connecting primaries

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Hi,

following on from a previous AMS post:

I have never been 100% sure what happens if you parallel the two primary
windings of a mains transformer in anti-phase. My instinct says a big bang,
but theoretically, won't both EMF's just oppose each other and you get
nothing?


Cheers,

Gareth.

Archived from groups: rec.audio.pro (More info?)

Gareth Magennis <sound.service@btconnect.com> wrote:
>
>I have never been 100% sure what happens if you parallel the two primary
>windings of a mains transformer in anti-phase. My instinct says a big bang,
>but theoretically, won't both EMF's just oppose each other and you get
>nothing?

Right, you get nothing. It's not very exciting.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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In article <3f250vF5lkm8U1@individual.net> philallison@tpg.com.au writes:

> >>I have never been 100% sure what happens if you parallel the two primary
> >>windings of a mains transformer in anti-phase.

> Depending on the VA rating of the transformer, you will blow the AC fuse
> OR have a fire on your hands real quick.

The purpose of a fuse is to prevent fires. If the unit is properly
fused for the current rating of the wires and transformer windings,
the fuse will blow and no fire will occur. If the unit is not fused,
there's likely to be a fire at some time in its life anyway, no matter
how the transformer is wired. Ohm's Law hasn't been repealed yet, as
far as I know.

I believe that you may be suggesting that if the windings are
connected in parallel "backwards" they'll draw more current from the
source than if they were connected for proper operation of the
transformer. If this is the reason for the potential disaster, please
explain (EM theory is OK with me) why this would be so.



--
I'm really Mike Rivers (mrivers@d-and-d.com)
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo

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In article <znr1116510689k@trad>, Mike Rivers <mrivers@d-and-d.com> wrote:
>
>
>I believe that you may be suggesting that if the windings are
>connected in parallel "backwards" they'll draw more current from the
>source than if they were connected for proper operation of the
>transformer. If this is the reason for the potential disaster, please
>explain (EM theory is OK with me) why this would be so.

I don't know, but I have accidentally done it and the described overcurrent
situation didn't happen. I think Phil's argument is that it basically will
act like a shorted secondary, which makes sense. In a perfect transformer
that -would- cause a fire.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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<bob_miller@spammenot.agilent.com> wrote:
>In article <d6idle$nl7$1@panix2.panix.com> Scott Dorsey <kludge@panix.com> wrote:
>
>: I don't know, but I have accidentally done it and the described overcurrent
>: situation didn't happen...
>
>Sure you were parallel'ing the reversed winding? Backwards on a series pair
>would be very boring indeed.

I was indeed, because I was changing from 230 to 120V operation and swapped
one of the pairs.

>I will readily admit to never having done this, so there is a non-zero chance
>I'm talking out of my ass. Anybody want to try it (with a protection fuse,
>please)...? I recommend using the panel breaker to energize it to avoid
>welding the power switch or the AC receptacle you plug into ;-)

Admittedly I was using one of the Talema toroids, which may well be
impedance-protected. I do know that the Talema toroids have a few
behaviours that are not typical of most filament transformers, like
a strong high-end rolloff above 200 Hz or so.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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bob_miller@agilent.com

>
> Hmm. You have a point. Any transformer that can gracefully handle a
> shorted
> secondary can probably survive just fine with a reversed primary winding.


** That is a stupid and totally wrong belief.

So called "impedance protected" transformers have loose coupling between the
primary and secondary - usually achieved by placing the secondary coil
away from the primary instead of winding it over the top, deliberately
increasing leakage inductance and so reducing the current flow if shorted.

Where split primaries for 120/240 operation are used, the two will likely
be wound one over the other and so magnetically close coupled - just like
a standard AC transformer.



> Given that a lot of appliance transformers are operated near enough to
> saturation to be already fairly lossy (saves mfg. cost) it probably could
> take a while to heat up enough to damage it anyway.


** Another totally false conclusion.

Operation close to core saturation indicates the minimum possible number of
turns have been used to create the primary - this reduces copper losses
( less resistance) and makes for a smaller transformer for a given VA
rating.

A real example:

I have a 15 VA e-core tranny on the bench, made in the USA by Pacific
Transformer of California.

The two primaries are rated for 120 volts @t 50 Hz and may be wired in
series or parallel.

One primary measures 202 ohms, the other 185 ohms - indicating the former
is wound on top of the latter.

If wired in parallel and anti-phase, all magnetic induction at 50 Hz
disappears and only the 96 ohms of resistance remains.

With a 120 volt supply, 96 ohms draws 1.25 amps.

The heat dissipation is therefore 150 watts, around 100 times the safe
normal.

Self destruction is guaranteed in a few seconds.

The situation is IDENTICAL to applying 120 volts **DC** to the primary
!!!!




............ Phil