A Dumb Idea
This may seem as an exercise in futility but, if you will, please imagine in your mind that there exists a generator that you could power from your household AC power and cause it to turn the shaft of a small gasoline engine. Admittedly, it’s a dumb idea but the purpose of this exercise is to better understand the nature of the generator. When you have a proper understanding of the nature of a generator you can then easily recognize problems either in ways to use the generator or its limitations.
While we are imagining a bi-modal generator, in fact there is one configuration. The bi-modality is determined by who is stronger. This is the concept that the reader is asked to grasp.
What is Strength?
“Strength” is measured in potential. When the armature of the generator turns it generates back EMF. It is here where “strength” is adjudicated. Who is stronger? On the one end we have power coming in from the national AC mains electric grid. Applied to the armature, this will cause the armature to turn and develop its own back EMF (electro-motive force). The armature will build up velocity turning faster and faster, building up ever greater EMF until it is able to counter the EMF coming in from the electric grid. At that point, the armature settles to a steady-state velocity.
Let us now suppose that the armature is directly and mechanically connected to the shaft of the small gas engine. The AC mains power is causing the generator armature to turn when is causing the small gas engine shaft to turn. This alone doesn’t accomplish much except run up your electric bill. But it sets the stage for bi-modality.
Moving on to Bi-Modality
We have the arrangement described above where the AC mains is powering the generator turning the shaft of a small gas engine. We are in a steady-state operation where the back EMF of the armature is countering the AC mains EMF. Let us suppose that the small gas engine is running but with the throttle not turned up high. It is in a way coasting along with the generator armature. But now we turn up the throttle of the small gas engine. We turn up the throttle such that the small gas engine is able to make the generator armature rotate faster. When the armature rotates faster it builds increased EMF. This changes the ball game. The generator back EMF is now “stronger” than that of the national electric grid. Our little household generator is now actually sending power to the AC mains. The power meter at the household service entrance actually begins to turn in reverse LOWERING your electric bill.
The stronger guy wins. Whenever the armature of the generator turns it develops back EMF. “Somebody” can mechanically turn the armature (a gas engine) producing EMF that can power a house. But likewise, if EMF is applied to the armature where it had previously been supplying power, the armature will begin to turn and build up velocity until its back EMF can counter the applied EMF. That mechanical energy can be used to do work such as maybe mechanically power a well pump.