Instability between a 16A triac 'dimmer' and 3300uF caps.

[Jerry]

You might try stage lighting places for big variacs. They still use them for controlling big incandescent lamps.

Put a scope across the two hot on your 240, you will see a sine wave. In reference to the neutral center tap they are just opposite polarities. That is single phase. You can try to rename it all you want but it is just wrong.

Two phase is not theoretical, it exists. A couple places in the US still use it to run old machinery from the turn of the last century.

4 wire two phase is no part of 4 phase. Each pair is referenced to only itself, it has no electrical connection to the other pair.

I have a +/- 15v common ground DC power supply, according to your definition that would make it a two phase DC power supply.

[chrismb]

I have a +/- 15v common ground DC power supply, according to your definition that would make it a two phase DC power supply.

Providing you are conceptually comfortable with a 0 Hz phase, then, yes, I'd happily call it a two phase 0 Hz supply! The fact that it is not '0 and X Volts' [two lines] is what makes it different to a '+X, -X and 0' supply [three lines].

Two phases that are 180 deg apart do, indeed, look like a sinusoid if you tap across them. So do two phase lines of a 3 phase supply! So? Any two lines of a poly-phase system will look like a one phase sinusoid!

The difference between a 230VAC single phase live and neutral, and a 'two phase' 180 deg separation is that the mid-point between those two phased lines is, nominally, always 'earth' potential, so you can send both lines down just two lines, with the centre tapped to earth and then remotely reference each to earth, without either actually being at earth potential. This is exactly the same as 3 phase (that is to say, the sum potential is always averaged to earth), just with one less phase.

Single phase = one line that is AC wrt a neutral [nominally earth] return line.
'Two phases' [but not 'what-is-know-as-two-phase'] = two lines that are AC wrt a neutral return line that are 360/2 separated in phase.
Three phases = three lines that are AC wrt a neutral return line that are 360/3 separated in phase.
What-was-called-two-phase = two lines that are AC wrt a neutral return line that are either 90 (or 270 depending on how you choose to look at it) separated.
Four phase would be 4 lines separated by 360/4 to each other.

etc.

[chrismb]

You might try stage lighting places for big variacs. They still use them for controlling big incandescent lamps.

I can definitely buy them (not from lighting places - we just don't have such helpful vendors here in UK, no way!!). But the price seems to be around the £500 mark.

[Doug Coulter]

If you only need a small variation around a voltage, you can use a variac/transformer of smaller size as a boost/buck to get there.
Yes, multi KW variacs aren't cheap or common. But you seem to be more vendor-challenged than most people in the UK - you need to take a page from Bills book and do more looking, it seems to be there for most other people in your country. You can also just rig up a tapped transformer with some switching, I've done that. Or build a proper switcher - or modify an existing one to get what you want, that's probably the cheapest way. There have been some pretty huge ones on the surplus market from old mainframe computers.

Your calling split phase two phase is just wrong. It's one phase, center-tapped. It's called split phase by everyone else on earth. If you're going to invent your own terminology for things, then no communication is possible unless we all want to learn your new language. That's kind of selfish, isn't it?

[Jerry]

You might try stage lighting places for big variacs. They still use them for controlling big incandescent lamps.

I can definitely buy them (not from lighting places - we just don't have such helpful vendors here in UK, no way!!). But the price seems to be around the £500 mark.

I am thinking more of the places that supply lighting equipment for movie productions. Rental houses and the like. Since a lot of places are going to HMI lighting the old stuff might be for sale.

[chrismb]

Your calling split phase two phase is just wrong.

I've over be-laboured the point already, so I'll withdraw from that line of discussion. But I'd just like to say I did not call split phase "Two Phase" [as in, nominative, definite article], I called it "two phases" [indefinite, descriptive]. (Well, if I did, I happily withdraw it.) I recognise conventional terms, and in this case I have learned that "Two Phase" means specifically phases that are 270 deg apart.

[chrismb]

I am thinking more of the places that supply lighting equipment for movie productions. Rental houses and the like. Since a lot of places are going to HMI lighting the old stuff might be for sale.

I see what you mean. I dare say here anything heavy as iron-cored transformers/variacs have been sold [or stolen!] for its copper by now, but maybe worth a look out.

Actually, there seems to be a very 'healthy' [for the vendors!] turnaround on ebay 'Business&Industrial' these days. So much stuff goes through I think everyone in the country uses it for tech disposal. Some of the cr@p that goes through is really a liability rather than valuable, but still attracts a premium. It takes a long time to wait in 'ambush' for the listing that has gotten little attention, like if the kit is a bit dirty looking or the listing is wrongly spelt. The transformer above is one I picked up locally for just GBP 5, just cos it's all rusty on the outside I guess, and it was a local pickup only.

[Starfire]

No, they are not. You cannot get two phase out of single phase. jerry

I have connected up many welders both static and rotary transformers in a machine shop. They are connected across two phase of a three phase supply - they do cause a balancing problem for power factor correction but with several they can be connected alternative pairs of phases to correct balance. PFC is with additional caps switched in with automatic correctors.

[Jerry]

No, they are not. You cannot get two phase out of single phase. jerry

I have connected up many welders both static and rotary transformers in a machine shop. They are connected across two phase of a three phase supply - they do cause a balancing problem for power factor correction but with several they can be connected alternative pairs of phases to correct balance. PFC is with additional caps switched in with automatic correctors.

I used to be a service tech for welding equipment. I have seen virtually every kind of welder out there. Still an authorized tech for Miller.

The power across two hot wires out of a three phase supply are single phase. The only way you could actually use two phases out of three phase is to be using a Wye system with a central neutral point like 208/120 or 480/277. Then the transformer in the machine would have to have two separate primaries and secondaries, basically 2/3rds of a three phase transformer. I have never seen a machine like this.

Of course, I have never seen an machine from the UK/Ireland.

[Starfire]