. The principles underlying radio communication . to run automatically, at theproper speed for the generator, or perhaps it would be betterto say that there are certain definite speeds at which motors have to run, and the generator has such a numberof poles that it gives the desired frequency when driven by amotor operating at one of these speeds. Voltage control of thegenerator is secured by means similar to those shown in Motor driven, by D. C. Motor.—When direct current at 110volts is available the arrangement is somewhat different. Theexciter is unnecessary, because c
. The principles underlying radio communication . to run automatically, at theproper speed for the generator, or perhaps it would be betterto say that there are certain definite speeds at which motors have to run, and the generator has such a numberof poles that it gives the desired frequency when driven by amotor operating at one of these speeds. Voltage control of thegenerator is secured by means similar to those shown in Motor driven, by D. C. Motor.—When direct current at 110volts is available the arrangement is somewhat different. Theexciter is unnecessary, because current for the field coil ofthe alternator may be taken directly from the line. It is then ALTERNATORS FOR RADIO USE. 197 possible to combine the generator and a 110-volt, direct current,shunt motor (see Sec. 97) into a very compact unit. The twoarmatures are on the same shaft and the two frames are joinedin one structure. Fig. 121 represents such a unit, which is shown partly disas-sembled in Fig. 122. The generator happens to be of the same. Fig. 121.—Small 500-cycle motor-generator set (2500 ; 24 poleson stator ; 24 teeth on rotor; 110 volts; amp.; kva.). 1. Field terminals. 2. Collector rings. 3. Armature terminals. 4. Shaft of both units. design as that shown in diagram in Fig. 103, but being builtfor nearly 50 per cent more power it is somewhat larger, hasmore poles, and runs at a correspondingly lower speed to givethe same frequency. The two armatures are seen on theircommon shaft; the collector rings are near one end and thecommutator near the other. 198 DYNAMO-ELECTRIC MACHINERY. One scheme of connections for such a unit is seen in Fig. 123,Which shows the d. c. motor connected to its line by way of aswitch and starting The rheostat shown in circuit withthe motor field, MF, may be omitted. Its purpose is to give con-trol of the motor speed,20 if such control is desired, in order toget some definite frequency quite accurately in the a. c. circ
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