Melted

[Diesel]

Nope. And here's why:

A transformer supplying a three-wire distribution system has a single- phase input (primary) winding. The output (secondary) winding is center-tapped and the center tap connected to a grounded neutral. As shown in Fig. 1. either end to center has half the voltage of end-to- end. Fig. 2 illustrates the phasor diagram of the output voltages for a split-phase transformer. Since the two phasors do not define a unique direction of rotation for a revolving magnetic field, a split single- phase is not a two-phase system.

You have two by splitting the single phase of 240 via one hot and one neutral, but you do NOT have two phases. No revolving magnetic field. No phase A, Phase B. Single phase is all you have.

[Diesel]

The ground center tap is misleading for many electricity 101 newbies. The ONLY way you're going to get multiple phases is to use multiple transformers and/or a phase converter, or a capacitor to offset the incoming phase feeding it. This is how you can run a 3phase motor from a single phase source. A single phase transformer is providing you a single split phase source. You can confirm that by following the instructions Rob already provided. You are NOT getting two seperate phases because you have two hot wires in that configuration. It's a split SINGLE phase source.

[Diesel]

No, you won't.

If you have a dual-trace scope, connect the common lead to L1, connect probe 1 to the center tap and probe 2 to L2.?? You'll see both traces are in-phase with each other. What you find is that you have a single phase that's been split down the middle. NOT two seperate phases. Credit to rob for the simple way in which to verify this. Phases are offset from one another. A split single phase setup is NOT offset. You incorrectly assume that because you have two

120volt wires feeding that outlet that you have two phases from a single phase pole transformer, but, you do not. OTH, If you have a hot coming from one transformer and another hot coming from yet another transformer, then, yes, you have phase A and phase B. They will be offset and you can confirm this with a dual trace scope.

[Diesel]

Thanks for confirming that what I've been writing is accurate when using a single phase transformer. Obviously, various individuals incorrectly assume that because they have two hot legs coming from that transformer that it must mean they have two actual phases. When in reality, they do not. It's amazing what people think they know about electricity sometimes, isn't it? :)

[NY]

No-one is disputing that if you measure end-to-end you get a single sine wave. But if you measure either end to the neutral, you get two separate sine waves (depending on which end you measure wrt neutral) and those two sine waves are 180 degrees out of phase with each other. If you connect the two leads of a twin-beam oscilloscope to the two ends of the cable, and assuming that the neutral is connected to mains earth by the house wiring installation, then you will see those two waves 180 degrees out of phase.

I joined the discussion half-way through. Was the OP saying that he had a mains socket where the line and neutral pins were connected to the two ends, with the centre tap left floating? I believe some shaver sockets (switchable between 120 and 240V) are wired like this: neither pin is connected to house neutral, instead teh centre tap is connected to neutral; both float at +/-

120 V or +/- 60 V wrt mains neutral (and mains earth, since neutral is normally bonded to earth at some point, either at the power station or in each house). Likewise for transformers used for building work: the thinking there is that it halves the voltage between either lead of the appliance and earth in case the cable sheathing gets cut.

[NY]

Ah, so you *are* saying that you only consider that you have separate phases if there is a 120 degree difference between them, and not if you have any other difference like 180 degrees. Obviously 180 wouldn't give you any rotation because the two voltages wrt centre tap and neutral always cancel each other out.

What we have established is that what you see depends on what you take as your frame of reference. If the centre tap is regarded as neutral/earth, then each leg is at 180 degrees to the other, wrt centre. If you perversely connect one of the scope leads the opposite way round (which requires everything on the secondary to be floating wrt mains earth) then obviously the traces will be in phase.

[Rob]

Yah, learning electricity can be a real forehead-slapper at times.

Reminds me of the old riddle about the hotel bellhop:

Three people check into a hotel room. The clerk says the bill is $30, so each guest pays $10. Later the clerk realizes the bill should only be $25. To rectify this, he gives the bellhop $5 to return to the guests. On the way to the room, the bellhop realizes that he cannot divide the money equally. As the guests didn't know the total of the revised bill, the bellhop decides to just give each guest $1 and keep $2 as a tip for himself. Each guest got $1 back, so now each guest only paid $9, bringing the total paid to $27. The bellhop has $2. And $27 + $2 = $29 so, if the guests originally handed over $30, what happened to the remaining $1?

[Rob]

Take a simple 2 cell flashlight.

Connect the common scope lead to the center tap of the two battery assembly.

Connect Probe 1 to either end of the battery assembly.

Connect Probe 2 to the remaining end of the battery assembly.

Observe the display on the scope and note that one probe/trace shows a positive voltage while the other probe/trace shows a negative voltage.

Have you just discovered two-phase DC batteries?   Or are you just playing games with your scope probes to amuse yourself?  LOL

[James Wilkinson Sword]

Don't connect a transformer the same way as you connect the strip. It r= eally won't like it.

-- =

Why is the front of an aeroplane called a cockpit? If you have female pilots do you call it a pussypit?

[James Wilkinson Sword]

On Fri, 01 Sep 2017 21:07:39 +0100, Tekkie=AE wrote= :

Funny, I've never had to do that.

You've not discovered LEDs?

-- =

Flatulence (n.), emergency vehicle that picks you up after you are run o= ver by a steamroller.

[James Wilkinson Sword]

If you look at each output on a scope, you can clearly see they're completely different. They're 180 degrees out of phase. And it's not just like connecting the output backwards, as you can add them together.

[Diesel]

That's an electrical 101 newbie mistake, actually. Indicating they not only don't understand what's going on, but, also don't know how to use a scope properly.

Rob wrote the best/easiest explanation I've seen so far in this thread, so I've taken the liberty of copying it below for you.

FWIW, the old "center tap as reference" is a trick test question instructors frequently use in Electricity 101 to see if students have been paying attention.By the time you've successfully completed Electricity 101 you realize how silly the two-phase argument/conclusion is. The fact that the center tap is grounded really confuses some new students. If you have a dual-trace scope, connect the common lead to L1, connect probe 1 to the center tap and probe 2 to L2. You'll see both traces are in-phase with each other.

You don't have a two phase output. You have a single phase output that's split in half. You don't gain an additional phase by the splitting. it's coming from the same secondary coil. Which is a single phase coil. If you want an additional phase, you have to add additional components. Another transformer, a capacitor, etc. Or, you can opt for a single phase motor to drive a three phase motor and gain two additional phases. But, either way, you're not going to get a second phase from a single phase source without doing some additional work.

[Diesel]

It's really the room 25 plus the bellhops two dollar tip, bringing it to 27. So when he gives the dollar back to each one, it equals out to

1. :) In other words, the already includes the bellhops tip and if we added it again, we'd be double tipping. So the three guests costs for the room along with the tip is already dollars. When he gives each one a dollar back, that's where we get the .

[Diesel]

That's because taking a measurement in that way from neutral is misleading. Infact, it's used as an have you been paying attention in class trick.

I'll ask this another way then.

Why do you think the secondary is providing two distinct phases when the primary only has one phase? Where is the second phase coming from?