142 ALTERNATING CURRENTS 



Hence, the more recent types of synchronizer are provided with 

 additional visual signals, in the form of lamps, which indicate to the 

 engine-driver whether the speed is too low or too high. 



77. Everett- Edgcumbe Rotary Synchronizer 



The principle underlying the action of this instrument is as 

 follows. Imagine a small induction motor, whose stator and rotor 

 are both provided with polyphase (two- or three-phase) windings, the 

 rotor winding being connected to slip-rings. If polyphase currents 

 of the same frequency be supplied to stator and rotor, the magnetic 

 fields due to them will revolve at the same speed, the rotor taking up 

 a position which gives coincidence of the two fields, and remaining 

 stationary. It is evident that any relative displacement of the fields 

 will result in a couple tending to turn the rotor one way or the other, 

 according to the direction of displacement. If, therefore, we suppose 

 the frequency of the rotor currents to be reduced below that of the 

 stator currents, the rotor field will tend to lag behind the stator 

 field, and the rotor will be pulled round by the stator in the direction 

 of rotation of the stator field, so that the two fields will keep in step 

 as before, the speed of rotation of the rotor corresponding to the 

 difference of the speeds of the stator and rotor fields. Similarly, if the 

 frequency of the rotor currents be increased above that of the stator 

 currents, the rotor field will tend to gain on the stator field, and the 

 rotor will be pulled back by the stator field, so as to make both fields 

 keep in step as before. The arrangement, therefore, furnishes an 

 extremely accurate method of comparing two nearly equal frequencies, 

 the rotor rotating one way or the other according as the frequency of 

 the rotor currents is below or above that of the stator currents, 

 and its speed of rotation being directly proportional to the frequency 

 difference. 



The stator and rotor cores of the Everett-Edgcumbe rotary 

 synchronizer are shown at the top of Fig. 105. The two-phase 

 currents for the stator and rotor are obtained by adopting the arrange- 

 ment of connections shown in Fig. 106. It will be seen that the two 

 phases are connected in parallel across the mains,* the necessary 

 phase difference being obtained by the insertion of a high non- 

 inductive resistance (an incandescent lamp) in series with one phase, 

 and a high reactance (a choking coil) in series with the other. The 

 core of the choking coil is shown in the middle of Fig. 105. The 

 rotor runs in ball bearings, and a pointer attached to it, and arranged 



* The connections are, however, made through two synchronizing transformers, one 

 for the 'bus bars and the other for the incoming machine. 



