How many phases? And another Lucien grumble: 'Amps Per Phase' | on ElectriciansForums

# How many phases? And another Lucien grumble: 'Amps Per Phase'

Discuss How many phases? And another Lucien grumble: 'Amps Per Phase' in the UK Electrical Forum area at ElectriciansForums.net

#### Lucien Nunes

##### -
Mentor
Arms
When I joined this forum a decade or so back, there was a discussion going on about a welder that was connected with a 3-core cable to L1, L2 and earth of a 3-phase supply, operating on the 400V present between L1 and L2. The advantage of choosing 400V is the reduced current requirement. My own portable stick welder has this option too with a voltage selector on the back. It will run on 16A if 400V is available, but 32A is needed if you switch it down to 230V.

According to the particiapnts in that discussion, the use of L1 and L2 made it a ‘2-phase welder’ because it was 'connected to two phases.'
'No,' I said:, 'it's single-phase' and was quickly admonished that I didn't know what I was talking about, and told to go and look up some definitions (that was the usual reaction back then to pretty much any newcomer.) I tried to make the distinction between various configurations:

a) A single phase device operating at the line-line voltage of a polyphase supply like this welder, using two line wires but no connection to the neutral.
b) A device that specifically needs two lines out of a 3-phase supply and its neutral.
c) A genuine two-phase supply and load that has two separate coils operating out-of-phase, historically usually 90 degrees apart, which would require two lines and a neutral, or two separate pairs of line and neutral.

Tony Smith knew I knew my onions here and didn't argue with me, but we moved on swiftly anyway and the point got brushed aside. It has resurfaced in a number of guises since, including a recent conversation, and I think I've finally realised where the misunderstanding comes from.

We often talk about the line conductors as phases and most people use 'phase' and 'line' interchangeably in many 3-phase contexts. E.g.
'Which phase is that lighting circuit on?'
'Put the immersion on a different phase to the lobby heater!'
‘He didn’t say which phase is dead but I think it’s L2.’
These sound completely natural and make sense when we consider loads that are star-connected, so that each phase (e.g. each coil on a motor or transformer) is powererd by one line conductor. E.g. motor coils A, B & C are respectively connected to L1, L2 and L3, or if you prefer it in European, U, V & W are respectively connected to R, S & T. In the case of a single-phase load, a single primary coil is connected to a single line conductor (and neutral)

As soon as we move from star-connected 230V windings to delta-connected 400V ones, things are a little more complex. Now, coil A is connected between L1 and L2, B between L2 and L3, C between L3 and L1. The load phases are no longer equivalent to the lines, but there are still three line wires, three coils, three phases. Moving now to the situation of the original welder and other single-phase 400V loads, UK electricians today are unfamiliar with connecting them between the lines. Looking back in time and across continents and oceans, this was not so unusual. e.g. aboard ship or in 1950s Europe. A 132/230V system with 132V present between line & neutral and 230V between lines has been widely used in the past. There, it would be perfectly normal to connect your 230V single-phase device across L1-L2, complete with double-pole switching and fusing, because that is how you get 230V from that system. That doesn't make the 230V load 'two-phase' any more than my single-phase portable welder turns into a 2-phase one when you flip the voltage selector.

Have you noticed what I haven't mentioned yet? Time. Phases are about time. In a 3-phase system, first one voltage peaks, then another peaks, then a third one peaks and we go back to the beginning. That is how a rotating magnetic field is produced inside a motor, how the neutral current sum to zero, how there are no power-zeroes etc. Phases of the moon, phases of the supply; it's a matter of things happening at different times. This is what the contributors to that old thread were not thinking about. If the welder only has one primary coil on its transformer, only one thing can happen at one time. To be '2-phase' it would need two primary coils, one coming to a voltage peak first, then the other one, then the first one again. Out of phase with each other. 100 years ago there were 2-phase motors which worked pretty well. They did have two coils just as a 3-phase motor has three. But ultimately the 3-phase system proved more versatile and the 2-phase was dropped. Americans / Hungarians, or Tesla / Westinghouse, if you want to look at it from a commercial angle.

Moving on to the actual gripe of the day, we have that horrible term 'amps per phase' most often used by people new to three-phase work or DIYers etc, although I do hear it from electricians. When we refer to amps in a circuit, invariably we are speaking of the current in each line conductor. That is the only measure that makes sense in every situation. A 32A 3-phase circuit can carry 32A in L1, L2, L3 and/or N (although not in all four of them at the same time unless you have a lot of nasty triplen harmonics.) Yet people seem to get confused... Do I need to say 32A per phase? Is 32A per phase a 96A circuit? To those people I say: 'Do you include the neutral current when you describe a single-phase circuit?' You wouldn't call a single-phase circuit fed from a C20 MCB a '40A circuit' because there's up to 20A flowing in each of two conductors. And it's the same with 3-phase.

The current is the line current. The number of phases is the number of separate loads with power peaks staggered in time through the AC cycle.

As for myself, they let me out of hospital under some pressure from me, with the proviso that I called in this morning for a blood test because they are worried about my kidneys. I did that and there is genuine cause for concern as the blood creatinine level is still rising. That could be because of the pressure on the left ureter causing hydronephrosis, or it could just be the result of them having to clear my blood of the large amount of contrast dye used in my many recent scans, which is a known kidney stressor. So, Dr. Mansukhani very kindly called me at lunchtime to say she's prepared to let me stay at home again tonight provided I return yet again tomorrow for a repeat test. If it is worse, I expect they will admit me as an in-patient so that I can have that second ureter stent put in. I'll talk more about the ongoing situation in another thread.

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Quite appropriately the above post requires acknowledgement with three separate emoticons, yet the forum only permits use of one.

As for myself, they let me out of hospital under some pressure from me,
I can imagine the conversation;

“Don’t you know who I am! I’ve been on TV, Daaaaaarling!”

As Lucien says it’s found on ships. The generators are wired delta with no star connection for neutral or earth. Not all ships have double pole switches so on a very fine vessel called the Arco Bourne is was good to watch the deck officer on watch get a belt from the chart table lamp after doing some repairs to it. We must have had a bad earth fault somewhere and he became our test device!! On the older ships, normally we would have two lamps in series across say L1 and L2 showing half brilliance with no fault , with the centre connected to the hull. This would be our earth fault indication but realistically it was just something else to ignore as anything on deck on a dredger would probably have an earth fault anyway!

Who the hell would be stupid enough to argue electrical science with lucien! …..I bet Nikola tesla is up there running ideas past him!

Who the hell would be stupid enough to argue electrical science with lucien! …..I bet Nikola tesla is up there running ideas past him!
How you keeping Lucky long time no see, hope your not having any more accidents

How you keeping Lucky long time no see, hope your not having any more accidents
Ok Dave thanks, no more accidents yet ….

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