Older Miller lectro spot model 11 spot welder

[Aeronca41]

I'm not tripping the breaker on the 15 amp circuit. Would my poor weld quality be explained by the 15 amp circuit? Is there such a thing as a 30 amp 110V circuit? I'm just aware of 15 amp and 20 amp.

Yes, there is such a thing as a 110v 30A circuit. You would probably begin to trip the 15 A breaker if you did a series of spot welds. I would certainly wire up a 30 A circuit with #10 and see if it helps. The owner manual link that H80N provided says your machine needs a 30A circuit. It would have a 30A single pole breaker and would be wired with #10 wire. They generally have a special outlet, NEMA 5-30. Your welder has a 5-15 series plug on it which allows you to plug it into a 5-15 or 5-20 outlet( standard home outlets). The 5-30 outlet has one prong "bent" at the top (Google nema 5-30) which I don't think will accept your 5-15 plug. But back when your welder was built they weren't so particular about such things, and may have expected the welder power circuit to just have a 5-20 receptacle. Hard to say what the thought process was behind the 5-15 plug with a 30A circuit requirement. Probably perfectly legit based on special allowances for welders due to non-continuous duty cycle in the NEC; I assume there was something like today's Article 630 in the code back then.

Also, as H80N and aametalmaster said, be sure your tips are not too large on the end. The manual calls for 1/8" diameter. Any larger and you will be losing heat concentration resulting in poor welds.

[miller_130xp]

Update: The manual reads 3.1 kva for 15% duty and 1.5 kva for 50% duty. So 15% duty would be 26.9 amps. If I understand correctly the duty effects the amount of time I have to let the machine rest. If I go with a 30 amp circuit and get 3.1 kva will that improve the spot welds?

The data plate reads 115volts, 1.5kva and open circuit volts 1.6. Using this calculator and 115 and 1,5 as inputs it works out to 13 amps. This is well within 15 amps. http://www.rapidtables.com/calc/elec...Calculator.htm

[Aeronca41]

You are completely correct, but I wonder why the manual specifies a #10, 30A circuit. My first thought is that there is a very large, but relatively short, current spike when you first make contact to weld that generates a lot of the heat. Would love to have a scope with a fast current probe on the input power line. However, thinking further, the transformer inductance would probably cause an exponential rise waveform rather that a very sharp pulse. Hard to say without some more data and calculations. Considering how simple these machines are, there really isn't much to go wrong on the weld side; any complexity is in the timer. I still suspect the contactor (relay). Do you have electrical and safety knowledge to work with live circuits?

[miller_130xp]

I have basic electrical and safety knowledge working on live circuits. For tools I have a multimeter.

Here is a photo of the timer and the two vacuum tubes.





Here is a photo of the 6c4 vacuum tube with the metal protective cover off.




Here is by far the best spot weld I was able to produce. The plugs for the other welds are significantly smaller in diameter.

[Aeronca41]

Can't tell from the pic if the contactor has a 2 or 3 contacts wired in parallel, or a single set. Can you see the contacts; are they clean, and is the contactor pulling in solidly when you start welding? Does it stay solidly in for the whole time? If there is any question, and If you're safe doing it, I would put the two voltmeter meter leads across each pair of contacts in turn (if there are multiple sets) on the contactor and look for zero volts when the contactor is closed and you are welding. Also conduct a very thorough search of the weld circuit's connections-wires, terminals, crimps, etc. this just sounds like you're not getting full power to the tongs for some reason.

[miller_130xp]

which part is the contractor? is that the relay part number 90 in the manual?

[Aeronca41]

Yes. 90 is the relay. It is a single pole (only one set of contacts). If you look at the schematic diagram on page 4 of the manual, the contacts are shown in the lower left side, labeled CR1. The coil for it is shown near the lower left as a circle with CR1 in it.