single to 3phase conversion

[Ed_Dinning]

Hi Folks, talking to some members at Golborne today who had built 1 to 3phase converters after my talk in April, it seems that there is a great difficulty in finding suitable transformers in scrap yards.

One member had used a welder transformer and was experiencing difficulties getting the system to work. This is because most welder transformers have an airgap of some sort in the iron of the transformer. This will prevent correct operation of the transformer and give it a "falling voltage" characteristic, idea for a welder, but not this application.

The ideal type is from a "yellowpot" safety or isolating transformer if it can be re-built and does not have a welded lamination stack. A welding transformer can be used if it is possible to re-build the lams without a gap.

Please feel free to mail me if you are having problems.


Ed

[Yorkie]

On a similar tack (transformer lams), many years ago when radio amateurs often actually built things, I recall that we used to use microwave oven transformers in which the secondary had burnt out and strip off all the old wiring and wind on 1.5mm (house wiring wire) to create a low voltage transformer for 13.5V power supplies. As I recall, it was about 1 turn per Volt, so only needed about 15 or so turns to get 15V. What I couldn't understand at the time, and still don't, is that the lams had weld runs across them to weld them together. I guess in part to stop them vibrating, but with my slim to non-existent knowledge of transformers, I couldn't understand why that didn't create a shorted turn. Obviously it doesn't, but I know for example, of people who have inadvertently created a shorted turn on a toroid transformer by allowing the top of the securing bolt to come into contact with the metal chassis to which the other end of the bolt is screw, with predictable consequences.

All of this only serves to remind me to steer clear of single/3-Phase conversion and to realise what an impressive job Colin ('retired') did of his set-up, which I've seen in operation!

[Ed_Dinning]

Hi David, yes microwave transformers were a good source of a core and the turns required were minimal as the primary was already wound.
TIG welded lams were a cheap production technique as the lams were all stacked in one direction, as they came from the box, without the bother of interleaving them.
The weld, which was on the outer corners only, did constitute a shorted turn; but it was in an area of low flux so the increase in losses was pretty small in practice.

Cheers, Ed

[Yorkie]

Thanks for the explanation Ed.

Considering what's in them, (transformer, magnetron, timer, cabinet, turntable motor etc) I can't believe that brand new microwaves can now be had for £35 or thereabouts.

Take off the VAT, the retailer's profit, the trip on a slow boat from China, the manufacturer's profit, I wonder what price the leave the factory gates for!?

[Skywave]

Bearing in mind that what appears in most of this Forum is readily available for anyone to view on the 'Net, I would just like to remind / inform anyone who is reading this that playing around with microwave oven transformers can be very dangerous. That high-voltage secondary winding is capable of passing enough current at that high voltage to kill you, should you come into contact with the lead-outs of that secondary winding.

Some of us are well aware of that fact - but I'm also sure that there will be some readers of this thread who are not.

Al. / Nov. 05, '13 //

[Yorkie]

Bearing in mind that what appears in most of this Forum is readily available for anyone to view on the 'Net, I would just like to remind / inform anyone who is reading this that playing around with microwave oven transformers can be very dangerous. That high-voltage secondary winding is capable of passing enough current at that high voltage to kill you, should you come into contact with the lead-outs of that secondary winding.

Some of us are well aware of that fact - but I'm also sure that there will be some readers of this thread who are not.

Al. / Nov. 05, '13 //

All forums have rules and anyone who views this forum, be it as a guest or as a member, should first read the rules and the disclaimer, rule 5 of which states:


5) Safety first:

When considering the information found in the forums, please remember that essential safety information may be omitted. Also, the information supplied has not been checked for accuracy. Electrical and electronic equipment can contain very high voltages that can cause serious loss or injury, or even death. If in any doubt, please verify the information and your planned course of action with a qualified third party. And, read the disclaimer.

Thus, anyone who is not aware of such dangers jolly well should be, and have not read the rules and disclaimer, which are clear, unambiguous, and succinct. Anyone who disregards that sensible advice, and is for example, idiotic enough to connect a microwave transformer or any other transformer or electric device to the mains, and then get hold of it, would be on a mission to prove Darwin's theory of the non-survival of the stupidest. As such, they're beyond redemption.

Or so it seems to me.

As this is primarily a vintage radio restoration forum, almost everything we deal with has at least mains voltage inside, or if TVs, 'scopes etc, EHT, in which case you don't actually have to physically touch connections to get a potentially lethal shock. If the Admin Team or others have serious concerns that guests or inexperienced members of the forum haven't read the rules and/or may not be aware of such risks, then rather than having to keep highlighting the risks in threads, maybe a prominent statement could be made at the top of the forum welcome page?

Just a thought - I don't personally have any concerns. Indeed, I'd be more concerned if forum members became so mindful of the risks posed to inept idiots that they were deterred from posting threads on potentially dangerous topics, which is a high proportion of the topics we discuss.

I hope that doesn't sound sniffy, flippant, dismissive or a rant - it isn't meant to.

[Joe]

Well said, David.

- Joe

[Retired]

Hi,

Many thanks David for your kind comments regarding my home made 3 phase.

I still use my home made 3 phase most days and so far it's been 100% reliable. I used a big welding transformer as a donor for the lamination's but these lamination's weren't welded. I've not come across welded lamination's but I would have thought an angle grinder could be used to remove the welds if deemed necessary?

I wouldn't recommend a novice to play around with mains electricity as it could be the last thing they do play around with. having said this I'm from a mechanical engineering background and am self taught regarding things electrical learning on a need to know basis but where mains electricity is concerned I'm extremely careful. Whilst winding on the primary on my transformer and adjusting the number of turns I tripped the mains 30A breaker a number of times which is rather disturbing.

Check everything is actually dead by switching off and disconnecting where possible from the supply before touching anything; unplug or pull fuses but be certain it is safe; in my case as I was unsure I took many multimeter readings; all it takes is one mistake to kill and above all whilst doing this kind of work avoid interruptions; how many have broken off mid way then returned to the job thinking it to be isolated?

Please be careful.

Kind regards, Col.

[Yorkie]

...all it takes is one mistake to kill and above all whilst doing this kind of work avoid interruptions; how many have broken off mid way then returned to the job thinking it to be isolated?

Please be careful.

Kind regards, Col.

Wise advice Colin - I've only had one nasty shock - luckily not from the mains but 500V from a cap reformer that I built. It was connected to a two-gang 13A socket outlet, and instead of pulling the plug out which I invariably do, I'd just switched off. But what had I switched off? The supply to my lathe, connected to the other socket!

I only briefly touched the voltage doubler, and boy did that liven me up. I can recall thinking 'hey - I'm too young to die and too old for nasty shocks'!! It's a lesson once learned that's never forgotten. I was really quite annoyed with myself because right back to my apprenticeship days I was always taught to do as much testing and fault finding by continuity tests with equipment isolated and I do that to this day as a matter of course. Most diagnostic and remedial work on radios can be carried out via continuity and resistance checks and only in the late stages need the mains be connected. I also use an isolating transformer when working on AC/DC radios.

[Skywave]

Oh dear! It wasn't my intention to upset anyone by my post and judging by the nature and length of your response, David, it seems that is exactly what I have managed to do. I also note that your post has Joe's approbation.

So, on the basis of those things, and the fact that my post was indeed superfluous, since our forum does indeed carry that warning message, I would like my post to be deleted please.

Thank you.

Al. / Nov. 6, '13 //