Hooking two (gasoline) generators together to make 220VAC
I think you have misinterpreted the original question, which was only about hooking two generators in parallel in order to get enough current. Hooking them in series to increase the voltage was not mentioned.

Of course it's possible &mdash; that's exactly how all of the multiple generators attached to the national distribution grid are connected! Alternators of this type are synchronous machines, and function equally well as motors as well as generators.

The key to making it work is to make sure that they are in phase before connecting them. Once you throw the switch, they are effectively "locked" together as if their shafts were physically coupled. Each one will then add or subtract power to this "mini-grid" according to the torque on its shaft. If one tries to run slower than the other, its generator will be driven by the other as a motor, keeping it up to speed.

One simple way to check the phase is to simply connect some light bulbs across the circuit breakers. Make sure that they're rated for 2&times; the phase voltage, because that's what they'll be getting when they're out of phase!







simulate this circuit &ndash; Schematic created using CircuitLab

Fire up the first generator and connect it to the output grid. For each subsequent generator, you fire it up and watch its light bulbs. They will flash at a rate that's equal to the frequency difference between that generator and the grid. Adjust the speed of the second generator until the flashing slows and the light bulbs go out. At that moment, the generators are at the same frequency and phase, and you can connect the new generator to the grid.

Of course, the small DIY generators that we're talking about aren't really meant to be controlled in this way. They generally have simple mechanical governors that keep the frequency approximately right, and voltage regulators that modulate the field current to keep the output voltage approximately in the right range. These mechanisms would probably be "confused" to some degree by such a hookup.

It's also possible that the engine is over-powered relative to the generator, and has more drag than the generator (when operating as a motor) can overcome. This would force the two generators out of sync, and large currents would flow, hopefully tripping their breakers and disconnecting them.