Centerless Transformers and Power Return

Open delta connection. Same as your stove!!

http://en.wikipedia.org/wiki/Two-phase_electric_power

Two-phase power can be derived from a three-phase source using two transformers in a Scott connection.

Your welding set is operating the same principle as the source of generation. The power supply that delivery to your household come from a transformer at the generation sub station. The hot wire will run thru your load (lamp, Air Cond, TV, and e.t.c.) and the neutral or common line is ostensibly there to complete the circuit back to the source of generation.

Your welding set consist of a transformer where the secondary side, the output is a low voltage output but with high ampere. It has 2 wires, where one of the wires is a hot wire and the other is the earth wire. Actually, the earth wires, is a neutral wire that return back to the transformer. The earth wire will be grounded to the metal parts where you want to do the welding works.

Your metal plate will be come the "load" as these metal plate have certain amount of resistance in it. When you do the welding works, the hot wire, will delivery the current thru your electrode and thru the metal plate. As soon your welding electrode put close to the metal plate (without touching it), the short circuit will occurs and the high current "jumps" across to the metal plate and return back to the transformer.

This creates a high arc and melts the electrode to the metal plate and the diffusion of the material started.

Grounding for the welding set is done at the primary side and not the secondary side. But one importance factors to keep in mind is that,

1) Do Not put your welding set on top of the metal plates where you want to do the welding.

2) Make sure that your welding set earth wire is grounded at the nearest point to the place where you want to do the welding works.

I have seen many earth wire burnt (earth wire at the primary side) due to the welding earth wire were NOT properly grounded. Most of them grounded the welding earth wire near to the welding set and do the welding few meters away and the welding current are flowing everywhere.

Hi Simon,

"The power supply that delivery to your household come from a transformer at the generation sub station."

The substation provides high voltage service to the transformers in the neighborhoods.

The household power comes from a single phase high voltage input transformer with an isolated, center-tapped, 240 Vac output winding that feeds his house. The transformer is a short distance from the house. Neutral is the center tap of the 240 Vac that gives 120 and 120 Vac to the lights and wall sockets and low power appliances in the house.

"Your welding set consist of a transformer where the secondary side, the output is a low voltage output but with high ampere. It has 2 wires, where one of the wires is a hot wire and the other is the earth wire. Actually, the earth wires, is a neutral wire that return back to the transformer. The earth wire will be grounded to the metal parts where you want to do the welding works."

The output winding of the welder is floating. Either end can be grounded to the work and the other for the welding rod. The work ground is not intended to carry welder current. The welder cabinet should be grounded locally long with the power ground near the location of the welder power connection. There is no "Neutral" wire.

"the hot wire, will delivery the current thru your electrode and thru the metal plate."

Both wires deliver current. It is a closed loop.

"As soon your welding electrode put close to the metal plate (without touching it), the short circuit will occurs and the high current "jumps" across to the metal plate"

Without touching nothing will happen as the voltage is not so high as to make a spark. Striking an arc requires a tap and pull to start the arc.

"melts the electrode to the metal plate and the diffusion of the material started."

Both the electrode and the part of the plate at the arc are liquifying and diffusing to make a bond.

The grounding problem you describe sounds like a defect in the equipment. The welder input and output windings should be isolated from each other and from ground. Grounding one side of the low voltage - high current winding should not effect the equipment ground.

I may be missing something though.

Jon

Most of the welder always put the welding on top of the metal plate and the welding set is not insulated from the metal plate. Please refer to the diagram attach.

So during the welding works, the high current flow back to the electrical earth wire and cause the cable always burnt.

Simon,

These are good things to be considered when Jerry set his up.

Thanks for the drawing.

The current from low Voltage transformer L1 through the welding point, work piece, metal plate back through L2 to the source nomally does not travel through the Welder cabinet. It is an isolated closed loop. I would be more inclined to think that part of the welding current is making its way to the ground and returning by way of the ground wire to L2. The welder should have insulating feet to prevent that path so that the 240Volt input side and cabinet stays safe from the welding side.

Jon

Per Kirchhoff's Current Law,

Current in L1 = Current in L2.

Current leaving in L1 terminal of the secondary coil must enters in the L2 terminal. Then how does the secondary current go back to Earth cable.

-MS

Msamad,

This is the reason not to put the barefoot welder on your work.

Part of the welding current goes through a parallel path by way of the ground system and burns it.

Jon

Thanks Jon, now the current path makes sense.

-MS

Hi Jerry,

In your single phase 240 Vac supply for your Welder you dont need a neutral.

Some may use 115 Vac for some control equipment and can get its 115 Vac from an internal 240 to 115 Vac isolation transformer.

A fellow I knew had a big Welder with holes for different voltage and current combinations. It must have had one for melting bulldozers and one for fine work.

Grounding your equipment is for your protection in case you spill your beer into the welder's input power.

How about a permanently wired hand operated breaker on the wall? No plug, no outlet to mess with.

In many countries outside of the US they only use floating 240 Vac. No neutral and ground was for safety with larger appliances.

So much simpler.

Jon

Thanks, but you don't seem to understand what I'm asking.

Why doesn't the primary side of the transformer require a neutral? How is the power returned to the power station to complete the circuit?

