276 SYNCHRONOUS MOTORS. 



because its speed depends only on the periodicity 

 and not the voltage of the supply. It cannot con- 

 sequently adjust the back electromotive force gene- 

 rated in the armature, since this depends only on the 

 product of speed and field strength. The motor can, 

 however, bring the current to the value required to over- 

 come the load by altering the phase angle between the 

 voltage applied and the back volts of the armature. This 

 changes the resultant voltage and consequently also the 

 armature current. At the same time the phase difference 

 between armature current and back voltage changes, and 

 consequently the amount of their mutual action changes 

 also. In general terms the effect of alteration in field 

 strength will be as follows : 



With excitation so adjusted that the back voltage and 

 applied voltage are equal, the armature of the motor will 

 assume a position such that the back voltage is not 

 exactly opposite in phase to the applied voltage, since in 

 this case the armature current would be zero, the two 

 voltages being equal and opposite. On the contrary, the 

 armature will fall behind, so that the back electromotive 

 force lags in phase until the product of the back voltage and 

 the component of the current in phase with the back 

 voltage produces the requisite torque. 



If the excitation is now decreased, the pull of the field 

 on the armature is lessened, and the armature lags still 

 further, thereby increasing the resultant electromotive 

 force in the armature and also the armature current, 

 until again the product of back electromotive force and 

 energy current produce the torque required to overcome 

 the load. 



If the excitation of the motor be increased instead o 

 decreased, the required armature current will flow with a 

 smaller angle of phase difference between applied and 

 back voltage, and the motor will consequently lag by a 

 smaller angle, and C cos will approach more nearly to C. 

 When the excitation has been increased just up to the 

 point where the armature current is exactly in phase 

 with the back electromotive force and cos <t> is consequently 

 equal to unity, the current required to drive the motor 

 will have its minimum value, since the whole of the 

 current acts usefully on the fields. On increasing the 



