INTRODUCTION TO SONAR 



check a circuit with high resistanca, select the high 

 scale, because the low range scale may indicate 

 Infinity when actually the resistance is less than 1 

 megohm. 



While conducting resistance tests, take into 

 account that other circuits containing resistance 

 and capacitance may be in parallel with the circuit 

 tested. To obtain a correct reading, the circuit 

 tested should be isolated from other parallel cir- 

 cuits. 



When measuring a resistor, remember that 

 most resistors have a tolerance of approximately 

 10 percent. If the measurement of a resistor falls 

 within this tolerance, it usually is considered to 

 maet requirements and need not be replaced unless 

 the circuit calls for a critical value. 



GENERATOR MAINTENANCE 



Minor routine m.^intenance of generators is 

 performed by the Sonar Technician to ensure a 

 proper voltage supply to the associated sonar 

 equipment. This part of the text is not intended to 

 teach the methods of generator repair; only those 

 procedures required as normal maintenance are 

 discussed. 



Comrautators 



The first requirement for good commutation is 

 continuous, close contact between the commutator 

 and the brushes. Successful commutation is not a 

 function of the electric circuit, or th3 brash, or the 

 commutator alone. It depends on all of these fac- 

 tors, and can be maintained only through proper 

 inspection, cleaning, and maintenance. 



• Inspection: Proper inspection of the commutator 

 consists of the following steps. 



1. Check the bars on the commutator for flat 

 spots, burned condition, and looseness indi- 

 cated by irregular liigh or low bars. 



2. Check for streaks and grooves. 



3. Check for dragged copper on the leading 

 edges of the bars and for filled slots caused 

 by the dragged copper. 



4. Check the riser connections for thrown 

 solder. 



5. Check the mica for a pitted condition and the 

 presence of oil, grease, dirt, and particles 

 of carbon or copper embedded in the mica. 



• Cleaning and maintenance: The following proce- 

 dures should be observed to maintain the commu- 

 tator in a good operating condition. 



1. Clean comravtators of dirt, oil, or grease 

 with a lintless cloth moistened with a safety- 

 type petroleum solvent. 



2. Clean scratches and rough edges from the 

 commutator bars by pressing a very fine 

 s andpaper against the surface with a block of 

 wood that has the same contour as the com- 

 mutator. A piece of canvas is sometimes 

 adequate for this purpose. The sandpaper or 

 canvas should be moved back and forth 

 across the surface, parallel to the shaft. 

 Remove all grit after this operation by 

 means of a lintless cloth. Under no circum- 

 stances should emery cloth be used for 

 cleaning. 



3. Clean high mica from 'oetween the commuta- 

 tor bars. This procedure consists of remov- 

 ing the mica protruding from the commuta- 

 tor slots, so that there is no contact 

 between the mJca and the brushes as the 

 commutator revolves. This process, called 

 undercutting, is accomplished by sawing or 

 scraping the mica from the slots. An ideal 

 tool for this purpose is a haclcsaw blade 

 that has been groimd down to remove all 

 teeth, so as to prevent burring of the 

 commutator bars. For small commutators, 

 a slotting file may be used to file and 

 scrape out the mica. C are must be taken not 

 to undercut too deeply. Although the square- 

 shaped slot usually is preferable, the V- 

 shaped slot may be more satisfactory where 

 the slots are likely to collect dirt. After 

 imdercutting Is completed, remove all burrs 

 and polish the commutator. Do not use any 

 lubricants when undercutting. 



Sliprings 



The principle of close electrical contact 

 applies to the sliprings as well as to the commu- 

 tator. With few minor exceptions, the procedure 

 for inspecting, cleaning, or minor resurfacing of 

 sliprings is identical to that for commutators. 



Brushes 



The brushes of a generator are the points of 

 contact between the external circuit conductors 

 and the commutator. These points "brush" the 

 commutator so as to take off the generated vol- 

 tage. The brushes ride on the surface of the com- 

 mutator and are held in place by brush holders. 

 Brushes usually are made of high-grade carbon. 

 They are insulated from the frame, and are free 



174 



