Hi -
"When I measure AC volts with voltmeter (test coil at some RPM), I can use the formula Vac x 1.4 = Vdc."
Yes - although in the end we'll need to subtract for rectifier losses but that comes off the competed stator so we don't worry too much about that with a single test coil.
"If I have 4 coils in one phase (all in series connection), my phase voltage would be 4x V(1coil)."
Yes - but remember that if we wire teh stator in Star, then we have to consider the other coils in series that are out of phase, so we have 4 coils in series, and then the 2nd 4 coils in series will offer 1.73x the voltage of a phase. So if we're building a 3 phase machine with 4 coils per phase (a total of 12 coils in the machine) then the final output will be from 8 coils, so the actual output will be 1.73 x the sum of your 4 coils.
"If I get, let's say 2.14V with one coil voltage with 60 RPM in a system with 4 coils per phase, I get 4x2.14=8.57, and than 8.57x1.4= 12V on 60 RPM in total system. Is that so?"
That would be true in a single phase machine wired in Delta. In star though you've actually got 8 coils in series to think about, and 4 of them are out of phase. So the final output is 1.73x the sum of 4 coils.
"I think I need to turn the generator to the speed (RPM) of desired cut-in speed to measure it correctly."
Not really - you can turn it at any speed so long as you know how fast its turning since voltage will increase directly with rpm. (if you have 10V @ 60 rpm then you'll surely have 20 volts at 120 rpm). So you just need to know the ratio of volts/rpm.
"Only, I don't know how danB measured all that with 110 turns on a coil, and how he find out that he'll need 44 turns (45 for avoid rectifier losses)?"
Because voltage is also directly related to the number of windings. So long as I know the wire gage, number of windings and the rpm at which we're testing, I can extrapolate and figure out how many windings I really want and I can know what size of wire will fit to make the proper sized coil.
If my desired cutin speed is 60 rpm, and I know I want 4 Volts but my test coil is of 110 windings is giving meme 10V, then I know I need 44 windings to get the right voltage. I also want my actual coils to be the same size, so I have to figure out what wire gage will achieve that. 3 strands of 14 with 44 windings would be the same size as a coil wound with a single strand of 14 and 132 windings. In this case though my test coil has only 110 windings, and I believe 3 strands of #15, 45 windings will be very close to the same size.