D.J. said:
can anyone give me a simple explanation of Phases of elctricity? I have looked it up before and just got more confused.
A single phase a.c. generator will produce one complete cycle of voltage for each revolution of its rotor. In other words, during each and every revolution the voltage increases to a maximum, decreases to zero, increases again to maximum but with the opposite polarity, decreases to zero, increases to maximum with the original polarity, etc etc.
The rotor moves through 360° in each revolution and we can identify points on the waveform it produce as it interacts with the fixed field by saying that each cycle takes, or lasts, 360°. We refer to these points as the phase or phase angle.
The value of voltage, expressed as a fraction of its peak, at any point in the cycle is equal to the sine of the phase angle.
For instance at one twelfth of a revolution, or 30°, the voltage will be 0.5 (sin30) of the peak. At one quarter of a revolution, or 90°, the voltage will be at its peak value or 1. If you check, by taking the sine of greater angles you will see that after 90° the amplitude decreases to zero at 180 and then increases in magnitude (size) but with a negative sign indicating that it is of the opposite polarity. The magnitude increases in a similar manner, rising to a maximum at 270 then decreasing to zero again at 360/0°.
In order to understand it, you might like to think of a 3 phase generator as being 3 single phase generators on the same shaft but one of them is physically oriented on the shaft 120° further round than the first one. And the 3rd one is mounted 120° from the 2nd (240° from the first.)
One end of each generator's winding is connected together, this being the common, or neutral, connection.
The three windings interact with the same field winding.
Now, when the output of the first winding, which we will call phase A, is at zero, because the other windings are symetrically displaced around the rotor shaft they will produce zero volts at symetrically different points of the shaft's rotation. For phase B to be at zero the shaft will have to rotate 120° from the point where phase A produced zero, and for phase C to produce zero the shaft will have to rotate a further 120°, or a total of 240°. 120° later phase A will have completed one complete cycle and the sequence repeats.
Maybe the picture will help.