'278 THE DIRECT-CURRENT MOTOR CH. XU 



when running a tramcar at 11 miles an hour, on a line 

 having a tension of 500 volts, the gear ratio being 4'78, 

 and the wheel diameter 33". From Equation 21 we 

 find that the induction factor has to be 51 '3, and that 

 the current when running at full speed will be 24 am- 

 peres. We have to determine the number of 

 surface conductors so that at full load the magnetisa- 

 tion under the tip of the pole due to the armature re- 

 action is not greater than 0'52 of that due to the magnets, 



]T * H = * * 

 52 " 298 S ' 



Since the armature is series-connected, the current per 

 conductor is 12 amperes; taking the larger pole of the 

 two, we find <f> to be 68, while 8 is 0-358 cm. Inserting 



A 



these values in our equation we find that = 0-132. 



If is greater than 0-132, the reaction will be too 



"a 

 great ; if it is smaller, the reaction will be less than the 



amount allowed. 



Now since M=pAN].Q~ s and p=2, we see that 



A = -^r x 10 8 . The area of the large pole is 714 square 



cm.; hence ^=71477,,, and we may write 477,, = 36 x 10 5 . 

 Now A must be a multiple of the number of bars 011 

 the commutator, so that 



If A = 4x120=480, E tJ = 7,500, and - \ =0-064. 



H 9 



If 4 = 6x120=720, 77^=5,000, and -^- = 0-144. 

 If 4 = 8x120=960, #,=3,750, and - ,-=0-256. 



