how 240 works

[mivey]

The 120 has two conductors, one of which is tied to ground (what we call "earth") at some point. We usually call the grounded conductor the neutral.

The single phase 240 normally does not have either of its two conductors tied to the earth.

Like this:

 

[crossman]

There ya go, Mivey. Good, straight-forward diagrams that tell the story.

 

[AKwiring]

Sometimes people think of AC power like a car battery with a positive and a negative and understand that current flow is from the "hot" to "ground" (we know better).

It appears to to explain the 120 volt circuits in a typical house.
Then we have this problem to explain how and where 240 comes from and where it goes now that the ground is not present.

Is this kinda of what you are asking AK?

This is exactly what I am asking. Thank you!

 

[gar]

080602-2050 EST

AKwiring:

A voltage source is a two terminal device that has a voltage difference between the terminals. I will include 0 V within this definition. The maximum voltage is any reasonable value.

For an ideal voltage source the voltage at its terminals will remain constant independent of load. In real life virtually all sources have an internal impedance that means under load that the terminal voltage usually drops with increasing load. However, there are some regulated supplies that have an extremely low internal impedance. Also some inductive and/or capacitive loads might cause higher terminal voltages. For theoretical purposes we can assume ideal voltage sources.

Neither terminal of a voltage source needs to be connected to a chassis, neutral, common, earth or any other voltage or current source. However, one terminal can be connected to any one or in some cases more than one of these.

In a common United States electrical distribution system to a residence you have two 120 V 60 Hz AC voltage sources connected together at a transformer center tap. Because the transformer is wired to add the two voltage sources together you have three possible ways to connect to the secondary.

1. Connect between the center tap and one outside lead of the secondary. This provides a nominal 120 V output with a phase of 0 (arbitrary choice) on the hot lead relative to the primary. I will call the center tap common. But below I will call one of the outside leads common.

2. Connect between the center tap and the other outside lead of the secondary. This provides a nominal 120 V output with a phase of 180 deg on this other hot lead relative to the primary (based upon the above arbitrary choice of 0 for connection 1).

Note: I have not connected any of the secondary leads to anything except my measuring instruments. To show phase relationships I would probably use an oscilloscope.

Quite often on transformer schematic diagrams a dot will be placed adjacent to a lead to indicate its phase relationship to other leads.

3. Make the second outside lead common (a reference point) and the first hot lead of item 1 the hot lead. Now the voltage is 240 V and of phase 0.

If the center tap did not exist as a terminal or I did not use the center tap, then I would call the combination of the two secondaries a single voltage source. It is really a matter of how you will use the device and how you want to analyze the circuit.

For some applications none of these leads would be connected to earth. However, in the conventional residential application the center tap will be called neutral and will be tied to earth. This earth grounding will at least be required at the main service entrance.

With this connection of the center tap to earth we have available in the home, under normal conditions, a voltage that does not exceed 120 V relative to any grounded surface. But we also have available 240 V between the two hot leads, neither of which is grounded, to use for higher wattage devices like stoves.

Note: when I say no voltage is greater than 120 V I am referring to the RMS voltage. The peak voltage of a 120 V RMS source is 1.414*120 = 170 V.

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