Chapter 20. -SHIPBOARD ELECTRICAL SYSTEMS 



LOAOQ 



SHUNT 



(A) 



SEPARATE EXCITATION 



SERIES COMPOUND 

 (B) 



SELF EXCITATION 



147.122 



Figure 20-15.— Types of d-c generators. 



The operation of a d-c motor depends on the 

 principle that a current-carrying conductor 

 placed in, and at right angles to, a magnetic 

 field tends to move at right angles to the direc- 

 tion of the field. A convenient method of deter- 

 mining the direction of motion of a current- 

 carrying conductor in a magnetic field is by 

 use of the right-hand motor rule for electron 

 flow (fig. 20-16). Extend the thumb, index finger, 

 and second finger of the right hand at right 

 angles to each other, with the index finger 

 pointed in the direction of the flux (toward the 

 south pole) and the second finger pointed in the 

 direction of electron flow. The thumb then points 

 in the direction of motion of the conductor with 

 respect to the field. 



ALTERNATING-CURRENT THEORY 



Just as a current flowing in a conductor 

 produces a magnetic field around the conductor, 

 the reverse of this process is true. A voltage 

 can be generated in a circuit by moving a con- 

 ductor so that it cuts across lines of magnetic 

 force or, conversely, by moving the lines of 

 force so that they cut across the conductor. An 

 a-c generator utilizes this principle of electro- 

 magnetic induction to convert mechanical energy 

 into electrical energy. 



In the case of alternating current, electrons 

 move first in one direction andtheninthe other. 

 Thus the direction of the current reverses 

 periodically and the magnitude of the voltage is 

 constantly changing. This variation in current is 

 represented graphically in sine waveform in 

 figure 20-17. 



The vertical projection (dotted line in fig. 

 20-17) of a rotating vector may be used to 

 represent the voltage at any instant. Vector Ej^^ 

 represents the maximum voltage induced in a 

 conductor rotating at uniform speed in a 2-pole 

 field (points 3 and 9). The vector is rotated 

 counterclockwise through one complete revolu- 

 tion (360°). The point of the vector describes a 

 circle. A line drawn from the point of the vector 

 perpendicular to the horizontal diameter of the 

 circle is the vertical projection of the vector. 



The circle also describes the path of the 

 conductor rotating in the bi-polar field. The 

 vertical projection of the vector represents the 

 voltage generated in the conductor at any instant 

 corresponding to the position of the rotating 

 vector as indicated by angled. Angle ^repre- 

 sents selected instants at which the generated 

 voltage is plotted. The sine curve plotted at the 



12.143 

 Figure 20-16.— Right-hand motor rulefor 

 electron flow. 



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