Wishing to observe first hand the effect of partially
interconnecting two out-of-phase AC power sources, I determined
to observe the effect via the 'Dark Lamp Method' often used to
determine when the full inter-connection of , say, two
generators, can be properly made.  In this technique, a lightbulb
is placed between the phase conductors to be mated- two
lightbulbs if it's a three phase situation.  The remaining leg is
interconnected (this is often done through transformers to limit
current).  In a single phase situation, the single light bulb
should pulse at the difference frequency (if there is a frequency
difference) or remain steady at some constant level if the
frequency is the same.  If the two sources are in syncronization,
that is, in phase, that steady level is zero, and the bulb will
be 'dark'.
 
So I hooked my PC-300 $70 inverter neutral to neutral with the
grid house current, and put a 40 watt light bulb between the 'hot
leg' of the inverter and the 'hot leg' from the wall outlet.
 
I expected to observe a slowly pulsing light bulb and thereby
know how closely the 60hz from each source matched, as well as
confirming my expectations in general.
 
Instead, the bulb grew quite bright over a period of about 1/2
second, culminating in a blown lightbulb and a $70 smoke signal
from the inverter.
 
What went wrong?:  I forgot to consider what I should have known! 
That when the two sources were 180 degrees out of sync, the
voltage difference would be 240V! ( [+]120V - [-]120V ) -not the
usual 120V that I was erroneously expecting. Dang!
 
I should have used two bulbs in series.  Hmmmm..., or maybe I
should have just watched a movie or read to the kids.
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