Polyphase Machines.—If two armatures, of the same number of turns each, be connected to the shaft at 90 degrees from each other, and re volved in a bi-polar field, and each terminal be joined to a collector ring, we have two separate electromotive forces differing in phase by 90 degrees or a two-phase machine. With 120 degrees phase difference and three sets of arma tures we have a three-phase winding. By prop erly interconnecting the three circiuts, we may use but three wires for transtnission, or four, in accordance with the system used. The con struction of multiphase machines is similar to that of the single-phase type, excepting that in the former we have as many armatures, series connected, as there are phases.
In the two-phase three-wire system, the wire from the common junctions of the phases car ries 1.414 times the current of the outer wires. The electromotive force between the outer wires is also V2 E, when E is the electromotive force per phase, or between either outer wire and the common return. When this system is used it is important that the load be carefully balanced on the phases and that the power factor be kept as high as possible in order to lceep the voltage on the phases nearly alike at the receiving end. Single phase motors or lamps may be connected to either or both phases, but it is very import ant that no load be connected between the outer wires, as the effect is to badly unbalance the voltages on the different phases.
In the three-phase star connected system the line voltage is 1/3---- 1.732 times the voltage on the coils of the machine, or the machine volt age, which is the pressure between any one of the three line connections and the common neu tral. The line current in this system is the cur renethat flows through any one of the machine windings. In the delta connection, the line voltage is tbe same as the voltage across any phase of the machine, while the line current, being the resultant of two currents, is NI1.732 times the current flowing through any phase of the machine.
Energy Polyphase.— In a two-phase cir cuit, whether three or four wire, the energy flowing is the sum of the products of each phase current by the phase pressure. Two wattmeters are used. In the three-phase system when E= volts between lines; I= amperes on lines; W= total watts output of machine,— then, whether the connection be star or delta, the total.
output X El, always supposing V3 the sYstem be balanced. Thus the output of the machine is not changed by changing the con nections from star to delta. In the balanced three-phase system, one wattmeter will register the total output if its constant be multiplied by 1.732. Two wattmeters are usually employed.
Regulation of Alternators.—The regulation of modern alternators varie.s from 5 to 6 per cent, which means that in case the full, non inductive load of an alternator be ta3cen off, the speed and excitation being kept constant, the terminal pressure will rise by an amount corre sponding to from 5 per cent to 6 per cent of its full load voltage. Close reg-ulation means a much better voltage-regulation on the system and stronger synchronizing. power. A certain amount of armature reaction is necessary to avoid large cross currents on changing the field of one or more machines operating in parallel, in the attempt to .preserve the same terminal voltage. The efficiency of large alternators is about 96 per cent to 97 per cent.
In regard to the frequency best adapted to transmission work, or to local distri bution, various factors enter into the problem. At 60 iv both arc and incandescent lamps can be operated satisfactorily. The transformers are smaller and cheaper than at 25 iv and motors are very satisfactory both as to low first cost, range of speed, and good starting torque (q.v.). Frequencies over 60N have been aban doned. The line drop, due to reaction, increases with the frequency: a change of frequency from 25 to 125 f%) would, on the same line, more than double the line drop. While as a rule 60 iv apparatus is cheaper than that for 25 N., yet the increase in polar speed often becomes difficult without increasing the number of poles to an undesirable extent, which, in 60 NJ apparatus, may be sufficient to malce the parallel operation of low speed direct-con nected alternators quite difficult.
When a current is intro duced into a circuit a magnetic field is produced, surrounding the conductor, the rise of which causes a counter electromotive force. This elec tromotive force is called the electromotive force of self-induction. The effect of self-induction upon electric currents is directly comparable to the effect of inertia on a material body. It is that quality that tends to hinder the introduction, variation or extinction of the current in a cir cuit As this effect is greatest at times of most rapid change of magnetism set up by the current, in alternating current circuits, it becomes a maxi mum when the inducing current is passing through zero, and, therefore, the counter elec tromotive force of self-induction lags 90 degrees behind the current in the circuit. It also follows the sine curve provided the current flowing is sinusoidal.