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flowing around the complete circuit in the manner indicated by the arrows in the diagram. This current reverses direction at the instant at which all the coils pass the midway points between the N and S poles. The number of alternations per second is equal to the number of poles multiplied by the number of revolutions per second. The field magnets N and S of such a dynamo are usually excited by a direct current from some other source. Fig. 311 represents an alternatingcurrent dynamo with revolving field and stationary armature connected directly to a tandem compound engine. Alternators of 5000kilowatt capacity (nearly 7000 horse power) 'have been built to run at the unusually high speed of 3600 revolutions per minute. Alternators of lower speed but of very much greater capacity are common (see huge rotor opposite p. 257).
355. The principle of the commutator. By the use of a socalled commutator it is possible to transform a current which
FIG. 310. Diagram of alternatingcurrent dynamo
FIG. 311. Alternating-current dynamo
is alternating in the coils of the armature to one which always flows in the same direction through the external portion of the circuit. The simplest possible form of such a commutator
is shown in Fig. 312.
It consists of a single metallic ring which is split into two equal insulated semicircular segments
FIG. 312. The simple commutator
a and c. One end of the rotating coil is soldered to one of these semicircles, and the other end to the other semicircle. Brushes b and b' are set in such positions that they lose contact with one semicircle and make contact with the other at the instant at which the current changes direction in the armature. The current, therefore, always passes out to the external circuit through the same brush. While a current from such a coil and commutator as that shown in the figure would always flow in the same direction through the external circuit, it would be of a pulsating rather than a steady character, for it would rise to a maximum and fall again to zero twice during each complete revolution of the armature. This effect is avoided in the commercial direct-current dynamo by building a commutator of a large number of segments instead of two, and connecting
FIG. 313. Two-pole direct-current dynamo with ring armature
each to a portion of the armature coil in the manner shown in Fig. 313. The result of using a simple split-ring commutator is shown graphically in Fig. 314.
356. The drum-armature direct-current dynamo. Fig. 315 is a diagram illustrating the construction of a commercial two-pole direct-current dynamo of the drum-armature type. At a given instant currents are being induced in the same direction in all the conductors on the left half of the armature. The cross on these conductors, representing the tail of a retreating arrow, is to indicate that these currents flow away from the reader. No E.M.F.'s are induced in the conductors at the top and bottom of the
armature, where the motion is parallel to the magnetic lines. On the right half of the ring, on the other hand, the induce currents are all in the opposite direction, that is, toward the reader, since the conductors are here all moving up instead of down. The dot in the middle of these conductors represents the head of an approaching arrow. It will be seen, however, in tracing out the connections 1, 11, 2, 21, 3, 31, etc., of Fig. 315 (the dotted lines representing connections at the back of the drum), that the coil is so wound about the drum that the currents in both halves are always flowing toward one brush b, from which they are led to the external circuit and back at b. This condition always exists, no matter how fast the rotation; for it will be seen that as each loop rotates into the position where the direction of its current reverses, it passes a brush and therefore at once becomes a part of the circuit on the other half of the drum where the currents are all flowing in the opposite direction. Fig. 316 shows a typical modern four-pole generator, and Fig. 317 the corresponding drum-wound armature. Fig. 326 (p. 310)
FIG. 315. The direct-current dynamo, drum winding
illustrates nicely the method of winding such an armature, each coil beginning on one segment of the commutator and ending on the adjacent segment.
357. Dynamo lighting circuit. The type of circuit generally used in D.C. incandescent lighting is shown in Fig. 318. The lamps are arranged
in parallel between the mains. The
field magnets are excited partly by FIG. 317. A modern drum armature a few series turns which carry the
whole current going to the lamps, and partly by a shunt coil consisting of many turns of fine wire (Fig. 318). This combination of series and shunt winding maintains the P.D. across the mains constant for a great range of loads. Such a machine is called a compound woun dynamo, to distinguish it from a series wound machine, for example, which dispenses with the shunt coil.
In all self-exciting machines there is enough residual magnetism left in the iron cores after stopping to start feeble induced currents when started up again. These currents immediately increase the strength of the magnetic field, and so the machine quickly builds up its current until the limit of magnetization is reached.
358. The electric motor. In construction the electric motor differs in no essential respect from the dynamo. To analyze the operation as a motor of such a machine as that shown in Fig. 313, suppose a current from an outside source is first sent around the coils of the field magnets and then into the armature at b'. Here it will divide and flow through all the conductors on the left half of the ring in one direction, and through all those on the right half in the opposite direction. Hence, in accordance with the motor rule, all the conductors on the left side are urged upward by the influence of the field, and all those on the right side are urged downward. The armature will therefore begin to rotate, and this rotation
will continue as long as the current is sent in at b' and out at b; for as fast as coils pass either bor b' the direction of the current flowing through them changes, and therefore the direction of the force acting on them changes. The left half is therefore always urged up and the right half down. The greater the strength of the current, the greater the force acting to produce rotation.
If the armature is of the drum type (Fig. 315), the conditions are not essentially different; for, as may be seen by following out the windings, the current entering at b' will flow through all the conductors on the left half in one direction and through those on the right half in the opposite direction. The commutator keeps keeps these conditions always fulfilled. The induction motor is pictured and described opposite page 291.
FIG. 319. Railway motor, upper field raised The electric motor is a device which receives electrical energy and converts it into mechanical energy. The dynamo is a device which receives mechanical energy from a steam engine, water wheel, or other source and converts it into electrical energy.
359. Street-car motors. Electric street cars are nearly all operated by direct-current series-wound motors placed under the cars and attached by gears to the axles. Fig. 319 shows a typical four-pole street-car motor. The two upper field poles are raised with the case when the motor is opened for inspection, as in the figure. The current is generally supplied by compound-wound dynamos which maintain a constant potential of about 500 volts between the trolley or third rail and the track which is used as the return circuit. The cars are always operated in parallel, as shown in Fig. 320. In a few instances street cars are operated upon