272 SYNCHRONOUS MOTORS. 



M v M., Source of alternating current. 



m v m z Source of direct current. 



M Motor armature. 



F Motor field windings. 



A l Ammeter reading armature current. 



A z Ammeter reading exciting current. 



F Voltmeter reading armature voltage. 



W Wattmeter reading power supplied to armature. 



R Field regulating resistance. 



S v S 2 Switches. 



L Synchronising lamp.* 



Instructions. Excite the motor field from a source of 

 direct current through a regulating resistance, ammeter, 

 and switch. Connect the armature to the source of alter- 

 nating current through a switch, ammeter, and series coil 

 of a wattmeter. Connect a synchroniser in parallel with the 

 switch for synchronising the motor before switching the 

 armature on to the supply. Connect a voltmeter and the 

 volt coil of the wattmeter across the terminals of the arma- 

 ture. 



Run the motor, close the switch in the exciting circuit, 

 and synchronise as described in the previous experiment, 

 closing the switch in the armature circuit when synchronism 

 is ob tamed. 



Next determine the most favourable excitation by varying 

 the exciting current until the armature current indicated 

 by A^ reaches its lowest value. 



The experiment should now be continued as follows: 

 With the motor running on no load, vary the excitation, 

 first decreasing and then increasing the exciting current, and 

 for each value of the excitation note the current and power 

 taken oy tne motor armature. If this can be done without 

 too great fluctuations in the source of supply and excessive 

 heating of the motor armature, the variations of excitation 

 should be taken so far in both directions that the motor 

 stops, the point of breakdown being carefully noted. 



The voltage applied to the motor and the periodicity of 

 the current must be maintained constant throughout the 

 experiment. If the current is derived from an alternator 

 which is not very large in comparison with the motor, it 



*The synchroniser will frequently be of the more complete type indicated 

 in Fig. 126. A double-pole switch will in this case generally be employed. 



