. Farm machinery and farm motors. extensively used in practice. 644. Self-exciting principle of dynamos.—In the earliertypes of dynamos the field magnets were always sepa-rately excited by either a battery or a magneto. Later itwas learned that the soft iron of the field magnet afteronce being excited retains some of the magnetism. Sincethen all direct-current dynamos are built on this is sufficient magnetism remaining in the fields sothat when the armature is up to speed it cuts enough 482 FARM MOTORS lines of force to induce a small current into the circuitaround the field co
. Farm machinery and farm motors. extensively used in practice. 644. Self-exciting principle of dynamos.—In the earliertypes of dynamos the field magnets were always sepa-rately excited by either a battery or a magneto. Later itwas learned that the soft iron of the field magnet afteronce being excited retains some of the magnetism. Sincethen all direct-current dynamos are built on this is sufficient magnetism remaining in the fields sothat when the armature is up to speed it cuts enough 482 FARM MOTORS lines of force to induce a small current into the circuitaround the field coils. This current more highly excitesthe field magnets until the dynamo soon picks up orestablishes its rated 645. Shunt dynamo.—In the so-called shunt-wounddynamo a small portion of the current is led ofT fromthe brushes bb (Fig. 363), and through a great manyturns of very fine wire which encircle the core of themagnet. In such a dynamo, as the load increases slightly decreases, and as the load decreases the. MAIN CIRCUIT FIG. 363—SHUNT-WOUND DYNAMO FIG. 364—SERIES-WOUND DYNAMO increases. Hence, if the current fluctuations aregreat and quite frecjuent it would keep an attendant oc-cupied to keep the field resistance regulated for the load.(See Fig. 368.) 646. Series-wound dynamo.—In the so-called series-wound machines the whole of the current is carried througha few turns of very coarse wire which encircles the field mag-nets (Fig. 364). Since every change of current alters thefield magnetizing current, consequently in the current in-duced in the armature the the brushes will varywith every change of resistance in the external circuit. 647. Compound-wound dynamo.—In the compound- ELECTRICAL MACHINERY 483 wound machines there is both a series and a shunt coilsurrounding- the cores of the field magnets. This styleof machine is designed to give automatically a betterregulation of voltage on constant-potential circuits thanis possible o
Size: 2366px × 1056px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookpublisheretcetc, booksubject
