PRINCIPLES OF NAVAL ENGINEERING 



Normally, when a circuit is overloaded or 

 when a fault develops, the fuse element melts 

 and opens the circuit that it is protecting. How- 

 ever, all fuse openings are not the result of 

 current overload or circuit faults. Abnormal 

 production of heat, aging of the fuse element, 

 poor contact due to loose connections, oxides or 

 other corrosion products forming within the fuse 

 holder, and unusually high ambient temperatures 

 will alter the heating conditions and thetime re- 

 quired for the element to melt. 



A more complex type of protective device 

 is the circuit breaker. In addition to acting as 

 protective devices, circuit breakers perform the 

 function of normal switching and are used to 

 isolate a defective circuit while repairs are 

 being made. 



Circuit breakers are available in many 

 types; some may be operated both manually 

 and electrically, while others are restricted to 

 one mode of operation. Figure 20-26 shows a 

 circuit breaker which may be operated either 

 manually or electrically. When operated elec- 

 trically, the operation is usually in conjunction 

 with a pilot device such as a relay or switch. 

 Electrically operated circuit breakers employ 

 an electromagnet, used as a solenoid, to trip a 

 release mechanism that causes the breaker 

 contacts to open. The energy to open the breaker 

 is derived from a coiled spring, and the electro- 

 magnet is controlled by the contacts in a pilot 

 device. 



Circuit breakers designed for high currents 

 have a double-contact arrangement, consisting 

 of the main bridging contacts and the arcing 

 contacts. When the circuit opens, the main con- 

 tacts open first, allowing the current to flow 

 through the arc contacts and thus preventing 

 burning of the main contacts. When the arc 

 contacts are open, they pass under the front 

 of the arc runner, causing a magnetic field to be 

 set up which blows the arc up into the arc 

 quencher and quickly opens the circuit. 



SYNCHROS AND SERVOMECHANISMS 



Synchros, as identified by the Armed Forces, 

 are a-c electromagnetic devices which are used 

 primarily for the transfer of angular-position 

 data. Synchros are, in effect, single-phase 

 transformers in which the primary-to- 

 secondary coupling may be varied by physically 

 changing the relative orientation of these two 

 windings. 



FLEXIBLE CONNECTION ^«< ARC QUENCHERS 



OVERCURRENT TRIP 



27.73 



Figure 20-26.— Circuit breaker. 



Synchro systems are used throughout the 

 Navy to provide a means of transmitting the 

 position of a remotely located device to one 

 or more indicators located away from the trans- 

 mitting area. 



Part A of figure 20-27 shows a simple 

 synchro system. When the handwheel is turned, 

 an electrical signal is generated by the synchro 

 transmitter and is transmitted through inter- 

 connecting leads to the synchro receivers. The 

 synchro receivers will always turn the same 

 amount and direction and at the same speed as 

 the synchro transmitter. 



Part B of figure 20-27 shows the same type 

 of system using mechanical linkage. As may be 

 readily seen, mechanical systems are imprac- 

 ticable because of the need for associated belts, 

 pulleys, gears, and rotating shafts. 



Synchro systems are widely used for input 

 control of electromechanical devices (servo- 

 mechanisms) that position an object in ac- 

 cordance with a variable signal. The essential 

 components of a servomechanism system are 

 the input controller and the output controller. 



The input controller provides the means, 

 either mechanical or electrical, whereby the 



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