. The principles underlying radio communication . ous by making the proper connec-tion to the conductors as they come into place. By a line of reasoning very much, like that for the d. c. gen-erator, we can pass from this simple case to that of a four-poledrum-wound motor, illustrated in Fig. 129. The directions ofcurrent and rotation are shown by arrows. Limiting Speed.—It might be expected that a shunt motorwould speed up indefinitely, but actually it soon comes to adefinite speed, and then continues to turn so fast, but no soon as the armature begins to rotate, it generates an em
. The principles underlying radio communication . ous by making the proper connec-tion to the conductors as they come into place. By a line of reasoning very much, like that for the d. c. gen-erator, we can pass from this simple case to that of a four-poledrum-wound motor, illustrated in Fig. 129. The directions ofcurrent and rotation are shown by arrows. Limiting Speed.—It might be expected that a shunt motorwould speed up indefinitely, but actually it soon comes to adefinite speed, and then continues to turn so fast, but no soon as the armature begins to rotate, it generates an to the right-hand This action is exactly the-same as in a generator. 21 See Section G3, page 153. 212 I) YNA MO-ELECTRIC MACHINERY. The emf. generated is opposite to the direction of currentshown by the arrows, and is for that reason called a counterelectromotive force. The faster the armature turns thegreater the counter emf. becomes. It cannot turn so fast thatthe counter emf. is as great as the line voltage, because then. Fig. 129.—Diagram of circuits of a 4-pole shunt motor. the two would balance; there would be nothing to make thecurrent flow through the armature, and consequently no pullto keep it turning. For example, suppose the armature resis-tance of a certain motor is ohm. and suppose that a currentof 4 amperes in the armature furnishes just enough pull to keepit rotating. If the speed is high enough to make the counter DIRECT-CURRENT MOTORS. 213 emf. 109 volts when the line voltage is 110, the current is 4 Next, suppose the motor is driving machinery that calls forfive times as great a pull. The speed falls off a little. Whenit has fallen enough to make the counter emf. 105 volts, thecurrent is 20 If that is enough to drive the load, thespeed will be steady at the new rate. So by changing its speeda very little the motor automatically takes more or less cur-rent, but always just enough to drive its load. The magnet
Size: 1541px × 1621px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1920, bookpublisherwashi, bookyear1922
