I have a 5 KVA toroidal transformer with a 208v primary and a 120v secondary. My goal is to convert the secondary to 240v center tapped. In it's original configuration, the secondary is wound with four separate wires. The Left 4 ends are crimped together to an output wire, and the Right 4 ends are crimped
together to the other output wire.

I want to separate the connections and rewire the coils in pairs with a center tap. The question I have is whether or not winding direction matters. I don't think so, but I wanted to be sure.

In the image below, I've just referenced one of the pairs for simplicity. Figure 2 shows the coils with one end conveniently joined to form the CT. The concern is that the direction from CT to Line oppose one another - See figure 1.



In the other way to form the CT, I could run a wire from the right side wires to the left side wires (Figure 3) which would keep the "flow" in one direction. But, does this actually matter in a mains power application? Efficiency is a concern in this case.

The second set of wires is going to parallel this first set, so if it's best they all just "flow" in the same direction, I would wire it like Figure 3. If it really doesn't matter, then I would go with Figure 2. What do you think?

 

Yes, it does. Windings of the same direction add together, windings of opposite direction "buck" each other, cancelling each other's voltage.

 

Figure 3 is the correct one.
If you use Figure 2, you will get exactly the same voltage on L1 and L2.

 

IOW the pair shown in fig 2, if wound in the same direction, the voltage between L1 & L2 will be zero, if the coils are identical..

 

You can always reverse the leads of one coil if the voltages cancel.

 

Got it guys. Thank you. I'll wire it so they run in the same direction.

 

Of course you need to understand that by splitting that secondary winding the current capacity is cut in half. Of course, if you are adding a center tap to feed a rectifier then it is OK, except for the insulation, which the four wires may not be adequately insulated from each other. So you might have a problem, or maybe not.

 

I was aware of the current change, it's not going to be a problem. I had also thought about the insulation, but I can't think of a way to test it to ensure it's going to be okay. I'm planning to bring the modified transformer up slowly with a variac, and take note of any peculiarities. If all goes well, I'll try it with some loads. Even if that works, there's still nothing saying that someday it might not just breakdown and fail. I hope it doesn't, but I have little control over that. It'll be in a metal box with suitable circuit breakers installed for safety.

 

If you are able to see the windings you could look for areas where the windings are much closer to each other. Those would be potential trouble spots. Are the windings varnished in place, or are the turns free to be moved? Or are they not even visible?

 

They are not visible, they're under layers of hard wrap. My guess is that all 4 wires were wrapped at the same time, parallel to each other. That would be the worst case, as you mentioned. It would be great if they individually wound them at different spots on the transformer. I don't have a means of sensing a signal injected into a winding, but I'm thinking I can use induction to hunt around the transformer and see it I can get a stronger voltage on a winding in any one particular area of the transformer. Maybe not, but it's worth a try.

 

They are not visible, they're under layers of hard wrap. My guess is that all 4 wires were wrapped at the same time, parallel to each other. That would be the worst case,

I would think that is pretty much the case from my experience with modifying transformers, it ensures that all windings have equal properties. rather that wound after each other.
At least the toroidal versions are generally easier to work with.

 

I am asking because I have two powerstat variable transformers that have a damaged section of winding. I also have two additional ones that are OK, so I am wondering about removing the damaged turns and adding a secondary winding, to use it as a power transformer. But not wanting to hijack the thread I will not go farther.

 

In my experience so far, have not seen a Toroidal that was varnished or dipped, generally the windings are just in their natural enameled state as wound.
.

 

Have you seen toroidal center tapped windings before? If so, were the windings on each side of the CT separated from each other with additional insulation efforts or did they rely on just the wire's enamel?

 

The ones I have seen were wound bifilar, i.e. together in order to achieve identical results.
Just the enamel insulation was used, no shellac etc.

 

Good to hear. That gives me more hope for my transformer. Thank you.

 

Presently I am dealing with two rather expensive transformers that have 120/240 connection options on the primaries. I also am a bit concerned because when operated on 240 volts it seems that there will be 120 volts between adjacent strands. That is not my favorite condition.