1 Gramme-ring single-phase armature 2 Gramme-ring three-phase armature
The armature of the alternator, like of the dc generator, is also a drum winding. Of course, the drum is now the inner surface of the stator. This is shown in Fig. 3.
The principles underlying armature construction are the same for both ac and dc generators. The sides of each coil lie in slots under adjacent poles, and the coils are so connected that their induced voltages are additive. The coils are form-wound, and the winding itself may be either lap- or wave-wound. Contrary to the dc generator, a wave winding on an alternator will produce the same voltage as a lap winding having the same number of coils.
The stationary armature has very pronounced advantages over the rotating one. Only two slip rings are required to provide direct current in the rotating field, while at least three would be required for a rotating armature. Since high voltages of 13,200 volts and greater are usually generated, great difficulty would be encountered in transmitting such voltages from the ends of the armature winding across the sliding conductors between slip rings and the stationary brushes. Insulating the slip rings from the shaft would present another problem. These problems are partly minimized with the comparatively low voltage of a few hundred volts used to excite the field.
The high-voltage insulation required for the armature conductors is more easily accomplished on a stationary member. Also, the heavier armature winding is more readily braced on the stator, and only the lighter field winding is subjected to centrifugal force. This is important when a high–speed turbine is the prime mover.
As more armature copper is required for greater capacity, deeper slots are also required. On a rotating armature, the deeper slot approaches the center of the armature, or a smaller diameter, and therefore the teeth providing the slot sides become narrower. On the stator, however, these slots approach the outer diameter, and hence the teeth get wider. This makes the stronger teeth on the stator as well as slower reluctance due to the increased cross section of the iron.
Frequency. Figure 4 shows the simplest possible polyphase alternator. It has just two poles and only one coil per phase.
As shown, points of one polarity, i.e., conductors a', b', and c' are tied together, this gives a connection most frequently used on generators. When the rotor makes one revolution, one cycle of voltage is generated in each conductor. The number of cycles per second, or frequency, is directly proportional to the speed of the rotor. Thus, if the speed is 60 rpm, the frequency is 1 cps, and when the rotor revolves at 3600 rpm, the frequency is 60 cps.
If a four-pole rotor is used, the two conductors of each coil must lie under adjacent poles. Otherwise, if the same armature winding as in Fig. 4 were used, the voltages generated in each conductor of a given coil would oppose each other, and the net voltage per coil would be zero. Therefore twice as many armature conductors are used, and are placed as shown in Fig, 5.
Fig.4 Fig.5
Coils of the same phase are usually connected in series in order to obtain the high voltages generated. One revolution of the four-pole rotor now generates two cycles in each conductor. A six-pole rotor would require an armature wound for six poles and would generate three cycles per revolution. Thus the frequency in the armature conductors is directly proportional to the number of pairs of poles, A four-pole rotor revolving at 3600 rpm would therefore generate three voltages, 120° apart in time and having a frequency of 120 cps.