Why Are Phases in a 3-Phase AC Generator Separated by 120 Degrees?

[jearls74]

Hi everyone, i have a question or two about the phases of a 3 phase A.C. generator. I have read about and seen several sine wave charts that state the phases are separated by 120 degrees, can someone explain this? i know the obvious that a circle is 360 degrees and that divided by 3 equals 120 degrees, but in my design the phases are 15 degrees apart. I am using a 8 magnet design ( following the rule there are one pair of magnets for every 3 coils ). The plotted results on a sine wave chart i drew give me 3 positive peaks followed by 3 negative peaks and repeats 3 more times for one revolution ( 4 voltage cycles per revolution). How does 120 degrees come into play? and my second question is: why is phase 2 connected in reverse? I will include a drawing of the stator so you can get an idea of what I am talking about on the 15 degrees apart. Can anyone shed a little light on this?

 

[famousken]

Im not sure about phase 2 being in reverse, but, to answer your question about the phases being 120 degrees apart. In your design they ARE 120 degrees apart, but only electrically. One cycle of ac electricity is going from 0v then swinging positive, negative, then 0 again. So for one mechanical cycle of your alternator, you have four electrical cycles. Your design is actually quite common in power generation plants to get the required 50 or 60Hz at a reasonably low generator shaft speed (they wouldn't last nearly as long if they had to spin at 3600 rpm)

 

[sophiecentaur]

Hi everyone, i have a question or two about the phases of a 3 phase A.C. generator. I have read about and seen several sine wave charts that state the phases are separated by 120 degrees, can someone explain this? i know the obvious that a circle is 360 degrees and that divided by 3 equals 120 degrees, but in my design the phases are 15 degrees apart. I am using a 8 magnet design ( following the rule there are one pair of magnets for every 3 coils ). The plotted results on a sine wave chart i drew give me 3 positive peaks followed by 3 negative peaks and repeats 3 more times for one revolution ( 4 voltage cycles per revolution). How does 120 degrees come into play? and my second question is: why is phase 2 connected in reverse? I will include a drawing of the stator so you can get an idea of what I am talking about on the 15 degrees apart. Can anyone shed a little light on this?

I'd need to see some more details but what you describe sounds either wrong or you are quoting out of the context. In a correctly wired alternator the phases should be even spaced.

 

[jearls74]

Here is a drawing of the stator and magnets

 

[sophiecentaur]

And . . . . .?
It looks ok. Did I miss something?
Where are the windings and how are they connected. Is there a picture of the waveforms you refer to?

 

[jearls74]

Im reworking my wave form chart, ill try to post it tommorrow, as for the windings, I am not that good at detailed drawing. Wouldnt all the 1's be i set of windings and all the 2's be the second and so on? Have i assumed wrong? The 3 phases would be connected in the delta set up. I also don't follow on how the phases are electrically separated by 120 degrees, i understand the mechanically they are separated by 120 degrees. if this could be explained in detail i would appreciate it. Also, thanks for the replies famousken and sophiecentaur.

 

[sophiecentaur]

The phase of the volts on each coil relates to its position in the rotation wrt the magnetic field. If you had three coils lined up, side by side, on a bench and moved a magnet past them, the voltage peaks would occur one after the other
Now consider an alternator with just three coils and a bar magnet rotating inside. Not necessarily efficient but it would give you a feel for what goes on as the magnet sweeps past each coil. The 120 degree relative phases from each coil would follow pretty logically from that setup.

 

[jearls74]

I reworked my sine wave chart and found out what i was doing wrong. Does this look right?