146 ALTERNATING CURRENTS 



lamp. A red light indicates to the engine-driver that his engine is 

 running too fast, a green light that it is running too slowly. The 

 motion of the plate is limited by means of a Fl -shaped block of brass 

 attached to the back of the plate (Fig. 105), which embraces a circular 

 strip of brass (shown dotted in Fig. 105), whose ends are bent so as 

 to form two stops. This strip is more clearly shown to a larger scale 

 in Fig. 108 ; it is screwed to the heavy brass plate which contains 

 the front ball bearing of the rotor. The mechanism which throws 

 the swing-plate against one or other of its stops will be understood 

 by reference to Fig. 108. Mounted loosely on the rotor spindle is a 

 striker-plate (this is also shown dotted in the lower part of Fig. 105), 

 which is placed between two pins projecting from the back of the 

 swing-plate (Fig. 105). The swing-plate is thereby carried round to 

 one side or the other, according to the motion of the striker-plate. 

 The latter is fitted with two pawls, which, by the action of two light 

 phosphor-bronze springs fixed into them, press against the edge of a 

 notched disc rigidly mounted on the rotor spindle (Fig. 108). Each 

 pawl is provided with a pin, which, in the extreme position of the 

 striker-plate, comes into contact with a cam-plate, thereby lifting the 

 pawl out of the notch in the disc and allowing the disc to revolve 

 without carrying the striker-plate beyond that position. In Fig. 108 

 the striker-plate is shown in the halfway position ; but" as soon as the 

 rotor begins to move, the striker-plate will be thrown to the right or 

 left, according to the direction of rotation, and will communicate its 

 motion to the swing-plate. 



When this synchronizer is used on a three-phase circuit, its stator 

 is connected across two of the 'bus bars, and its rotor across two of 

 the terminals of the incoming machine. 



78. Siemens and Halske Three-phase 

 Synchronizer 



Messrs. Siemens and Halske some time ago introduced a form of 

 synchronizer for three-phase machines which has largely come into use. 

 The principle of this synchronizer will be understood by reference to 

 Fig. 109. Let OA, OB, OC be the secondary windings of a transformer 

 whose primaries are across the 'bus bars, and OA',OB',OC'the secondary 

 windings of a transformer whose primaries are across the terminals of 

 the incoming alternator ; the connections being such that when the 

 machine is ready to be switched in, the e.m.f.'s in OA and O'A', OB 

 and O'B', and OC and O'C' respectively, are in phase with each other. 

 In that case, it is evident, there will be no p.d. between A and A', B 

 and B', and C and C', so that three lamps connected between these 

 pairs of points would all remain dark. If the incoming machine 



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