In the last chapter the generating of a current by means of a dynamo-electric machine was briefly considered. The reversal of the direction of the current induced by the motion of the coil of wire, as illustrated in Fig. 25, is true of all the coils of wire comprising in part the armature of a dynamo. This is further illustrated in Fig. 26, which shows the ends of the wire coil C - C connected with two semicircular pieces of brass, A and B, representing the commutator, which are in contact with flat pieces of copper, E and F, representing the brushes of a dynamo. Assuming that the coil of wire is revolving clockwise, and cutting the lines of force from the N to the S poles of the magnet, a current induced in the part of the coil C is in the reverse direction from that in the part C, and only requires a closed circuit to flow around the coil in the direction shown by the arrows. As the coil continues to revolve until the position of the parts C and C are reversed, the current still flows around the circuit L in the same direction. The direction of the current in the coil has been reversed, but the pieces E and F are now in contact with different brushes, so the current still flows in the same direction around the main circuit. By having a large number of coils of wire in the armature and a corresponding number of sections in the commutator, the current in the main circuit is made practicallv uniform, the current from one coil rapidly succeeding that from the preceding coil.

Studies In Electricity X The Dynamo 259

Fig. 26.

In commercial dynamos the practice is to have from 24 to 50 coils, each coil having several turns of wire, or the equivalent to several turns, as, to save labor, several lengths of insulated wire are wound together and the ends soldered at the proper section of the commutator. The greater the number of coils the more uniform the current, but the size of the machine and its uses regulate the number that are mechanically desirable.

The sections of the commutator are insulated from each other by mica or other nonconductor.

In addition to the coils of wire in the armature of a dynamo is an iron core, the purpose of which is to make a good magnetic path for the lines of force passing through it from the N to the S pole of the field magnets, as the core concentrates these lines of force, so increasing the number cut by the coils of wire, and consequently increasing the efficiency of the dynamo. The magnets between which the armature revolves are called the field magnets. The function of the field magnets is to provide the magnetic lines of force, through which the armature coils revolve. They may be permanent magnets or electro-magnets, the latter being universally used when other than very light work is required. The reason for this is that electro-magnets are capable of giving a much more powerful current than permanent magnets.

Studies In Electricity X The Dynamo 260

Fig. 27.

In the earliest forms of dynamos the field magnets were excited by a current from an outside source; but this form was soon superseded by the self-exciting dynamo. One form, known as the series dynamo, is shown in Fig. 27. The iron cores of the field magnets, after being once excited, retain a certain amount of magnetism, termed residual magnetism. While small in amount, it is yet sufficient to produce some electro-motive force, so that when the armature revolves, a feeble current is produced, which, passing through the field coils, increases the magnetism, which, in turn, increases the magnetic lines of force and the resulting current from the armature coils. This continues until the armature core and field cores are thoroughly saturated with magnetism, and the dynamo reaches its maximum efficiency. By experiment and calculation the size and wiring of the several parts of a dynamo are carefully determined, tha the greatest output may be obtained from a given expenditure of power, and yet not reach a point where excessive or injurious E. M. F. is generated. The series dynamo is a form not much used, as it is not self-regulating under a varying load. If underloaded, the E. M. F. increases excessively ; if overloaded, it decreases rapidly, - the reverse of which is desirable under those conditions.

Studies In Electricity X The Dynamo 261

Fig. 28.

The wiring of the field coils is in series with the outside circuit, and the armature and the whole current passes through them. This necessitates a few turns of large wire for the fields. The load of a series dynamo is usually connected in series.

Another form of wiring which overcomes certain of the objections of the series dynamo is that known as the shunt-wound dynamo, shown in Fig. 28. In this type the field coils form a shunt to the main circuit, only a portion of the current from the armature passing through them. The current, therefore, is divided or shunted, the larger part going directly to the outside circuit, and the balance around the field coils. As this latter current is small in amount, the wire for the field coils of a shunt-wound dynamo is small in size, but consists of many turns. The magnetism produced by the field coils is proportional to the current and the turns of wire, ampere turns, as they are called. Thus 10 turns of a large wire carrying 10 amperes is the equal of 100 turns of smaller wire carrying 1 ampere, and each will exert the same magnetizing force. By reducing the size of the wire, the ampere turns of a shunt-wound dynamo is made equal to the ampere turns of a series dynamo of the same size. The amount of energy required to magnetize the fields, and the efficiency of the two types of dynamos under a normal load, should be the same.

Studies In Electricity X The Dynamo 262

Fig. 29.

The shunt dynamo is more nearly self-regulating under a varying load than a series dynamo, the load being usually in parallel. Therefore, as additional branches in parallel in the main circuit are closed, the resistance falls, and more current is supplied by the armature. This decreases the amount received in the shunt or field coils, thus reducing the magnetism, which in turn slightly reduces the current of the armature, and so regulates the output of the dynamo. A low resistance in the armature is desirable in this type, and also an even strength of magnetism in the fields. To regulate the voltage of a shunt dynamo, a rheostat is generally inserted in the shunt circuit. A rheostat is an instrument containing circuits of varying resistance, with a switch for disconnecting any or all of them.

Another type of dynamo which is self-regulating under wide variations of load is that known as the compound dynamo, shown in Fig. 29. This is a combination of the two previous forms of winding. In addition to the shunt winding of the fields, a few coils of thick wire in series with the main circuit are added. The effect of this is to make the current in the field winding, and consequently the magnetism produced proportional to the current flowing from the armature. The shunt winding maintains the proper voltage and the series winding the volume of current. It is customary, when using this form of dynamo for electric lighting work, to have the series winding slightly in excess of the theoretical requirements, that the voltage of the current may be fully maintained at all parts of the main circuit. This is called over compounding. The various parts of the above types of dynamos will be more fully considered in subsequent chapters.