PRINCIPLES OF NAVAL ENGINEERING 



20-14 shows the reduction in ripple obtained by 

 the use of two ci -Is instead of one. Since there 

 are now four commutator segments and only 

 two brushes, the voltage cannot fall any lower 

 than point A; therefore, the ripple is limited by 

 the rise and fall between points A and B. By add- 

 ing still more armature coils, the ripple can be 

 reduced still more. 



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REVOLUTIONS 



41.9 



Figure 20-14.— Voltage from atwo-coil 

 armature. 



TYPES OF D-C GENERATORS 



D-c generators are usually classified ac- 

 cording to the manner in which the field windings 

 are connected to the armature circuit (fig. 20- 

 15). 



A separately excited d-c generator is indi- 

 cated in part A of figure 20-15. In this machine 

 the field windings are energized from a d-c 

 source other than its own armature. 



Self-excited d-c generators may be of three 

 types, as indicated in part B of figure 20-15. 

 A shunt generator has its field windings con- 

 nected parallel with the armature, whereas the 

 field windings of a series generator are con- 

 nected in series with the armature. The com- 

 pound d-c generator employs both shunt and 

 series field windings. 



The d-c generator most widely used in the 

 Navy is the stabilized shunt generator , which 

 employs a light series field winding on the 

 same poles with the shunt field windings. This 



type of generator has good voltage regulation 

 characteristics and at the same time ensures 

 good parallel operation. 



VOLTAGE CONTROL 



Voltage control is either manual or auto- 

 matic. In most cases, the process involves 

 changing the resistance of the field circuit, 

 thus controlling the field current which permits 

 control of the terminal voltage. The major dif- 

 ference between the various voltage regulator 

 systems is merely the method by which the 

 field circuit resistance is controlled. 



DIRECT-CURRENT MOTORS 



The construction of a d-c motor is essen- 

 tially the same as that of a d-c generator. The 

 d-c generator converts mechanical energy into 

 electrical energy, and the d-c motor converts 

 the electrical energy into mechanical energy. 

 A d-c generator may be made to function as a 

 motor by applying a suitable source of direct 

 voltage across the normal output electrical 

 terminals. 



There are various types of d-c motors, 

 depending upon the way in which the field coils 

 are connected. Each type has characteristics 

 that are advantageous under given load condi- 

 tions. 



Shunt motors have the field coils connected 

 in parallel with the armature circuit. This type 

 of motor, with constant potential applied, de- 

 velops variable torque at an essentially constant 

 speed, even under changingload conditions. Such 

 loads are found in drives for such machine shop 

 equipment as lathes, milling machines, drills, 

 planers, and shapers. 



Series motors have the field coils connected 

 in series with the armature circuit. This type of 

 motor, with constant potential applied, develops 

 variable torque but itsspeedvaries widely under 

 changing load conditions. The speed of a series 

 motor is low under heavy loads but becomes 

 excessively high under light loads. Series motors 

 are commonly used to drive electric cranes, 

 hoists, and winches. 



Compound motors are a compromise between 

 shunt and series motors, having one set of field 

 coils in parallel with the armature circuit and 

 another set of field coils in series with the ar- 

 mature circuit. The compound motor develops 

 an increased starting torque over the shunt motor 

 and has less variation in speed than the series 

 motor. 



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