(My stove is gas.)

It seems the primary of the welding transformer is Line to Line voltage - so, there shouldn't be any neutral. The current is coming from one Line and returning back through another Line. Not necessarily the return path has to be neutral.

-MS

Jerry,

Neither transformer winding "needs" a neutral. other countries use untapped 240 Vac in their homes and everything runs off 240 volts.

The current in the loop in the European home goes from one end of the transformer winding the through the stuff you plug in and turn on and returns to the other end of the transformer. No ground, no neutral and no physical connection to the primary side.

The transformer is electromagnetically coupled to hi Voltage primary source "on the street". The primary source from the substation is it's own loop and has no real connection to the household except by electromagnetic induction in the transformer.

America chose to center tap and ground the 240 Volt winding to provide lower voltage in our home appliances that we come into contact with.

Other home appliances use 240 V as their primary power and 115 Vac for user interfaces the we come into contact with.

The three power lines in my neighborhood don't have ground or neutral line. When one of them goes to a transformer primary winding the other end of the winding is connected to the earth by way of a grounding rod.

Jon

The primary doesn't need a neutral because the transformer is the load. If it is three phase then the load is from phase to phase and not phase to neutral. If it is single phase then the transformer is connected to the hot and neutral and the frame is grounded. The load is then phase to neutral.

240 Vac to the primary of the transformer has no neutral.

The welder only needs the 2 hot leads from 240Vac to supply arc current. If the internal welding transformer is not properly isolated, a missing neutral or ground becomes a safety issue. If there is ANY current leakage through direct OR capacitive coupling, you may get lethal voltages impressed on the cables or case of the welder. That is the MAIN reason to always provide a neutral or ground as recommended by the manufacturer. BE SAFE!

240 Vac to the primary of the transformer has no neutral.

Capacitive coupling?

Don't spill the beer on the welder.

Capacitive coupling from motor and transformer windings may cause a shock hazard on ungrounded machines. This is one example specific to motor windings.

www.pacsci.com/support/documents/mtd/d-10.pdf

Jerry, perhaps I can answer your original question in a way you might understand.

You are in New Hampshire, USA by your title, so the references to open delta and Scott's' 90⁰ 2 phase miss the point, so don't let us confuse you with a lot of technical missed shots.

To your question. You asked about 2 hot lines and a neutral from a center-tapped transformer, (which is quite common in the US,) and specifically, how a circuit not using the neutral can work.

The simple fact is that ALL power in this system flows from one hot leg to the other hot leg.

As an example, let's consider a situation where line A supplies a 100 watt 120v light ending on the neutral, and line B supplies an identical 100 watt 120v light ending at the same neutral connection point. The power flows from line A through the light to the neutral connection point and then on through the other light to line B. Nothing flows from that neutral common connection point back to the transformer, everything flows from line A to Line B through the 2 lights across the common N tie point.

The supply neutral provides a 'shortcut' return path only if the loads from line A to N are not identical to the loads from Line B to N, (which is usually the case BTW). The neutral amp flow in fact is measured to identify a seriously un-balanced system.

To unbalance this system, we could switch 'off' the light from B to N. Now the amps from Line A through it's light to the neutral tie point cannot get to Line B through the other light, so the supply Neutral then acts as a 'shortcut' back to the transformer center tap connection where it continues through the other 1/2 coil of the transformer to line B.

Now to your welder. The welder is connected in your stated case from Line A to Line B, with no load to neutral to unbalance the flow from line A to line B. Your welder is designed to use the energy from both halves of the transformer coil, no 'shortcut' needed.

For the stories about equipment grounds getting involved in the welding circuit and burning, let's just state for the record that a ground bond is ONLY energized when something goes wrong. A Neutral is expected to carry some current regularly, but an earth ground bond is not.

In the case drawn above, the 'something wrong' is missing from the drawing. The short circuit path from the return welding lead to the case ground is not indicated, (which is how the ground bond gets into the circuit).

I hope this will prove useful. Regards, CJM

Here is a simple non technical answer to why it can work with just 2 wires and not ground or return to source of generation. At the power station, the wires leave the generator and go to the primary side of a transformer. One wires goes in and the other comes back to the generator, completing the circuit. From that point on, all the power that is transmitted on the power lines is through induction. If you do a quick read up on magnetism and induction you'll see how it works. So, the return to source has been done at the first stage of power generation. Everything from then on is induced current and isolated from the source. There's a bid of a misnomer in the idea that when you plug something into an outlet it goes back directly to the power generation station. It only goes back to the last transformer it is connected to. In the case of the welder, the secondary side of the transformer is the source. Another way to think of it is that each transformer secondary is it own generator.

Hope that helps, someone can correct me if my analogy is incorrect.

Thank you all again. I was unaware that the neutral only functioned in unbalanced situations. During my power plant days, I was told that the reason the blackout of 65 spread so quickly was that the generators (alternators) in trying to hold up the grid went into motor from too much return and activated the overspeed trips, creating a cascade of failure.

The difference between a motor and a generator is a few degrees of phase angle.

Pedaling a bicycle uphill you are the generator. Pedaling a bicycle downhill you are the driven load and gravity is the generator. It's all about a very slight change in timing (phase angle) between the feet and pedals. CJM