AC Basic Questions

Thread Starter

dw85745

Joined Oct 5, 2015
32
Given a 240 volt circuit:
If we measure the voltage between L1 and L2 we get 240V.
If we measure the voltage between L1 and neutral OR L2 and neutral we get 120 Volts.
Neutral is secured a frame.
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Questions
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GIVEN: We have L1 connected to a Load. The Load then connects to L2.
1) Since L1 and L2 are 180 degrees out of phase, then I assume the Load is seeing 120 volts?
2) Because of the phase difference is L2 just acting like "the neutral" to L1 ?
3) Are the electrons just following back and forth down L1 and L2 OR
are the electrons flowing in both directions such that L2 is the return path for L1 AND
L1 is the return path for L2?
4) If NOT, since the Neutral is connected a frame, how are the electrons getting from L1 OR L2 over to the Neutral?
Thanks
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Answer (included here to highlight for those of us trying to wrap our head around AC ) Please ignore the OP questions as originally stated above, and focus on the following.
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One needs to look at AC as a transformer. For this discussion one needs to ignore the fact that a transformer can be a step up or step down transformer.
The primary is the service connect coming from the power company. The secondary is our supply. The secondary can be equal to the primary -- e.g. 240 volts in (primary) and 240 volts out (secondary). The secondary can also be tapped at various points to obtain any desired voltage. If tapped in the center, each leg would yield 120 volts. Tapping the secondary at some other point may give 24v, 22, 12, or whatever voltage is needed. In a circuit diagram Line1 and Line2 just represents the two legs of the secondary OR the tapped voltage (whatever is that voltage). The circuit diagram will govern as to what is that voltage, or if multiple taps are used, the nomenclature will need to so note.
For a US household with 120 volts, the black wire is one leg of the tap, and the white wire is the other leg of the tap (center assumed). If the circuit diagram shows 240 volts with a third wire going to the chassis, then the 240 volts would represent the end points of the secondary. The third wire going to the chassis is the center tap. HENCE each leg of EITHER secondary wire to that chassis wire will give 120 volts.


Thanks to ALL who responded.
 
Last edited:

Jon Chandler

Joined Jun 12, 2008
433
Think of the AC supply(US) as the output of a center-tapped transformer. The transformer output is L1 and L2, with the center-tap being the neutral. Between L1 – L2, the voltage is 240 volts. Between L1 – neutral or L2 – neutral the voltage is 120 volts.
 

crutschow

Joined Mar 14, 2008
31,129
Since L1 and L2 are 180 degrees out of phase, then I assume the Load is seeing 120 volts?
Why do you say that, when in your second sentence you say the voltage between the two is 240V (which is the correct value)?
180 degrees out-of-phase means the two voltages add.
 

Thread Starter

dw85745

Joined Oct 5, 2015
32
Thanks both for responding.
Volts center tapped (VCT) describes the voltage output of a center tapped transformer. For example, a 24 VCT transformer will measure 24 VAC across the outer two taps (winding as a whole), and 12 VAC from each outer tap to the center-tap (half winding). These two 12 VAC supplies are 180 degrees out of phase with each other, measured with respect to the tap, thus making it easy to derive positive and negative 12 volt DC power supplies from them.

Jon Chandler: No offense, but your response in effect mirrored by OP.
The concept of transformer or buss bar are the same IMHO.
But doesn't clear up the 120 V electron flow back to source, if the neutral is attached to the frame unless L1 OR L2 is being used -- HENCE my confusion. In the schematic I'm looking at (neutral to frame), there are NO traces (wires) going back to the frame. So for the 120 to flow back (because its AC), IMHO it either has to be going back down the same wire (say L1 in this case) or flowing down the other Line wire (L2 in this case).

crutschow:
I used 180 out of phase, because that is the only thing that came to mind that would allow the electrons to flow back down L1 or L2 without colliding?
 
