Principles of naval engineering - Historic Naval Ships Association

Chapter 20. -SHIPBOARD ELECTRICAL SYSTEMSLOAOQSHUNTSERIESCOMPOUND(A)SEPARATE EXCITATION(B)SELF EXCITATIONFigure 20-15.— Types of d-c generators.147.122The operation of a d-c motor depends on theprinciple that a current-carrying conductorplaced in, and at right angles to, a magneticfield tends to move at right angles to the directionof the field. A convenient method of determiningthe direction of motion of a currentcarryingconductor in a magnetic field is byuse of the right-hand motor rule for electronflow (fig. 20-16). Extend the thumb, index finger,and second finger of the right hand at rightangles to each other, with the index fingerpointed in the direction of the flux (toward thesouth pole) and the second finger pointed in thedirection of electron flow. The thumb then pointsin the direction of motion of the conductor withrespect to the field.ALTERNATING-CURRENT THEORYJust as a current flowing in a conductorproduces a magnetic field around the conductor,the reverse of this process is true. A voltagecan be generated in a circuit by moving a conductorso that it cuts across lines of magneticforce or, conversely, by moving the lines offorce so that they cut across the conductor. Ana-c generator utilizes this principle of electromagneticinduction to convert mechanical energyinto electrical energy.In the case of alternating current, electronsmove first in one direction andtheninthe other.Thus the direction of the current reversesperiodically and the magnitude of the voltage isconstantly changing. This variation in current isrepresented graphically in sine waveform infigure 20-17.The vertical projection (dotted line in fig.20-17) of a rotating vector may be used torepresent the voltage at any instant. Vector Ej^^represents the maximum voltage induced in aconductor rotating at uniform speed in a 2-polefield (points 3 and 9). The vector is rotatedcounterclockwise through one complete revolution(360°). The point of the vector describes acircle. A line drawn from the point of the vectorperpendicular to the horizontal diameter of thecircle is the vertical projection of the vector.The circle also describes the path of theconductor rotating in the bi-polar field. Thevertical projection of the vector represents thevoltage generated in the conductor at any instantcorresponding to the position of the rotatingvector as indicated by angled. Angle ^representsselected instants at which the generatedvoltage is plotted. The sine curve plotted at the12.143Figure 20-16.—Right-hand motor ruleforelectron flow.503