"I think the reality of the situation is that, when the AC induction motor is running unloaded, it's armature speed is synchronous to that of the incoming AC line. Under that scenario, the current flow becomes a totally reactive load 90-degrees out-of-phase with the driving field's phase. Essentially, the armature's current is zero... the only time the armature's current builds to any respectable level, is when SLIP occurs, in which event, armature current rises dramatically because the resonance is now lost... XL doesn't equal XC."
I'll put this another, possibly simpler way.
An ac power system cannot transmit any power without there being at least some phase shift.
For, at 0 degrees phase shift, there can be no power transferred, while at 90 degrees phase shift, the power transferred is maximized.
However, every ac system everywhere must meet a stability criteria, which is perhaps best summed up in the so-called "equal area criteria", heavily paraphrased thusly:
If the area under the curve on the leading side is less than the area under the curve on the lagging side, then the system is unconditionally stable.
If the area under the curve on the leading side is equal to the area under the curve on the lagging side, then the system is conditionally stable.
However, if the area under the curve on the leading side is greater than the area under the curve on the lagging side, then the system is unconditionally unstable.
What this is saying is if you operate synchronous ac machines below 90 degrees, you will transmit power; but should you operate these machines above 90 degrees, some generators will become motors, and you will more than likely break their shafts.
At the electric utility where I was an EE more than thirty years ago, we actually had this happen. Twice!
General Electric couldn't explain this phenomenon, and our operating people couldn't either.
The group of which I was a part got involved and we developed mathematical models of the system, and of the turbine-generator, and we isolated the problem to a resonance between the "series compensated" [ * ] transmission line, which was very long, and ultra-high voltage, and the turbine-generator's shaft.
For, when the transmission line became cold enough, hence its resistance became low enough, the series capacitors, combined with the self-impedance of the T-G could cause the T-G to become a motor, and instead of converting 750 MW of mechanical power from the turbine into 750 MW of electrical power onto the line, a tremendous amount of electric power from the system, through the line, would flow into the T-G, and the excess mechanical power, having no where to go, was forced into the T-G's shaft, causing it to break.
That's what can happen if the "equal area criteria" is violated ... 1 million horses [ ** ] pulled apart that turbine-generator. Twice.
Under conditions of the second warrantee settlement with G-E, we agreed not to operate this line using "series compensation", under specified very cold weather conditions.
I guess the moral of this story, and, indeed, this thread, is when you operate ac power systems on the "bleeding edge", perhaps something, or someone is going to bleed.
I prefer not to bleed, so I exited the discussion.
[ * ] Here is a case where capacitors are in series with the line. The intended purpose is to compensate (that is, eliminate), to the greatest degree possible, the effects of the inductance of the line. However, by the Law of Unintended Consequences, if you don't know what you are doing, you can overcompensate, thereby creating a potentially unstable condition.
[ ** ] The actual value is more like 1,005,326 HP, all of this being loosed into the T-G's shaft, thereby over stressing it, and ultimately breaking it. This causes a big, very expensive KABOOM!
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