Fig. 2,069.—General Electric frequency changer set, consisting of a 11,000 volt synchronous motor with direct connected exciter and a 2,300 volt alternator.
Frequency Changing Sets.—A frequency of 25 cycles is generally used on railway work and in large cities using the Edison three wire system, and as a 25 cycle current is not desirable for electric lighting it is necessary to change it to 60 cycles by means of a frequency changer shown in fig. 2,069 for distribution in the outlying districts.
The two machines in this combination are of the same construction, only the synchronous motor would have eight poles and have the 25 cycle current passing through it, while the generator would have 20 poles and produce 62½ cycles per second at 300 revolutions per minute. By supplying the motor with 24 cycles, the generator would produce 60 cycles.
Fig. 2,070.—General Electric four unit frequency changer set consisting of a 11,000 volt synchronous motor, 13,200 volt alternator, 250 volt exciter, and 440 volt starting induction motor. Where parallel operation is required between synchronous motor driven frequency changers, a mechanical adjustment is necessary between the fields or armatures of the alternator and motor to obtain equal division of the load. The adjustment can be obtained by shifting the keyway, or by special cradle construction. In the latter method, one machine is bolted to a cradle fastened to the base. By taking out the bolts, the frame can be turned around through a small angle relative to the cradle and therefore to the armature frame of the other machine, when the bolts can be replaced.
It will be seen from the figure that the separate exciter is fastened on the base plate and has its armature directly connected to the shaft.
Parallel Operation of Frequency Changers.—It is very difficult to construct two or more frequency changers and join them to synchronous motors so that the current wave of one machine will be in phase with the other, since the speed of the motor will depend on the frequency of the line and be independent of the load thrown on it.
When alternators are run in parallel, if one machine lag behind, the other carries the load with the result that the lightly loaded machine will speed up and get in step with the other, or in other words a synchronizing current will flow between the two alternators and tend to keep them in proper relation with respect to phase and load.