Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERING20-14 shows the reduction in ripple obtained bythe use of two ci -Is instead of one. Since thereare now four commutator segments and onlytwo brushes, the voltage cannot fall any lowerthan point A; therefore, the ripple is limited bythe rise and fall between points A and B. By addingstill more armature coils, the ripple can bereduced still more.u.o111oREVOLUTIONS41.9Figure 20-14.— Voltage from atwo-coilarmature.TYPES OF D-C GENERATORSD-c generators are usually classified accordingto the manner in which the field windingsare connected to the armature circuit (fig. 20-15).A separately excited d-c generator is indicatedin part A of figure 20-15. In this machinethe field windings are energized from a d-csource other than its own armature.Self-excited d-c generators may be of threetypes, as indicated in part B of figure 20-15.A shunt generator has its field windings connectedparallel with the armature, whereas thefield windings of a series generator are connectedin series with the armature. The compoundd-c generator employs both shunt andseries field windings.The d-c generator most widely used in theNavy is the stabilized shunt generator , whichemploys a light series field winding on thesame poles with the shunt field windings. Thistype of generator has good voltage regulationcharacteristics and at the same time ensuresgood parallel operation.VOLTAGE CONTROLVoltage control is either manual or automatic.In most cases, the process involveschanging the resistance of the field circuit,thus controlling the field current which permitscontrol of the terminal voltage. The major differencebetween the various voltage regulatorsystems is merely the method by which thefield circuit resistance is controlled.DIRECT-CURRENT MOTORSThe construction of a d-c motor is essentiallythe same as that of a d-c generator. Thed-c generator converts mechanical energy intoelectrical energy, and the d-c motor convertsthe electrical energy into mechanical energy.A d-c generator may be made to function as amotor by applying a suitable source of directvoltage across the normal output electricalterminals.There are various types of d-c motors,depending upon the way in which the field coilsare connected. Each type has characteristicsthat are advantageous under given load conditions.Shunt motors have the field coils connectedin parallel with the armature circuit. This typeof motor, with constant potential applied, developsvariable torque at an essentially constantspeed, even under changingload conditions. Suchloads are found in drives for such machine shopequipment as lathes, milling machines, drills,planers, and shapers.Series motors have the field coils connectedin series with the armature circuit. This type ofmotor, with constant potential applied, developsvariable torque but itsspeedvaries widely underchanging load conditions. The speed of a seriesmotor is low under heavy loads but becomesexcessively high under light loads. Series motorsare commonly used to drive electric cranes,hoists, and winches.Compound motors are a compromise betweenshunt and series motors, having one set of fieldcoils in parallel with the armature circuit andanother set of field coils in series with the armaturecircuit. The compound motor developsan increased starting torque over the shunt motorand has less variation in speed than the seriesmotor.502