Single phase or three phase?

[Copybell]

Can someone tell me how to tell if a generator is single phase or three phase by looking at the insides.  The nameplate is gone.

[Jim Mc]

Short of a nameplate, and without running the machine, I can think of only two ways.  First would be to count the number of stator main output leads:

If it has -

12 leads - 3 phase
10 leads - 3 phase
9 leads - 3 phase
6 leads - 3 phase
4 leads - could be either 3 or single phase - let us know how they are marked
3 leads - could be either – again, how are they marked?
2 leads - single phase

The second way to tell a single phase from a 3 phase is to inspect the stator windings from one of the ends of the machine.  If they appear to be essentially uniformly spread around the circumference of the stator, it’s probably 3 phase.  If there appear to be a pair of ‘gaps’  in the windings 180 degrees apart, it’s single phase.

Neither way is 100 % certain - tell us more about what you have.

If the machine can be spun up, and the field energized, it's also possible to tell by making voltage measurements across the output leads, but I'm not gonna take the time to explain that until I get more info about what you have...

[Doug]

One more thing, looking at the windings if one group of coils is of a different size of smaller wire its single phase. If all the windings appear exactly the same size and lay around the stator in exactly the same way it poly phase( probaly a lap winding if you look close each coil lays on lop of one slot where it starts and lay in the bottom of the next slot where it ends ).

Generaly single phase machines are wound with the concentric coil system where on coil group is wound with a wire or group of wires that change in span( distance between start and end of coils ) and number of turns per coi.

Poly phase windings are generaly wound "lap" this is where each coil is exactly the same size and the ends overlap each other starting on top and end on bottom.

That much said some eroupean companise ( ABB is a good example ) will use concentric coils in 3 phase machines because its faster and easier to wind. Some english companies made lap wound single phase alternators in the 60s but they were rare and large.

The important thing to look for is a visual difference between coils means single phase and there is probaly a capacitor in there some place too....

Doug

[Copybell]

Doug and Jim
   Thank you for the help.

[Jim Mc]

...if one group of coils is of a different size of smaller wire its single phase...

Why? 

(I'd expect that on a single phase induction motor for starting, but not on an alternator.)

[Doug]

Some brushless alternators are wound almost like an induction motor...
A good common example of this are the older Carrier AC units used on trucks had this winding type. The rotor had diodes in it to generate a field, the AUX winding (with cap) induced the rotor when it resonated at the right speed for 60 hz. It made for a simple alternator but the voltage regulation was a little lack luster

Doug