120 ALTERNATING CURRENTS 



forces, when attempt is made to adapt this type of rotor to more 

 than two poles. 



Figure 126 shows a four-pole, radial-slot rotor. Although the 

 coil-ends are not held as strongly in this type of rotor as they are 

 in the parallel-slot type, it is better adapted to four-pole and six- 

 pole rotors, because there is not the reduction of iron section 

 back of the slots with increase in the number of poles, such as 

 occurs in the parallel-slot type. 



The winding on these types of rotor is called a distributed field 

 winding. (See Fig. 131.) 



The field connections are usually carried out through the center 

 of the shaft to slip-rings. Two or more carbon brushes resting 

 on the slip-rings carry the current to the winding. The excita- 

 tion voltage is usually 120 volts or 250 volts and in the larger 

 stations is supplied by bus-bars devoted to excitation only. In 

 smaller installations, the exciter is mounted directly on the alter- 

 nator shaft, Fig. 127, or else is belt-driven from the alternator 

 shaft. Large central stations usually have a storage battery 

 floating on the exciter bus, and in addition, may have steam- 

 driven exciters to be used in emergencies. 



ALTERNATOR ELECTROMOTIVE FORCES AND OUTPUTS 



58. Generated Electromotive Force. Figure 128 (a) shows 

 the magnetic flux between the armature surface and a north 

 and a south pole of an alternator. Assume that the flux distribu- 

 tion is sinusoidal, Fig. 128 (6), the flux density being a maximum 

 under the center of the pole. Let B' be the average value of 



2 

 the flux density. B' is equal to - times the maximum value B. 



Let a be a conductor cutting this flux with a velocity of v cm. 

 per second. This conductor a has a length of I cm. perpen- 

 dicular to the plane of the paper. 



The average voltage from equation (93), Vol. I, page 217, is 



e' = B'lvW-* volts. 



Let D be the pole pitch in centimeters and / the frequency in 

 cycles per second. 



