Dual Primary Windings

[4krow]

   Every so often, I see a power transformer that has 'dual primary windings'. Ok, so that is to make it compatible with 110-220 volt AC.  So far, I get it. Then, they are very specific, or NOT as to what the wiring combination is if you choose 110 volt. It seems that you might wire the two primary windings in parallel, but I assure you, that is a mistake. And I have received the instruction that when wiring the two windings together (in parallel), make sure that they the correct 'polarity', i.e. red to red, black to black. Again, all this good advice is false, as an overheated transformer taught me recently. Now, having said all of this, I am thinking that I will be just fine by using only ONE primary winding, and capping off the second winding, for safety's sake.  Correct? I'm thinking that this makes the best sense.

[Doc B.]

Not enough information. You need to ask the transformer manufacturer about your specific application, as the primaries can be wound and tapped in different ways and overheating can be caused by other things in the circuit.

[mcandmar]

As above, the answer depends on the transformer in question but i cant imagine any scenario where you would leave one primary disconnected, they should always be wired in series or in parallel depending on your line voltage.  They key is wiring them in the correct phase in relation to each other, which in some cases is not marked or obvious in any way, you will just have to measure them to figure it out.

e.g. i have come across some vintage iron with four black wires on the primary, a simple resistance check with tell you which pair is each winding, then i use a small low voltage ac supply or variac and measure the secondary voltage with them joined each way around, the one with the higher output voltage is correct.   If you wire them out of phase you will get very little output as i believe the two windings just cancel each other out and heat up the core.  They key is a low voltage, dumping full mains into an incorrectly wired transformer will probably kill it.

[4krow]

That's good idea about using a low voltage AC supply. Now I can try the wiring without worrying about instant death of the tranny. Thanks

[Paul Birkeland]

It seems that you might wire the two primary windings in parallel, but I assure you, that is a mistake. And I have received the instruction that when wiring the two windings together (in parallel), make sure that they the correct 'polarity', i.e. red to red, black to black. Again, all this good advice is false, as an overheated transformer taught me recently.

Hmm, you should double check the wiring diagram and the ratings of the transformer.  This is almost always how you wire a dual primary transformer for 110V operation.  (For example, the Stereomour II, Seductor, and Kaiju all have this arrangement for 110-125V wiring)

Now, having said all of this, I am thinking that I will be just fine by using only ONE primary winding, and capping off the second winding, for safety's sake.  Correct? I'm thinking that this makes the best sense.

No, this is bad advice based on false premises.  This will cause loading issues and damage your transformer, as you need the rest of that copper on the primary. 

[4krow]

Well, As it turns out, the wiring is fine after establishing the higher voltage on the secondary side, per the polarity of the primary windings. Good common sense advice about using a low AC voltage to test this in the first place. Why didn't I think of that? So, no scorched transformer. It will be easier for me in the future just to use a 120 volt transformer in the first place, or at least have knowledge from the manufacturer as to the proper primary winding.

  Here is a shot of the nearly completed project: