CHAPTER IV.

Types of Field Magnets-Types of Armatures-Exciting Power-Magnetic Circuit-Magnetic Resistance-Formulas for strength of Field-Single and Double Magnets-Difficulty in Small Dynamos-Characteristic Curves -Horse-power Curves-Speed Characteristics-Application to Electric Tramcars-Static, Dynamic, and Counter-Electro-motive Force.

IN the preceding chapter it has been shown how the electro-motive force of an armature can be found if the total number of lines passing through its core be known. It will now be necessary to determine the number of lines, that is the strength of the magnetic field, from the constructive data of the machine. Before entering into a scientific investigation of the subject a cursory glance at the different types of field magnets adopted by the various makers of dynamos and motors, will be of interest. These are shown in Figs. 27 to 51. To make the classification comprehensive the type of armature is written beneath each field and the maker's or designer's name is written above it. We distinguish three types of armature. 1. The Drum, wound on the Hefner-Alteneck principle, as explained in Chapter II., and shown in Figs. 18 and 19; 2. The Cylinder, wound on the Pacinotti or Gramme principle, also explained in Chapter II., and shown in Figs. 25 and 20; and 3. The Disc, wound on the Pacinotti or Gramme principle and only differing from the cylinder by the shape of the core. It is a cylinder of considerable diameter and small length, in fact a flat ring or disc.

All the magnets employed in dynamos or motors are horse-shoes; straight-bar magnets with poles at the ends being never used. The reason is obvious. We must in all cases bring opposite poles to the same armature, and that necessitates the employment of a bent magnet. It is necessary to distinguish between single, double, and multiple magnets. In the single horse-shoe magnet all the lines passing across the armature go through the magnet in the same direction. As an example we may take the Edison-Hopkinson dynamo, Fig. 27. The lines passing across the armature from N to S continue all in the same direction, viz., vertically upwards from S to B, thence across the yoke from B to A, and finally vertically, downwards from A to N. A free unit pole would be urged along the closed magnetic circuit NSBAN, and there is no other way along which it could travel. Now in a double horse-shoe, as represented for instance by the Weston machine, Fig. 41, there are two ways along which a unit pole might travel. One of these is N S BAN, and the other N SD CN, or in other words, of the total number of lines passing across the armature, one half will go through the horse-shoe NA B S, and the other half will go through the horse-shoe N CDS. We may consider the field magnets to consist of these two horse-shoes placed with like poles in contact to the left and right of the vertical center line. The arrangement of the "Manchester" dynamo is similar, but in this case the portions A B and C D, which in the Weston dynamo constitute the yokes, form the excited or active parts of the magnets and are surrounded by the magnetizing coils. The field magnets of the original Gramme dynamo (or motor) also belong to the double horse-shoe pattern. But in this case a plane laid through the center lines of the cores of the

TYPES OF FIELDS.

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magnets is parallel to and contains the center line of armature shaft, whereas in the Weston type it is at right angles to it. Here, again, the lines are split up to the right and left of the vertical center line into two distinct circuits. Fig. 37 shows a similar arrangement, but with a single magnet. Figs. 39, 40, and 50 show single magnets, the plane of the horse-shoe being at right angles to the armature. Fig. 48 shows a quadruple horse-shoe magnet. Here the lines of force passing across the armature belong to four distinct circuits: S DAN, SD CN, SBA N, and S BCN. The field magnets of the Brush (Victoria) machine shown in Figs. 46 and 47 consist of 8 complete horse-shoes, four on each side of the disc, and in some multipolar machines even a larger number of magnetic circuits is sometimes employed. The latest machines of M. Marcel Deprez (Fig. 51) have two cylinder armatures mounted on the same spindle a b, and around them are placed eight horse-shoes, of which two, SBAN and SCD N, are shown in the illustration. It is not necessary to enter into a detailed description of all the types shown, as the diagrams are alone sufficiently clear.

After what has been said above it will be evident that the proper function of the field magnets in a dynamo or motor is to produce lines of force which pass across the armature core. All other lines which miss the armature are useless and may even be detrimental to the working of the machine. The greater the number of useful lines the greater will be the electro-motive force generated at a given speed and with a given armature. Our aim should therefore be to produce a maximum number of lines, and as a first step towards the realization of this object we must determine the relation between the number

of lines and the constructive data of the machine. of these data is the exciting power, that is the prod the number of turns of wire wound on the magne the magnetizing current sent through the wire. customary to reckon the exciting power in Ampere-1 and it is shown by experiment and theory that the m in which the product is made up is quite imma We may have a large number of turns of fine wire small current, or we may have few turns of stout and a large current. The effect will always be the if the product of amperes and turns be the same. periment also shows that for low degrees of magnetiz the electro-motive force produced in the armature i portional, or nearly so, to the exciting power P a] to the field magnets; and since electro-motive forc strength of field Z are always proportional, we fin in these cases Z is proportional to P. We can rep this relation mathematically by introducing the conce of magnetic resistance. According to this there every magnetic circuit a passive force opposing the tion of lines, and the number of lines which are nev less created is the quotient of the magnetizing forc this resistance. Calling the latter R, we have

This formula is rigorously correct, provided we su in determining the magnetic resistance for every con of magnetization. For low degrees of magnetizati resistance is nearly constant, and in these cases exists simple proportionality between Zand P; for degrees of magnetization the resistance increases an relation between Z and P becomes more compli A limit is ultimately approached beyond whic