Need help identifying 3-phase connection

I am trying to construct a circuit diagram for a diesel generator power system. I have read quite a bit online about 3-phase, but I haven't found an explanation for what I am seeing:

The generator has only three leads. Between lead "A" and lead "B" I read 208 volts. Between "A" and "C" I read 120 volts and between "B" and "C" I read 120 volts.

I don't think these voltages are consistent with normal delta, high-leg delta or open delta, delta with a center tap on one phase, nor with a wye system. Incidentally, the generator powers 120V lights, some baseboard heaters of unknown voltage, and a big motor that is labelled "3 phase."

Can anybody tell me what connection arrangement I have?

Thanks in advance.

There is on more lead someplace. Follow it back from the three leads to the motor.

And the generator model is what?

Good idea to start with the model number! I'll scrape off some of the dirt and grime and try to find it.

It would appear you have a three phase four wire 120/208 'Wye' connected genset but you are missing one leg.

Connection 'C' appears to be your common center point for the wye and you are missing a 'D' connection for the third phase.

I have a old military genset that can be reconfigured for a number of different voltage and phase combinations and if I am remembering right in one of the 3ph connections for the 120/208 output one of the lugs that is a phase in another configuration becomes the center point connection for the wye setting.

It seems that your C leg has blown a fuse or something. If you had a WYE configuration you would have 4 wires total. I am thinking you are missing the fourth wire though somewhere.

tcmtech said:

It would appear you have a three phase four wire 120/208 'Wye' connected genset but you are missing one leg.

Connection 'C' appears to be your common center point for the wye and you are missing a 'D' connection for the third phase.

I have a old military genset that can be reconfigured for a number of different voltage and phase combinations and if I am remembering right in one of the 3ph connections for the 120/208 output one of the lugs that is a phase in another configuration becomes the center point connection for the wye setting.

Click to expand...

Right you are.
I learned something today.

I don't understand it though.
Would need more than the 6 leads to get 120/240 single phase.

"Phase Conversion in Generators:
(1) Reconfiguring Coil Connection
A three-phase generator can be converted to a single-phase one by altering the connection between its stator windings inside or outside the generator head. For instance, in the case of a 3-phase generator, you would have 6 leads. Larger generators commonly have 12 leads from six coils and all the wires come out of the generator making it easier to configure the generator in a variety of ways as follows –

• Connecting the coils in series will convert the generator to a single-phase one.
• By connecting opposite coils in series, you can double the output voltage.
• A parallel connection will double the flow of current. "

http://www.dieselserviceandsupply.com/Generator_Phase_Conversions.aspx

I don't understand it though.
Would need more than the 6 leads to get 120/240 single phase.

Click to expand...

Six leads is all you need to get 120/240 single phase from a 120/208 or 120/120/120 three phase genset.

For the 120/240 single phase setup all you do is configure it into the 120/120/120 delta setup then shift your bottom phase left or right. (_/\ or /\_)
From that the bottom phase connections that were moved left or right make up one side of the 120/240 and the other two, still in their open ended delta configuration, make up the opposing 120 so when connected end to end you get 120 - 0 - 120 with the proper 240 across the two ends.

Glad you got this figured out!