I am 100% certain there used to be toroidal cores (50/60Hz electrical steel, not ferrite) available on ebay and/or Amazon, and 80% certain there were also cores available which had a primary winding but no secondary. A "wind your own secondary" type of thing. I need that now but can't find it anywhere on the internet or even just the cores. Maybe I've forgotten the right combination of words to search. Any leads?

 

Toroidal coil ?

----> https://www.ecosia.org/images?q=toroidal coil#id=A6E3277F73282FE9766FB9A9B6690A4784CE5EF0

----> https://www.nicore.com.cn/toroidal-core-html-2

 

Toroidal coil ?

----> https://www.ecosia.org/images?q=toroidal coil#id=A6E3277F73282FE9766FB9A9B6690A4784CE5EF0

These are ferrite core inductors, would not make good mains transformers. Needs to be silicon steel/electrical steel, like these:

----> https://www.nicore.com.cn/toroidal-core-html-2

But this seems like one of those "fill out inquiry form with how many thousand units per month you need and if your answer excites us we might respond, otherwise screw you"-type of scenarios.

I am looking to buy one or a few, no more than 5. 1.5kVA, 120V primary. This something you could simply order online before, PayPal or CC; no inquiry, no RFQ, quote, PO, "setting you up in our system" nonsense. If I have to go through that rigmarole I will, but would much rather just order the thing and get a tracking number.

 

This guy custom builds them and sells them complete also, he may oblige you!
https://www.antekinc.com/transformers/,

 

I am 100% certain there used to be toroidal cores (50/60Hz electrical steel, not ferrite) available on ebay and/or Amazon, and 80% certain there were also cores available which had a primary winding but no secondary. A "wind your own secondary" type of thing. I need that now but can't find it anywhere on the internet or even just the cores. Maybe I've forgotten the right combination of words to search. Any leads?

AIrlink Transformers makes transformers to order, and supplies cores, so I should think they would supply a single winding transformer. (After all, if it is a single winding with 110V and 230V taps, it's an autotransformer.)

But winding toroids is fiddly, and winding a single winding makes it a "special" with a higher prices, so why not get the secondary wound as well?

If you want toroidal inductors on gapped cores, or sectional windings with specified leakage inductances, then https://www.tigertoroids.co.uk/

 

AIrlink Transformers makes transformers to order, and supplies cores, so I should think they would supply a single winding transformer. (After all, if it is a single winding with 110V and 230V taps, it's an autotransformer.)

Thanks!

But winding toroids is fiddly, and winding a single winding makes it a "special" with a higher prices, so why not get the secondary wound as well?

The application is an existing hot wire cutter that is powered from a variac. Looking to use a thicker wire because existing wire is too fragile. I want to play with different sized wires to find something that is ideal, so I don't know my requirements. I would just rewind a MOT if this were my own one-off pet project but it's for work and there are multiple machines so it needs to be repeatable and obtainable. I need to provide instructions for replacement like "purchase transformer part number 123-4567-89 and pass 14 turns of 8AWG wire through the center." It would be a bit much to say "purchase obsolete GE microwave model #12-3456 and remove transformer, rewind bla bla bla."

Though I suppose I could use a MOT for experimentation, find the appropriate voltage output, then order transformers with that output.

 

An interesting option is one of the variable devices like a "Variac " or a "Powerstat", which have a toroid with a variable tap. I have a couple of 230 volt ones that have burned sectionstoward one end. So they can be cleaned up, the burned segment removed, and used as the primary for your high current secondary. It would take a bit of effort, and shipping will cost because they weigh a bit over ten pounds. You might find a reasonably priced similar device as surplus closer by..

 

I have some of these that need a home. https://www.bridgeportmagnetics.com/bmg-product/o-cores/
No windings at all.

 

The application is an existing hot wire cutter that is powered from a variac. Looking to use a thicker wire because existing wire is too fragile.

I used for hot wires and hot knives such transformer:
= = = = = = = = = = = = = =
RS12-300 (12V/300W) HATCH Halogen Lighting Transformer
• Dimensions (L x W x H): 6.41" x 1.70" x 1.27"
• Input: 120V AC - 50/60 Hz
• Output: 11.5V/AC
• Enclosure Material: Metal
• Weight: 12.8 oz.
• Power Factor: > 0.95
• Operating Frequency: 25KHz
• Works with most commercially available dimmers
= = = = = = = = = = = = = =
Info about: https://forum.allaboutcircuits.com/threads/5-v-80-a-ac-power.186273/post-1725854

 

I have some of these that need a home. https://www.bridgeportmagnetics.com/bmg-product/o-cores/
No windings at all.

How many do you have and what VA rating are they?

 

An interesting option is one of the variable devices like a "Variac " or a "Powerstat", which have a toroid with a variable tap. I have a couple of 230 volt ones that have burned sectionstoward one end. So they can be cleaned up, the burned segment removed, and used as the primary for your high current secondary. It would take a bit of effort, and shipping will cost because they weigh a bit over ten pounds. You might find a reasonably priced similar device as surplus closer by..

