240 V split phase and 240 V single phase

Question

All right, I don't get it. I tried and tried, but I just don't understand. I have a dozen questions about this so somebody just pick one out all of these, I don't care.

What's inside a North American residential 240 V pole transformer?

Is it (secondary) double iron core windings with 120 V on each core and each end of the coil has one hot and one neutral wire that get tied in series to add up to 240 V?

OR

Is it a single iron core winding with two hot wires on each end and a center wire tapped at the 120th winding to make a 120 V wire?

Are there two sine waves in a (secondary) double iron core transformer or one?

OR

Are there two sine waves in a (secondary) single iron core center tapped transformer or one?

If either of the two types of transformers do truly have two sine waves, are they 180° opposite from each other at peak?

OR

Are they 90° out of step from each other at peak?

OR

Are they on top of each other?

Please don't use to many equations or symbols, because I've exhausted my explanation abilities as it is with these questions and might not understand much more than the simple words that were already used here.

Answer 1

The power feed to a North American residence is normally provided by a center-tapped transformer, with the center tap of the transformer secondary grounded. the full secondary winding produces 240 Volts.

the connection is like so:

^{simulate this circuit – Schematic created using CircuitLab}

Arghhh! The shcematic editor won't let me draw what I want!

The two ends or the transformer winding (120 V A and 120 V B) are each 120 Volts from Neutral, but 180 degrees out-of-phase, so you get 240 volts between them.

Most outlets in a home will connect between 120 V A and Neutral, or between 120 V B and neutral, so will get 120 volts.

Certain heavy loads (electric stove, electric water heater, electric clothes dryer) will connect between 120 V A and 120 V B to get 240 volts.

Answer 2

I designed electrical systems for buildings since 1983. I think I can make sense of this for you!

Forget the 180 degrees out of phase that you are being told. You have a single phase sine wave on the primary side of the transformer. One sine wave. Let's assume for a minute that it is 1 volt per turn on the secondary of the transformer. You have three terminal connection points L1, L2, and N. You have 240 total windings with a conductor also connected at 120 turns. As this is AC, let's assume first half of secondary sine wave travels from L1 to L2. At the same exact time, same direction it travels from L1 to N and N to L2. So two sines at 120 windings and volts and one at 240 windings and volts.

The second half of sine wave, again at exactly the same point in time, goes from L2 to L1. etc. This is called split phase because there is more than one voltage from a single transformer. This is not what is taught in engineering classes or looks like on an oscilloscope because an oscilloscope has one common lead and reads from L1 to L2, from L1 to N, and from L2 to N. This is what actually happens inside the coil of a transformer. Please ask those saying 180 degrees out of phase to explain how the sine waves can be out of phase! All the secondary sine waves MUST happen at exactly the same time as the primary and the rules of electricity say in the same direction through the coil.

Answer 3

The secondary side is basically 2 windings connected in series that each put out 120 V, but each winding is opposite phased relative to the other secondary winding, so the sine waves are 180 degrees out of phase. There is a wire connected at the middle point where the two windings are connected together and this serves as the "neutral" N conductor.
When you tap between L1 and N or L2 and N, you only get one winding used so it only outputs 120 V, when you tap between L1 and L2, you are using both windings, and since they are phased opposite, you get 240 V difference between the two legs.

If you need additional information look up "transformer phasing" to get more information about the secondary configuration.

Answer 4

Dunno about the other stuff posted on this answer but these are the facts:

Middle wire (center tap on the transformer, usually colored white) is grounded so that wire is at 0 volts.

One end tap of the transformer output (usually a black wire) is 120 volts AC RMS (root mean squared), so a sine wave with a frequency of 60 cycles per second and an amplitude of 120 times square root of 2 = 169.7 volts. Basically it varies from + 120 sqrt(2) = 169.7 volts to minus 120 sqrt(2) = -169.7 volts 60 times per second.

The other end tap of the transformer output (usually a red wire) is also 120 volts AC RMS (root mean squared). However it is the negative of the voltage of the other side (black wire) so when the black wire is say 100 volts, the red wire will be -100 volts. Now it turns out, shifting a sine by 180 degrees makes it the negative sine (sin(x+180 degrees) = -sin(x)), so saying that the voltage on the red wire is the negative of the voltage on the black wire, that the red wire is phase shifted by 180 degrees compared to the black wire or that the red wire is "out of phase" with the black wire all mean exactly the same thing, that one is the negative of the other.

Since the red and black wires have opposite voltages, this means the voltage difference is twice as great, so the RMS voltage across a connection between the red and black wires is 2*120v = 240v. So a connection of this sort is used to run high power appliances like electric stoves and central air conditioners.