ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 5 
pole core section and the resulting pole width, taking into 
consideration that about 5% of the pole-core length is 
taken up by insulation material. 
292. With an average magnetic density in the cast iron rim of B,= 
45,000 lines per square inch, calculate its necessary section. 
293. Determine the rim dimensions by allowing 2.25 inches pro- 
jection on each side of the magnet pole for support and 
protection of the field coils. 
294. By means of Equation $1 of the Instruction Book, calculate 
the value of effective back-ampere-turns. 
295. Calculate the value of the necessary field ampere turns per 
pole corresponding to the normal open circuit pressure, 
considering that with regard to the specified momentary 
overload capacity the normal excitation ampere turns 
must be about 2.25 times greater than the normal effective 
back-ampere-turns. 
296. The current density in the field winding being chosen at 1,300 
amperes per square inch and the space factor assumed at 
.40, calculate the approximate necessary winding space on 
one side of an excitation coil. 
297. Assuming a maximum allowable winding depth of 1.5 inch, 
calculate the useful pole height. 
298. Make the poleshoe height c=1.2 inch and collect in tabular 
form the principal dimensions of this synchronous motor 
thus far established, in a manner similar to that of the two 
preceding alternator designs. Give also a dimensioned 
sketch similar to Fig. 74 of the Instruction Book. 
299. Asa first approximation, calculate the air gap value by means 
of Equation 21 of the Instruction Book, assuming that 
about 70% of the field ampere turns are wasted in the air 
and allowing a reduction of the calculated air gap of about 
15% on account of the effective iron section being reduced 
by the slot openings, which consideration is entirely neglect- 
ed by‘the application of the Formula 21. 
