Take apart entire transformer. Examine core construction. If core has shorting welds on it, forget it.
Better off going to an industrial liquidator or such and buying an actual AC transformer, and winding your welding winding onto that transformer.They are meant to convert AC line voltage and frequency to other AC voltages with maximum efficiency. A microwave oven transformer is not made for any such purpose.
I don't understand your comment about "circulating magnetic shunt power", since MOT's are not being used in ferroresonant applications. However, I can understand heating from excessive core hysteresis losses because of the high peak flux levels used in these transformers.
In MOTS, NST's, and Class 2 and 3 transformers, magnetic shunts merely alter the transformer's magnetic circuit and should not significantly impact no-load losses one way or another. Proof of this can be seen in lower operating temperatures by simply adding more turns to the primary winding. This allow the core to operate at a lower peak (non-saturating) flux density, and a lower magnetizing current. Oven manufacturers push their designs in order to save on copper or silicon steel costs. With a fixed core cross sectional area, increasing primary turns would also require that they proportionally increase secondary turns to reduce core losses - adding costs and potentially reducing margins and competitiveness.
A shunted transformer does not HAVE to run hot and, in fact, most Class
2 or 3 transformers don't. But a cheaply made transformer that partially saturates the core by design (whether shunted or not) WILL run hot...Bert
08_06....
ok they actually INCREASE the leakage inductance on purpose.. thanks Mark
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The cheapest, best route is to simply go to the car parts store, or Harbor Freight, and buy a cheap arc welding outfit. Doh!
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Its a way cheap way to limit the voltage in a high line case.
Newer ones use switches because the price of copper went above the price of transistors.
Some years back I ran into a TV that was about the first to use a switcher instead of a transformer to make the voltages. The switcher ran at the Horz sweep frequency because, it was in fact also the Horz sweep circuit.
It worked fine unless the line voltage changed. The picture changed size a bit when the line voltage changed.
[snip]
Cheaper still, take your project to a sheet metal fab shop and have them weld it. No! Better yet, don't build anything. Buy it all at WalMart.
Some people just like to roll their own.
The capacitor is there because the circuit is a half wave doubler, with the magnetron as the second diode. They do use the size of the capacitor to limit the current/power.
Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)
Although I think I understand what you're trying to say, the only thing in your description that doesn't apply to a MOT is the "maximum efficiency" part, which for a spot welder in occasional use for a second or two, doesn't matter much at all. The main advantage of using a surplus transformer that is not from a microwave oven is that it might be available in a larger core size (so more power to the weld) and that the core is less likely to be welded together so it is easier to rewind or alter.
Chris
I wouldn't be surprised if 5A were normal. Did you leave in the magnetic shunts? The high magnetizing current plus the shunts make it practically short-circuit proof.
If it's only "fairly warm" after 10 minutes, I'd go ahead and make the welder - you'll only be running it a for few seconds at a time, after all.
Have Fun! Rich
Yup!
Don't worry - if there were a shorted winding, it would have already blown up. ;-)
Cheers! Rich
Transformer cores that operate efficiently, and "roll your own" do not generally go together well with laymen. It takes someone that knows what the hell is going on to go out and source the right materials to "roll your own" transformer at that power level, and not pay twice over for the power required to use it.
Again... BEST BET is to find an old transformer that has not rusted itself to the ground, and unwind the secondary, and add your own, high current, low turns secondary. The whole thing would have to be fed by a variac if he actually wanted to have an adjustable current output rate.
Its not like making a efficient transformer for mains frequency use is any sort of rocket science. The hardest part is likely to be finding the core material in the small quantities a hobbiest needs. Most of the production of transformers is offshore these days.
Once the material question is solved, the transformer design is well within the grasp of even an experienced electronics technician or hobbiest. It is a bit out of the reach of someone who just now built his first a crystal radio.
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As others have said, the no-secondary-current condition is not normal operation, and may (due to core saturation) draw excessive current. It isn't gonna kill your project, just ignore it. When the secondary is putting a few hundred watts into your spot weld, the saturation won't happen.
For testing, it might be prudent to put a ballast resistor (like
100W incandescent light bulb) in series with the AC input.For best welding, consider using a one-shot timer to deliver an exact duration of power to the primary. Something that sets from 0.1 second to 10.0 seconds with thumbwheels might be very useful, and these are common surplus items.
Gosh, you made me curious. Just how does the magnetron act as the second diode of the doubler of its own cathode-anode supply? Circuit please.
Here's the basic circuit:
|| +------------------------+ ||( 3.3 VAC, 10 A, typical | TP Relay or || +------------+------+ | Magnetron _ Fuse I __ Triac || | +-|----|-+ o------- _---+---/ -- ----/ ----+ || +------||----+ | |_ _| | | )||( HV Cap | | \\/ | AC I \\ I=Interlock )||( __|__ | ___ | Line | TP=Thermal Prot. )||( 2,000 VAC _\\_/_ +----|:--+ o------------+-------------------+ ||( .25 A | HV |'--> Micro- ||( typical | Diode | waves (Controller not shown) || +------------+---------+ _|_ - Chassis ground
A fair description is here:
Ok i see the voltage boost, but it is not what i ordinarily consider a doubler topology. More of a funky half wave rectifier with a half wave load.
Model the "half wave load" as a perfect diode with a series resistor and it looks like the standard half wave doubler circuit.
Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)
I think the magnetron would be more correctly modeled as a Zener in series with a resistance, since it requires about 3KV anode-cathode potential before it will conduct.
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=A0 =A0|| +------------------------+
=A0 =A0||( 3.3 VAC, 10 A, typical =A0|
------------+------+ =A0 =A0| Magnetron
=A0 =A0 =A0 =A0 =A0| =A0 =A0+-|----|-+
_ =A0_| |
=A0)||( HV Cap =A0 =A0 =A0| =A0 =A0| =A0 \\/ =A0 |
=A0 =A0 =A0 __|__ =A0| =A0 ___ =A0|
_\\_/_ =A0+----|:--+
=A0 =A0|'--> Micro-
=A0 =A0||( typical =A0 =A0 | Diode =A0 | =A0 =A0 =A0waves
--------+
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 _|_
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0- =A0Chassis ground
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A magnetron will conduct for any positive voltage. It just doesn't conduct very much at low voltages. The magnet causes the electrons to spiral out from the cathode. It doesn't stop the outwards travel.
The voltage on the tube controls the gain. There is a threshold of oscillation where the gain becomes enough for the RF to start being made. This makes the duty cycle on the RF generation below 50%
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