Good morning, another day with plenty of showers for us. Now trying to second guess when the rain will ease long enough for us all to go up to the pond and be entertained by feeding the small fry.
In my misadventure with the generator yesterday, I tried using a multi-meter set on AC to determine if the generator was putting out 240 volts. But when I measured between the two terminals that were supposed to have the 240 volts on them, I only got about 7 volts. Yet each of those terminals measured 120 volts with respect to the neutral terminal. I've seen this before with the 240 volt service to the house, and am curious as to why that is.
Most single phase generators I have seen with dual voltage use 2 windings. If they have a selector switch then in principle, when set to the lowest voltage the windings are connected in parallel (the sinusoidal waveforms from each are then in phase with one another), allowing the output current to come equally from each winding, giving maximum current output.
In the highest voltage position the windings are connected in series to make them 180 degrees out of phase with one another, producing 240V. A 120 V socket outlet may also be connected across one the windings in this configuration, allowing 120 appliances to be used at the same time, albeit at a lower current than in the low voltage position.
My guess is that you were taking a measurement at a place where the windings were in phase instead of out of phase. The 7V discrepancy( instead of it being exactly 0V), could be explained by the windings occupying different physical positions in the stator slots. The revolving magnetic field passes each winding at a slightly different instance in time, hence their AC waveforms are never perfectly 0/180 degrees in/out of phase with each other.
In practice, generator windings are often complicated by having 3 wires instead of just one at each end (with the extra wire being taken from a tap part way into the winding), if you ever need to disconnect them from the terminal board, make sure they are well marked and go back in the same position.