-what determines how much voltage at what cycle you get?
-can you get AC and DC from the same powerhead or is it one or the other?
-split voltages from an alternator.. say 230/115.. is that just from how many legs you use?
All generators generate AC power. DC generators have a quite intuitive mechanical rectifier in the form of a commutator and brushes.
Split phase power (US 120/240V) is generated by a generator that has two coil groups 180 degrees out of phase with another in relation to the generator poles, and connected together by a common center tap to the winding called the neutral. Three phase in its various voltage forms is basically the same, except that the three phases are displaced by 120 degrees.
The dual voltages comes from leg to leg vs leg to neutral in both types. For instance the old 120/208 or the wild leg 120/240 three-phase systems are 120 leg to neutral (except for the wild leg on high leg delta systems will be 208V to neutral) and 208 leg to leg (120/208) or 240 leg to leg (high-leg delta).
The more common these days is 277/480 wye configuration which allows you to run large motors like this without breaking the bank for wiring to the load:
For 120/240 split phase loads powered off 277/480 you have to install transformers in your shop, either in the equipment or standalone for things like 120V wall outlets. All my machine tools (lathe, vertical mill, welders, air compressor, etc..) are all 480V three-phase. But we do have a big "box" transformer for 120V power in the shop, that converts the center leg of of the 277/480 to 120V single phase.
So it all depends on the size of your loads and how much you want to spend on wiring. If you have a big shop you'll use 277/480. If you have a lot of split-phase equipment (240V air compressor, plaz, machine tools, etc.) you'll likely use US 120/240 split phase power. If you have a lot of smaller three-phase machine tools you'll use 120/208.
It all depends on your major draw equipment. If the majority of your big equipment is 120/240 split phase then installing a 120/240 split phase generator is the best, and install a phase converter to run your three-phase stuff. If the majority is three-phase, then install a three-phase generator and transformers to run your split phase stuff.
I don't know what you have, so I can only guess and provide all the scenarios I can think of.
In the case of generators to run your shop, the generators that are available are not permanent magnet. They are self-exciting wound field type. They have to run at a specific speed to produce 60 Hz current, and a device called an AVR regulates the field current to produce the correct generator output voltage.
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Chris