Calling all transformer gurus

The only way to tell is to estimate the # of turns on the primary them measure the inductance. Then wind enough turns on the secondary to get a good reading on your inductance meter. Compare the ration of the two to see if things are out of whack.

5A of magnetizing current seems rediculously high though.

"BobW"

** Reeeaaallyyyy ???

Shows just how TOTALLY CLUELESS you are - pal.

** Lotsa luck - pal.

ROTFL ....

** Sounds about right.

Microwave trannies were NEVER meant to be run with no load.

They are all purpose built, special design freaks among mains transformers.

** Wanker.

** Funny how microwave trannies ALL have fans blowing on them.

Why do ya think that is - eh ??

** Shame there is SFA current available from that one turn - even when shorted.

Got a clue what " leakage inductance " is ???

** Shows just how ** TOTALLY CLUELESS ** you are - pal.

Primary winding shorts have much the same effect as secondary winding ones.

** Ya don't saaay ????

( Wot a bloody hoot !!!! )

** Low primary L is not the real issue.

Own a variac ?

.... Phil

Get the volume 215 June 2008 Issue of Circuit Cellar and read the article on phase controlled resistive soldering using a rewound MO transformer. Author is Ed Nisley. PGS 30-35 He is getting 150-250 amps with his design. Actual title is the "Above the Ground Plane" column.

Since the things are common as rain, just get another one.

Steve Roberts

ng

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I have heard you can also make a spot welder from an old weller soldering gun. Put a pulse network on the input and change the heat tip to a clamping style tip as found on a spot welder.

Sorry to hear about your google group spam problem.........

al

I've rewound MOT's a few times & the secondary was always about 1v/turn. Implying that your primary is OK. The "fairly warm" might be relevant to diagnosing the primary, but it's not relevant to it's use as a spot welder - it's only on for a few seconds per weld.

My MOT spotwelder:

Bob

Your transformer is perfectly fine... for a microwave oven transformer. These are very cheaply made transformers - they use just enough copper and iron to allow them to do the task, but not enough to allow them to run efficiently, especially under no load, as are operating with the core in partial saturation. They are designed to be cheap, not efficient. However, you could add more turns to the primary or drive it at a lower voltage to reduce no-load current via a Variac or a buck transformer.

These transformers also use magnetic shunts between the primary and secondary windings. These may be left in to help limit short circuit current, or removed for higher output current and to give you a bit larger winding window.

Bert

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ng

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Define "fairly warm". Human body temperature is likely OK. Above that is likely a problem.

e

That isn't out of line for a transformer like this. Even with one shorted turn in primary, you would still see that so it doesn't mean a lot.

the

nt

The extra current drain will be as an in phase component. It is too bad you don't have a known good example to get the known good numbers from.

ing

The main clue is the "Q" or phase angle.

=3D

The primary doesn't have shorted turns. 5A into an unloaded MOT is about right, and 1V per secondary turn is, too. YMMV If you want to reduce the current, add some windings in phase with the existing primary. The added windings will reduce the current. You don't need a lot of turns - try 10. You can experiment. It will change your turns ratio and lower the secondary volts per turn.

For use as a spot welder, you probably don't need to cool the transformer - I'm thinking of a very low duty cycle. However, they do heat up, so you may want to blow a fan on the thing if you'll have it powered on for a long time.

There's a bunch of stuff on youtube from people who have done what you're doing, so you might find some information there.

Ed

Vague memory of the thread about those things that said that some of them are sooooo cheap that they're running saturated in normal operation.

Stick it on a variac and see if the current/voltage relationship gets a kink in it near normal line voltage.

Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)

Very nice. Your welder is mas macho!

From the other replies, I think my MOT is okay, too. I'm pretty sure I can get away with just using one because I only plan to weld very thin material.

Thanks for posting your pdf.

Bob

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Thanks, Bert. I've done a little more reading on these transformers and I think you're exactly right.

