Three phase AC
Single phase AC has the advantage that it only requires 2 wires. Its disadvantage is seen in the graph at the top of this page: twice every cycle V goes to zero. If you connect a phototransistor circuit to an oscilloscope, you will see that fluorescent lights turn off 100 times per second (or 120, if you are on 60 Hz supply). What if you need a more even supply of electricity? One can store energy in capacitors, of course, but with high power circuits this would require big, expensive capacitors. What to do?An AC generator may have more than one coil. If it has three coils, mounted at relative angles of 120°, then it will produce three sinusoidal emfs with relative phases of 120°, as shown in the upper figure at right. The power delivered to a resistive load by each of these is proportional to V2. The sum of the three V2 terms is a constant. We saw above that the average of V2 is half the peak value, so this constant is 1.5 times the peak amplitude for any one circuit, as is shown in the lower figure at right.
Do you need four wires? In principle, no. The sum of the three V terms is zero so, provided that the loads on each phase are identical, the currents drawn from the three lines add to zero. In practice, the current in the neutral wire is usually not quite zero. Further, it should be the same guage as the other wires because, if one of the loads were to fail and form an open circuit, the neutral would carry a current similar to that in the remaining two loads.
The voltage (top) and square of the voltage (bottom) in the three active lines of 3 phase supply.
Do you need four wires? In principle, no. The sum of the three V terms is zero so, provided that the loads on each phase are identical, the currents drawn from the three lines add to zero. In practice, the current in the neutral wire is usually not quite zero. Further, it should be the same guage as the other wires because, if one of the loads were to fail and form an open circuit, the neutral would carry a current similar to that in the remaining two loads.
