# Error on AC Diagram Vol 2, Ch.10, P1

Discussion in 'Feedback and Suggestions' started by sashery, Jan 3, 2008.

1. ### sashery Thread Starter New Member

Jan 3, 2008
1
0

has an error.

If you have 2 identical 120 VAC sources, perfectly in-phase referenced to the same ground, both outputting identical waveforms, if you read across the 2 hot wires in this case you should see 0 Volts AC. They are parallel identical circuits and since they are in phase, the difference between them at any instant in time is 0 Volts, not 240 V as indicated on the diagram.

2. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Thanks for the suggestion.

I have limited understanding of power systems engineering in general, therefore will need to seek advice on this.

Comments from members with experience in this area are welcome.

Dave

3. ### hgmjr Moderator

Jan 28, 2005
9,030
214
There are two different diagrams that involve dual 120VAC voltage sources.

In one case the two AC voltage sources are in-phase with one AC source's +terminal connected to the other AC sources -terminal. I believe this diagram is accurate because if you imagine that you are taking a snapshot of the output voltage at a given instant, say at the 120V peak, you can imagine that you have two 120VDC sources in series with the postive terminal of one connected to the negative terminal of the other. This would produce for that brief instant in time 240VDC out.

The other diagram has the two 120VAC sources connected minus to minus. One of the AC voltage sources is clearly labeled as having a phase of 180 degrees. This 180 degree phase shift has the effect of inverting that AC voltage source with respect to the other and therefore it is as if the two AC voltage sources were connected minus to plus as in the first diagram.

In the end I don't see anything wrong with these two diagrams. Possibly sashery is referring to a different diagram than the two I have just described.

hgmjr

4. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Thanks hgmjr, I will await Dennis' input on this.

Dave

5. ### Dcrunkilton E-book Co-ordinator

Jul 31, 2004
416
11
I agree with hgmjr

6. ### sfb New Member

Nov 20, 2007
1
0
I too have some difficulty understanding this. Please bear with me, if you will, my aim is to learn. I address two concerns below:

1) In the fifth paragraph from the end of 10.1, it reads, "Here is where the (+) and (-) polarity markings really become important. This notation is often used to reference the phasings of multiple AC voltage sources, so it is clear whether they are aiding ("boosting") each other or opposing ("bucking") each other. If not for these polarity markings, phase relations between multiple AC sources might be very confusing. Note that the split-phase sources in the schematic (each one 120 volts 0[degrees]), with polarity marks (+) to (-) just like series-aiding batteries can alternatively be represented as such: (Figure 10.7)"

I think this description better fits Figure 10.5 than 10.7, since 10.5 shows two AC generators developing a zero-degree phase shift. 10.7 shows two 120 VAC generators, producing two signals 180 degrees out of phase with each other.

2) There are two signals in 10.7 that are 180 degrees out of phase with one another and 240 VAC across the hot legs.

Towards the end of 10.1 it reads, "If we mark the two sources' common connection point (the neutral wire) with the same polarity mark (-), we must express their relative phase shifts as being 180 [degrees] apart. Otherwise, we'd be denoting two voltage sources in direct opposition with each other, which would give 0 volts between the two "hot" conductors."

The signals are 180 degrees out of phase (as shown in Fig. 10.7). They are also shown "series-aiding" by the polarity marks which to me, means they cannot be 180 degrees out of phase. If they are 180 out how can you have 240VAC across their legs?

Figure 10.7 shows two generators developing one signal 180 out from the other yet the output is labeled in phase?

Thank you very much for this invaluable resource,

sfb

7. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
The diagram is accurate. They are series identical circuits, not parallel.

The description fits both 10.5 and 10.7. The two "AC generators" are not necessarily generators, but rather "generic" AC sources.

Those are in phase, not out of phase.

Indeed! This is a re-iteration of the importance of knowing whether the legs are in-phase or 180 out of phase.

I know all of this can be confusing, but remember the following:

When the common lead is labeled with a "+" from one source and a "-" from the other, we have in-phase series aiding, and voltage between the hot leads will be the sum of the sources.

When the common lead is labeled identically from both sources, we have 180 degree out of phase bucking, and the hot leads will be the difference of the two sources.

The text and diagrams are accurate.

8. ### Dcrunkilton E-book Co-ordinator

Jul 31, 2004
416
11
I agree that the diagrams are correct

Dennis Crunkilton