Ques. How is three phase current generated?
Ans. It requires three equal windings on the alternator armature, and they must be spaced out over its surface so as to be successively ⅓ and ⅔ of the period (that is, of the double pole pitch) apart from one another.
Ques. How many wires are used for three phase distribution?
Ans. Either six wires or three wires.
Six wires, as in fig. 1,260, might be used where it is desired to supply entirely independent circuits, or as is more usual only three wires are used as shown in fig. 1,261. In this case it should be observed that if the voltage generated in each one of the three phases separately E (virtual) volts, the voltage generated between any two of the terminals will be equal to √3 × E. Thus, if each of the three phases generate 100 volts, the voltage from the terminal of the A phase to that of the B phase will be 173 volts.
Fig. 1,264.—Experiment illustrating self-induction in an alternating current circuit. If an incandescent lamp be connected in series with a coil made of one pound of No. 20 magnet wire, and connected to the circuit, the current through the lamp will be decreased due to the self-induction of the coil. If now an iron core be gradually pushed into the coil, the self-induction will be greatly increased and the lamp will go out, thus showing the great importance which self-induction plays in alternating current work.
Inductance.—Each time a direct current is started, stopped or varied in strength, the magnetism changes, and induces or tends to induce a pressure in the wire which always has a direction opposing the pressure which originally produced the current. This self-induced pressure tends to weaken the main current at the start and prolong it when the circuit is opened.
The expression inductance is frequently used in the same sense as coefficient of self-induction, which is a quantity pertaining to an electric circuit depending on its geometrical form and the nature of the surrounding medium.
If the direct current maintains the same strength and flow steadily, there will be no variations in the magnetic field surrounding the wire and no self-induction, consequently the only retarding effect of the current will be the "ohmic resistance" of the wire.