# Digging today and learning - Voltage selection on some rotary generators - 120v only or 120/240v split



## FlyFisher (Jun 30, 2018)

Based on the questioning in another thread quoted below for reference:



FlyFisher said:


> Browse Deweb said:
> 
> 
> > You can reduce your inrush current with a soft start kit for your air conditioner. Most larger generators that can supply both 120V and 240V can be configured to supply a higher 120V only plug which puts the generator windings in parallel, doubling your available current on that leg. For example, on the Honda EU7000is, the 120V Only switch position results in the diagram on the bottom of this image:
> ...


I decided to open up the electrical panel on my 2600w rotary and trace it a bit. It does have a voltage selection switch. See pictures:



















I never studied this one in much depth. I always set the switch to the 120/240v position and ran off the L14-30 for both voltages.

My assumption was that the duplex 5-20's were wired one per leg/phase.

One point from the above quotes to emphasize here:



FlyFisher said:


> For example - a rotary generator with a neutral bonded to ground/frame - the center tap on the alternator (neutral) is pulled down to ground (which technically it should be, in the vast majority of scenarios) and the end taps are your two poles - L1 and L2. What your schematic shows is you simply tie L1 and L2 together and you double the amperage at 120v, while taking out the 240v ability. This is absolutely wrong. L1 and L2 are 180 degrees out of phase. If you short L1 and L2 you blow up your generator. What ever phase L1 is in, you need to shift L2 180 degrees so that it synchronizes back to L1.


After tracing the wiring and opening up the box that was debunked. This is quite a mess of wires but you'll notice a terminal block in the back with all the screw terminals and wires. That is the back side of the voltage selector switch - 3 pole, 3 position (I believe center is "off" - so "on-off-on" are the 3 positions).










If you look to the bottom left of the image that is the input side of the double pole 30a breaker. The blue and brown wires are the extremities of the alternator windings - so on 240v these would be your L1 and L2 lines. Those go into the switch and then either get sent direct to the L14-30 in the 120/240v position, or they get combined at the switch with neutral to parallel the 2 poles of the alternator.

See crude schematics below:

The first schematic is the 120/240v operation. Note the coil representing the alternator on the left. This is how all alternators, speaking of a schematic representation of their wiring, work with 3600rpm 120/240v capable units. On top of it - when a generator is said to have "neutral bonded to frame" that is exactly what is going on - the center tap, neutral, is taken right down to ground and they become one in the same - just like neutral and ground in your split phase electrical box at the service entrance of your home's power distribution system (it is not common for sub panels, but I believe it is required at the service entrance).









2nd schematic:

Note the separation between the alternator poles. Neutral can not be bonded to ground because what was neutral on the bottom pole is now hot. You can't short hot to neutral. What was L2 on the previous schematic is now neutral on the bottom pole. Again, you can't short hot to neutral.

Thus for this selectable voltage and paralleling poles it is impossible to have a neutral bonded to frame/ground on the alternator poles. The alternator MUST be floating above ground.

Yes, I did use gray to represent both neutral on the top pole and hot on the lower pole - I did that on purpose because that is the same end of where the poles are joined in the 240v config. It should be easy to see why bonding the 2nd pole's hot side to ground would be a problem.









If you look at the following waveform traces - each waveform is 1 leg of a split-phase circuit, so each waveform is 120v. The dim/bright spots in the traces are just the refresh/scan rate on the 'scope - the camera speed is too quick.










If you combine those waves in SERIES - that is measure across both poles combined you get the following waveform:










Without floating neutral to change it being at 0vAC (center tapped alternator neutral) to 120vAC (end of the pole tap, L1 or L2) you can't get the power waves to lay on top of each other - they are 180deg out of phase and cancel each other out.

The below waveforms simulate this. It is physically impossible to create an actual power wave form like this, thus the simulation - its done with 2 channels on the oscilloscope. In a 120v power circuit that would be akin to a single channel (one set of hot and ground), whereas the image is of 2 waveforms, or 2 sets of hot and ground, phased to simulate:











Go back to my 2nd crude schematic above for the 120v only switch position on the generator. The split alternator allows you to reverse the wires - to bring what was neutral on the 2nd pole to hot and what was the 2nd hot on the 2nd pole to neutral. That is where you get the 180deg phase shift that would take the above waveform and lay both channels (each channel representing a "phase" in this simulation) on top of each other in-phase. Only then do you get the "double in amperage".


----------



## iowagold (Jan 22, 2020)

yea there are some weird science things that can work if the stator is set up right.
gen heads are not all the same...

i guess for me this is why i like the eu7000is honda inverter gen set.
it does all of the weird science stuff inside the inverter...
so you can do things like 120 vac only and parallel...

the mode i like for the gen set is to do a 50 amp marine outlet on the gen set...
that way in the 120 vac mode you can get all of the power out through one outlet.
pm for details on that mod.

cool find on the stator setup in this old school gen!


----------

