Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGNormally, when a circuit is overloaded orwhen a fault develops, the fuse element meltsand opens the circuit that it is protecting. However,all fuse openings are not the result ofcurrent overload or circuit faults. Abnormalproduction of heat, aging of the fuse element,poor contact due to loose connections, oxides orother corrosion products forming within the fuseholder, and unusually high ambient temperatureswill alter the heating conditions and thetime requiredfor the element to melt.A more complex type of protective deviceis the circuit breaker. In addition to acting asprotective devices, circuit breakers perform thefunction of normal switching and are used toisolate a defective circuit while repairs arebeing made.Circuit breakers are available in manytypes; some may be operated both manuallyand electrically, while others are restricted toone mode of operation. Figure 20-26 shows acircuit breaker which may be operated eithermanually or electrically. When operated electrically,the operation is usually in conjunctionwith a pilot device such as a relay or switch.Electrically operated circuit breakers employan electromagnet, used as a solenoid, to trip arelease mechanism that causes the breakercontacts to open. The energy to open the breakeris derived from a coiled spring, and the electromagnetis controlled by the contacts in a pilotdevice.Circuit breakers designed for high currentshave a double-contact arrangement, consistingof the main bridging contacts and the arcingcontacts. When the circuit opens, the main contactsopen first, allowing the current to flowthrough the arc contacts and thus preventingburning of the main contacts. When the arccontacts are open, they pass under the frontof the arc runner, causing a magnetic field to beset up which blows the arc up into the arcquencher and quickly opens the circuit.SYNCHROS AND SERVOMECHANISMSSynchros, as identified by the Armed Forces,are a-c electromagnetic devices which are usedprimarily for the transfer of angular-positiondata. Synchros are, in effect, single-phasetransformers in which the primary-tosecondarycoupling may be varied by physicallychanging the relative orientation of these twowindings.FLEXIBLE CONNECTIONFigure 20-26.— Circuit breaker.^«< ARC QUENCHERSOVERCURRENT TRIP27.73Synchro systems are used throughout theNavy to provide a means of transmitting theposition of a remotely located device to oneor more indicators located away from the transmittingarea.Part A of figure 20-27 shows a simplesynchro system. When the handwheel is turned,an electrical signal is generated by the synchrotransmitter and is transmitted through interconnectingleads to the synchro receivers. Thesynchro receivers will always turn the sameamount and direction and at the same speed asthe synchro transmitter.Part B of figure 20-27 shows the same typeof system using mechanical linkage. As may bereadily seen, mechanical systems are impracticablebecause of the need for associated belts,pulleys, gears, and rotating shafts.Synchro systems are widely used for inputcontrol of electromechanical devices (servomechanisms)that position an object in accordancewith a variable signal. The essentialcomponents of a servomechanism system arethe input controller and the output controller.The input controller provides the means,either mechanical or electrical, whereby the516