Last edited:

crutschow

Joined Mar 14, 2008
31,129
If the load is connected to L1 nd L2, then the electrons just move between L1 and L2.
Since there is no connection to neutral, there are no electrons flowing in the neutral.
 

Thread Starter

dw85745

Joined Oct 5, 2015
32
Taking this a bit further.
Example1:

L1 (at source) > Load > L1 (back to source)
So measuring voltage across the load we should get 120 volts?
Conversely:
L2 (at source) > Load > L2 (back to source)
So measuring voltage across the load we should get 120 volts?

BUT IF we have:
L1 (at source) > Load > L2 (back to source)
I ASSUME we would still measure 120 volts across the load?
So to keep the electrons from colliding since L2 is also cycling,
then would not L1 and L2 need to be out of phase?
 

WBahn

Joined Mar 31, 2012
27,903
Taking this a bit further.
Example1:

L1 (at source) > Load > L1 (back to source)
So measuring voltage across the load we should get 120 volts?
Conversely:
L2 (at source) > Load > L2 (back to source)
So measuring voltage across the load we should get 120 volts?

BUT IF we have:
L1 (at source) > Load > L2 (back to source)
I ASSUME we would still measure 120 volts across the load?
So to keep the electrons from colliding since L2 is also cycling,
then would not L1 and L2 need to be out of phase?
Draw a picture of what you mean and you should see that in your first two cases you have a short across your load that is connected to a single point.

You keep talking about electrons colliding. This is not a good view of how electricity flows, though it could just be a translation-to-English artifact.
 

Thread Starter

dw85745

Joined Oct 5, 2015
32
Thanks to all for the additional responses. I guess I'm slow on the uptake, but while sleeping last night it clicked. I want to give a big THANK YOU to Jon Chandler's post regarding transformers (echo by KeithWalker I found this AM). I have edited my original Post so others do NOT need to read through this entire thread.
 

MrChips

Joined Oct 2, 2009
27,695
No. Please do not edit posts. You can add edits as a separate paragraph beginning with the word Edit:.

If you alter the wording then it changes the significance of all subsequent replies.

Leave it up to each member to choose whether or not to read through all the posts in a thread.
 

MisterBill2

Joined Jan 23, 2018
13,808
Post #9, (from KW) nails the issue exactly.
Typical residential power IS INDEED from a transformer, 240 volts with the center point common AND tied to earth ground.
AND the voltage between L1 and L2 is constantly the same as the vector sum of those two voltages relative to the center point neutral.
So now you have the detailed description of how it works. This duplicates what others have said, but with a vector notation it may be clearer.
 

Thread Starter

dw85745

Joined Oct 5, 2015
32
Thanks all for responding.
MrChips: My apologizes, won't do it in the future.
MisterBill2: I believe I gave KW credit.
MaxHeadRoom: If you are referring to your picture. In the US I've seen this done where they want one outlet hot and the other controlled by a switch. HOWEVER, never realized until now that there was 240 between the hot legs. This supports the concept Not to work on a hot circuit.
In regard to your question (my guess is):
If Load 1 and Load 2 are exactly the same size the Neutral current will be Zero.
 

MisterBill2

Joined Jan 23, 2018
13,808
The original concept for separate circuits in a kitchen outlet was because of the proliferation of resistance heating element appliances, coupled with the intention of saving builders the expense of installing more outlets in kitchens.
The reality is that GFI (GFCI) outlets occupy a whole standard outlet box made the two-circuit concept unworkable. Now multiple outlets on separate circuits are common, each protected by a GFCI. This is good because an unknowing individual attempting to replace one of those dual-feed outlets would probably receive a very rude shock after only removing power from one of them. And a very loud tripping of the overload protection if they did succeed in replacing the outlet without removing that part that ties the two individual outlets in parallel.
The fact is that some ideas and concepts have very unfortunate secondary effects. Failure to consider secondary effects is a serious flaw suffered by many governing individuals.
 
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