The first time I read this, I misunderstood you. But I get it now. Thanks for the suggestion; the more I think about it, this is actually starting to seem like maybe my best option. Variacs can be easily ordered, even up to 1500VA that I'm shooting for, and if the wiper is removed and power connected to the ends, it's pretty much exactly what I asked for; a toroidal transformer with a primary but no secondary. And it even comes with a handy-dandy ventilated enclosure (Assuming it still fits with heavy gauge wire added).

 

I would have though any off the shelf toroid of the required Va with a LV secondary could easily be used, if necessary.
The secondary is often just a few turns and very easy to remove. !

 

Probably a toroid transformer with a six or twelve volt secondary and the desired wattage rating would be the cheap way to go.And probably there will be enough room for a few added turns already. The new price for a variac will be much more because of the brush assembly and the case. But damaged ones will be a lot cheaper..
Do you know the voltage across your heated wire presently in use??

 

do you really need partially completed transformer or you just cannot find the ready made one for your specs? what are the specs btw?

 

do you really need partially completed transformer or you just cannot find the ready made one for your specs? what are the specs btw?

Specs are TBD following experiments with thicker wire. Here's what I know: existing hot wires are controlled by 1.5kVA variacs and they can't handle the current of the existing wires, which are too thin and fragile. The only reason (i think) these variac are smoking is because they are autotransformers, using the same gauge wire throughout. When the last one smoked I had an amp clamp on the input and it was under 5A so the VA rating isn't the issue, just wire gauge.

I have a broken variac to play with, wrap a secondary on it, do my experiments, develop my specs, and from there you're right, I can just order proper transformers.

 

OK, and one thing you need to know is that the very weakest point of a variac is the brush contact point. So if you can avoid current in that weal point you will do all right. What sort of wires are being used?What metal alloy? Some alloys have mucg greater hot strength than others. ALSO consider that there may be other choices. Water Jet cutting is one and laser beam cutting is another,So the question becomes just what sort of cutting is to be done and how fast.Laser cutters cut metal at fairly high speeds, and the water jet I saw was cutting several inches per second. And water is a non-polluting material, chep to purchase.

 

OK, and one thing you need to know is that the very weakest point of a variac is the brush contact point. So if you can avoid current in that weal point you will do all right.

I'm planning to keep the existing variac in place but instead of going straight to the hot wire it will go to the new transformer, and from that transformer to the wire. This way the variac will be set somewhere between 50% and 100% instead of between 0% and 20%

What sort of wires are being used?What metal alloy? Some alloys have mucg greater hot strength than others.

Right now I think it's just steel wire, ("bailing wire"). I would expect it to be kanthal or nichrome or similar; that's probably part of the problem. I intend to test those alloys too, part of why I expect a different operating voltage will be necessary.

So the question becomes just what sort of cutting is to be done and how fast.Laser cutters cut metal at fairly high speeds, and the water jet I saw was cutting several inches per second. And water is a non-polluting material, chep to purchase.

The application is a "topsheeter." A roller conveyor is under the machine and as pallets pass beneath it, it unrolls a 4-5ft wide plastic sheet and then the wire heats up and is advanced into the payed-out sheet, cutting it. Machines by other manufacturers use a razor on a linear bearing to accomplish this task, which works great until the razor is dull. The hot wire would be a better solution if the issue of weak wire is solved. The secondary issue of burning plastic smell might be able to be addressed too; if the wire can be made hot enough to quickly zip through the plastic instead of pausing against it, slowly melting through it and getting covered in molten plastic ooze.

 

Definitely Nichrome wire will be a better choice, and probably a scheme to have the wire fully heated before it touches the plastic sheet. but one other option for the knife scheme is a rotary blade, like a pizza cutter disc, except spun by a motor. That scheme can easily have an automated sharpening after some number of cuts.
The hot wire scheme will probably have some issues with both the health vapor control police and the fire safety folks. But at least you will be able to avoid burning out variable transformers. One other option is a switching-mode power supply with an adjustable voltage.

 

Definitely Nichrome wire will be a better choice, and probably a scheme to have the wire fully heated before it touches the plastic sheet. but one other option for the knife scheme is a rotary blade, like a pizza cutter disc, except spun by a motor. That scheme can easily have an automated sharpening after some number of cuts.
The hot wire scheme will probably have some issues with both the health vapor control police and the fire safety folks. But at least you will be able to avoid burning out variable transformers. One other option is a switching-mode power supply with an adjustable voltage.

In the past I have used phase angle power controls with current feedback, closed loop for constant current mode for hot wire applications. This works well. CC applies low voltage in the beginning when the wire is cold and resistance low; as it heats up and resistance increases, the voltage does as well. This prevents overloading the transformer initially. Probably DC SMPS in CC mode would work well too.

But this is not my design. This is production equipment that has already been in service for years, and has always had issues with wire longevity and burning out variacs. I can't afford to take it out of service for the amount of time it would take to implement a superior solution so something like the pizza cutter is definitely off the table.

Putting a transformer between the variac and the hot wire seems to me like the quickest, least invasive option.

 

I suggest trying actual nichrome wire of a size anticipated to be strong enough. And for an experiment you could power a similar length of wire in a test not associated with the machine and observe the voltage required to produce the desired temperature. That would help select the rating of the transformer you need to purchase without any guesswork. And it will avoid disrupting the machine operation.