be drawn will be limited by the copper losses in the windings. Operation at a lower voltage does not change the copper loss for a given current. It may allow a slight increase in copper loss due to lower core loss, and this may allow a slight increase in output current. The original volt-ampere rating would not apply to these new operating conditions and should not be used to calculate the allowable output current.

As MrAl points out, if isolation is not required then there are several autotransformer connections which will give a variety of output voltages. Determining the allowable output current for these connections can be quite complicated since the magnetizing and output currents may flow through both windings in either an aiding or opposing direction.

[Dean Huster]

However, by using boosting and bucking techniques with multiple windings, including (actually, especially including the primary), you'll end up with a "softer" transformer with lousy regulation.

Dean

[MrAl]

However, by using boosting and bucking techniques with multiple
windings, including (actually, especially including the primary),
you'll end up with a "softer" transformer with lousy regulation.
Dean

Well Dean, i have to respectfully disagree, and here's why...

Say we have a 12vac output transformer with a 120vac primary,
and we always connect the 120v primary to the 120vac line,
and the line is fairly stiff.
Lets say this 12v output with a 10 amp load sags down to
10.8 volts. It's supposed to be 12v, but it loads down to
10.8 volts, so the voltage error is -10 percent. Another way
of stating this is that with the load, we only get 90 percent
of the no load output voltage.
Next we connect the secondary to the primary in a series
aiding connection so that the primary voltage (120v) gets
boosted by an anticipated 12v, and we wish to use again
a 10 amp resistive load. Without load, the voltage is
120v+12v=132 volts. Once we connect the 10 amp load however,
the 12v winding output drops again to 10.8 volts. Now we
have the 120v primary in series with the 10.8v secondary
and so the output is now 130.8 volts. Since the no load
voltage is 132 volts and the loaded voltage is 130.8 volts
we are now getting 99 percent of the no load voltage on
the output when the circuit is loaded, which is the same
as a -1 percent sag. This is not only acceptable, it's
very good, and it's much better than the -10 percent we saw
when the transformer was used as a 12v step down transformer.
The reason this works so good is that in the autotransformer mode
the stiff line is making up the bulk of the output voltage and that
doesnt sag much. In real life the line would sag a bit too and
that would affect the total output, but then that would affect the
output even with a perfect transformer with no losses at all.

[Robert Reed]

But MrAl - the primary was only designed for a one or two amp current in your hypothetical transformer. Nice regulation for a couple of seconds and then POOF!

[MrAl]

But MrAl - the primary was only designed for a one or two amp current in your hypothetical transformer. Nice regulation for a couple of seconds and then POOF!

Hi Robert,

Well, im not sure what would give you that idea as the primary does
not draw any more current than it would when it was hooked up in
a step down configuration (120vac to 12vac).

When the transformer is connected as a step down the primary
current is about 1 amp when the secondary is 10 amps (approx).
When it is connected as a boost autotransformer the primary current
is still 1 amp and the secondary current still 10 amps (for a 10 amp
load at approx 130 volts). The reason it works this way is because
in addition to the 12v secondary the line feeds the load, not the
primary. The primary only has to draw enough current to feed
the secondary, which means it's still going to be 1 amp (approx).

Note also...
In order for the primary to draw 10 amps the secondary would
have to draw 100 amps (10:1 voltage ratio, 1:10 current ratio).

[Robert Reed]

MrAL
Didn't mean to step on your toes. I misread your previous reply and this one cleared things up. You are absolutely right.

[MrAl]

Hi Robert,

Hey no problem bud. If i say something that isnt correct i like
to know about it so i can find out what went wrong. As i noted
in my post before i may not have gotten all the combinations
of windings and i did the voltage calculations kind of quick, so
there is always the possibility of an error. I believe that with
discussion these errors will surface and then they can be eliminated
so it's good to have feedback.