I only plan to weld very thin material so I'm going to leave the shunts in as this will, as you say, provide built-in current limiting without the need for an external ballast.

Bob

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Ed,

I think you're right about the 5A being normal for a MOT. Also, as you say, the duty cycle will be very low so there's no need to worry about long-term heating.

Yep, YouTube is where I got the idea from. I first saw the capacitive discharge welders that a few people had videos for. They'll do the job, too, but a bit more complicated than the MOT version of the spot welder.

Thanks.

Bob

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From more reading and from some other replies to my post, it seems that 5A is normal for these cheaply-built microwave oven transformers.

I had thought about creating a secondary and measuring its inductance to see if it jibed with the measured primary inductance (about 83mH). The only problem with this technique is that if there was a shorted primary winding, it might affect the inductance reading of the secondary.

However, as it turns out, there are "shunts" that are designed into these transformers in order to create an intentionally-large primary-to-secondary leakage inductance. This is done to provide built-in current limiting. So, the secondary inductance measuring technique probably *would* work because any short at the primary would be somewhat decoupled from the secondary.

Anyway, the 5A doesn't really bother me. I was just curious if it was normal or not. It appears that it is normal and life is good.

Thanks.

Bob

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=3D- Hide quoted text -

what is a magnetic shunt?

Mark

You should probably knock out the magnetic shunts, if you want to make good welds. It is possible to make sort of welds with relatively low currents if you don't clamp the electrodes hard, so that the resistance of the joint is high and the I^2*R loss in the joint is higher. But when the electrodes are deliberately not clamped hard, the resistance is unpredictable and so is the weld quality. Also you can't then do a second weld close to the first one, because the current will preferentially flow through the first weld. If you want reliable welds you need much higher clamping force and clean metal that is being joined, and then you need much more current because of the low (but predictable) resistance, in the range of 5000 Amps and more. To get this much current you'll have to do all you can to reduce the leakage inductance, e.g. knocking out the shunts, and also keep the secondary wiring very short and stout to keep the resistance low. If limiting the primary current requirement is something that has been concerning you, I think you probably need a bigger electrical supply with a higher rated circuit breaker.

Chris

A magnetic shunt in transformers often consists of small transformer laminations that purposely bypass (leaks) part of the magnetic flux that would otherwise link primary and secondary windings. Adding leakage flux has the effect of reducing the voltage regulation of the transformer - output voltage will decline significantly with increasing load, and output current will rise to a well-defined maximum when the transformer is short-circuited.

Although leakage flux (i.e., magnetic flux from one winding that does not link to the other winding) is usually undesirable in transformers, some types of transformers rely on it to limit short circuit current to safe/controlled levels. For example, transformers and ballasts used with arc or neon lighting, welding transformers, and all class 2 transformers rely on magnetic shuts to stabilize current flow when driving glow or arc discharge loads, and preventing transformer damage or overheating during prolonged or unexpected short circuits. Another example, Class 2 doorbell transformers, can accidentally be safely short-circuited indefinitely, while a normal transformer would quickly burn up.

A brief discussion and a diagram showing one method of adding magnetic shunts between primary and secondary windings in a transformer can be seen here:

Bert

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***************************************************
We specialize in UNIQUE items! Coins shrunk by huge
magnetic fields, Lichtenberg Figures (our "Captured
Lightning") and out of print technical Books. Visit
Stoneridge Engineering at http://www.teslamania.com***************************************************

In this case, i do not think you can determine if there is or isnot a shorted turn. I am airly certain that the transformer used is of a sola-like design, meaning an extra magnetic path that is resonant (note the capacitor) to give regulation - and in this application, current limiting. Think of it..the maggie sees all kinds of loading fomthe equivalent of an open thru semi-matched to almost short; that highly variable load is seenby the tranformer.

They may be "cheap", but the heat comes from the neceassary circulating magnetic shunt power...

Adding some turns might provide an added benefit: You can bring out some taps and fine tune the output power by selecting the appropriate one.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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