Understanding Residential AC Phases

Most people assume that single phase means that both 120v legs in residential application are of the same phase.  This is a complete misconception.  If the two 120v legs were in phase, the difference between the two would never change, giving you no power between legs:

imageimage

Note that as the purple line moves from left to right (time) on the graph, it's "length" never varies so we have no potential or power.  At the current position of the purple line, both waveforms are at zero volts.  The POTENTIAL between the two waveforms is zero.  If both waveforms are at 120v, there is also no POTENTIAL between them.

Potential is the DIFFERENCE between the two waveforms, which is subtractive. The purple line must vary in length for there to be power.  Imagine the graph in three dimensions like the triangle on the end of this graphic:

image

The actual system used in homes is a split phase electricity distribution system.

A split phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial (up to about 100 kVA) applications. It is the AC equivalent of the former Edison direct current distribution system. Like that system, it has the advantage of saving the weight of conductors for the installation. Since there are two live conductors in the system, it is sometimes incorrectly referred to as "two phase".

Here is a simplistic graphic of what is happening:

image

A transformer supplying a 3-wire distribution system has a single phase input (primary) winding. The output (secondary) winding is center-tapped with a conductor called the neutral on the center tap, which is normally connected to earth ground. In this case, the transformer is rated at 120 V on either side of the center tap, giving 240 V between the two ungrounded terminals.  This also produces phase inversion, since the signals at the ends of the secondary are in antiphase with respect to the center tap:

image

Notice that Neutral to L1 or Neutral to L2 is 120v and L1 to L2 is 240v.  The 240v collapses to zero every cycle.  This is very convenient, as the Neutral line carries NO current if the loads are balanced.

image

So how do I really know that my model graphic is correct?  I never liked calculus, but I remember how to test a theory.  What I'll do is draw many vertical purple lines, representing multiple test points in time.  This will show me the potential difference between the waveforms:

image

The next step is to isolate those lines and align them along their bottom edge.  Now if I take my beginning waveform half - which represents 120v - and stretch it vertically 200%, I can see if the calculations add up - and they do!  We really do have 240v.

BUT WAIT

If the waveforms are out of phase - like when we hook up a 3-phase generator to a residence - all that goes out the window.  Having the waveforms out of PHASE 120 degrees results in the peak L1-L2 voltage dropping and current ALWAYS being carried on the neutral line.   Furthermore - as we learned from UNDERSTANDING AC POWER - we now have a distorted waveform, which means that most meters can't properly measure the voltage. Again, this is a simplistic graphic of what is happening:

image

Note that as the purple line moves from left to right (time) on the graph, it's "length" varies so we have power.  It never gets as long as in the previous graphic, so we never have a 240v potential between the phases.  Let's test our theory with those vertical lines again:

image

We know that where the two waveforms meet, there is no potential.  So we'll fill in between two such points.  After we isolate our lines and align them along their bottom, we see that we do have a sinusoidal waveform.  It just isn't as big as our previous one.  If we shrink our 240v waveform by 86.6%, it fits our test results.  It just so happens that 86.6% of 240v is 208v !  Nifty, huh?

BUT DEFINITELY NOT GOOD!

Since most AC motors rely on phase shift to produce their rotation, this distorted waveform makes the motor very inefficient.  The magnetic fields that induce rotation partially cancel each other out, and don't produce the proper push/pull timing.  The motor has to draw far more current to do the same work, and will quickly overheat.  The only 240v motor generally found in residential applications is the air-conditioner/heat-pump compressor.  The only reason most people purchase large, three-phase capable generators for residential use is so they can run said compressor.  If you hook it up incorrectly, however, you are going to destroy that which you most wish to run!

Water heaters, clothes dryers, and stoves - the other 240v appliances in most residences - won't notice a problem.  Their heating elements are simple resistive loads, and the motors/blowers that exist in most of these devices are only 120v.

None of your 120v consumer items will notice the problem either.  Unless, of course, they have inter-connects.  This happens with audio/video systems and computer systems.  If two inter-connected items are plugged into different electrical outlets, and those outlets are out of phase, there is a very large and damaging voltage potential between the two. Remember, in out-of-phase systems neutral is always carrying load and voltage.  Since neutral is often used as a ground reference, inter-connected equipment can be quickly destroyed by the difference in potential.

Partial source:

http://www.answers.com/topic/split-phase

Top