An elementary book on electricity and magnetism and their applications . ough the armature. It isthe leakage, or stray lines of force, which sometimes magnet-izes watches when they are carried near a dynamo. 146. Multipolar generators. The machines which have beendescribed have two poles and are called bipolar commercial purposes, especially in machines above 10kw. capacity, it is common practice to use four, six, eight, oreven more poles. Such machines are called multipolar. Figure 208 ELECTRICITY AND MAGNETISM 143(a) gives the external view and figure 143(6) gives a diagramof th
An elementary book on electricity and magnetism and their applications . ough the armature. It isthe leakage, or stray lines of force, which sometimes magnet-izes watches when they are carried near a dynamo. 146. Multipolar generators. The machines which have beendescribed have two poles and are called bipolar commercial purposes, especially in machines above 10kw. capacity, it is common practice to use four, six, eight, oreven more poles. Such machines are called multipolar. Figure 208 ELECTRICITY AND MAGNETISM 143(a) gives the external view and figure 143(6) gives a diagramof the electric and magnetic circuits of a four-pole increasing the number of poles we can get the commercialvoltages (110, 220, or 500 volts) at much slower speeds thanwould be necessary in bipolar machines. Since the voltagedepends on the rate at which the wires of the armature cutthe lines of magnetic force, in a four-pole machine each wire on the armature cuts a{ j complete set of lines of force four times in eachrevolution instead oftwice as in a two-pole. Fig. 143. — Four-pole generator (a) and its diagrammatic cross section (&). machine. For this reason the speed of a four-pole machine isone half the speed required in a two-pole machine for the samevoltage. Furthermore, the multipolar machine is more eco-nomical to build, because it requires less iron to carry themagnetic flux. The same principles govern the winding of the armaturesfor multipolar machines as those already described for bipolarmachines, but each coil reaches around a chord of the armatureinstead of across the diameter. The chord must always be DIRECT-CURRENT GENERATORS 209 such that when one conductor of a coil is under one pole thereturn conductor of the same coil must be under a pole ofopposite sign; and the chord is usually the same as the chordfrom the center of one pole face to the center of the next poleface, although it may be an odd multiple of the same. It will be observed from the diag
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmagnetism, bookyear19
