Principles of naval engineering - Historic Naval Ships Association

Chapter 20. -SHIPBOARD ELECTRICAL SYSTEMSPb + PbOg + 2H2SODISCHARGING_2PbSO^ + 2H2OCHARGINGThe capacity of a battery is measured inampere-hours . The capacity is equal to theproduct of the current (in amperes) and thetime (in hours) during which the battery is supplyingthis current to a given load. The capacitydepends upon many factors, the most importantof which are (1) the area of the plates in contactwith the electrolyte, (2) the quantity and specificgravity of the electrolyte, (3) the general conditionof the battery, and (4) the final limitingvoltage.Voltage Produced by MagnetismCONDUCTOR MOVEDDOWNCONDUCTORMOTIONCONDUCTOR MOVEDUPOne of the most useful and widely employedapplications of magnets is in the production ofvast quantities of electric power from mechanicalsources. The mechanical power may be providedby a number of different devices, includinggasoline engines, diesel engines, water turbines,steam turbines, and gas turbines. Thefinal conversion of these energies to electricityis done by generators employing the principleof electromagnetic induction.There are three conditions which must existbefore a voltage can be produced by electromagneticinduction. First, we must have amagnetic field; second, a conductor; and third,relative motion between the field and the conductor.In accordance with these conditions,when a conductor is moved across a magneticfield so as to cut the lines of force, electronswithin the conductor are forced to move; thusa voltage is produced. ,Producing a voltage by magnetic inductionis illustrated in figure 20-3. If the ends of aconductor are connected to a low-reading voltmeteror galvanometer and the conductor ismoved rapidly down through a magnetic field,there is a momentary reading on the meter.When the conductor is moved up through thefield, the meter deflects in the opposite direction.If the conductor is held stationary and themagnet is moved so that the field cuts acrossthe conductor, the meter is deflected in the samemanner as when the conductor was moved andthe field was stationary.The voltage developed across the conductorterminals by electromagnetic induction is knownas an induced emf , and the resulting current thatflows is called induced current. The inducedLEFT-HAND GENERATOR RULEFigure 20-3.— Left-hand generator rule.12.143emf exists only so long as relative motionoccurs between the conductor and the field.There is a definite relationship between thedirection of flux, the direction of motion of theconductor, and the direction of the induced emf.When two of these directions are known, thethird can be found by applying the left-hand rulefor generators . To find the direction of the emfinduced in a conductor, extend the thumb, theindex finger, and the second finger of the lefthand at right angles to each other, as shown infigure 20-3. Point the index finger in the directionof the flux (toward the south pole) and thethumb in the direction in which the conductoris moving in respect to the fields. The secondfinger then points in the direction in which theinduced emf will cause the electrons to flow.DIRECT-CURRENT CIRCUITSAn electric circuit is a complete path throughwhich electrons can flow from the negative terminalof the voltage source, through the connectingwires (conductors), through the load, andback to the positive terminal of the voltage495