BIPOLAR DIAGRAM 163 



Fig. 81 OB is a measure of the armature M.M.F., i.e., the strength 

 of the armature considered as an electromagnet lying across the 

 magnetic field whose flux is $ and whose M.M.F. is R (see Fig. 83). 

 The resulting torque is then proportional to yli$cos^ or approxi- 

 mately proportional to A\R cos fy; but in this approximate analysis, 

 $ and therefore R is determined by the impressed E.M.F., and is 

 in any given case a rough measure of the exciting M.M.'F of the 

 generator which supplies the power, while F is the exciting M.M.F. 

 of the synchronous motor. 



Thus the two crank pin radii correspond roughly to the exciting 

 M.M.F.'s of the generator (driver) and the motor (driven), the 

 tension OB corresponds to the armature M/M.F., the angle <J> to the 

 phase difference between current and E.M.F. or to the angle by 

 which the armature M.M.F. differs from its position of maximum 



FIG. 83. 



torque-producing effect, and the coupling angle 6 roughly to the 

 mechanical phase difference (measured in electrical degrees) between 

 the revolving parts of the generator and motor. 



The electrical angular velocity of the motor is the same as that 

 of the generator, but their mechanical angular velocities are inversely 

 as their numbers of poles; while in the mechanical analogue the 

 angular velocities of the two parts of the coupling must be the same. 



In order to see what determines the stiffness of the electro- 

 magnetic coupling between alternator and synchronous motor, it is 

 necessary to remember that the torque is strictly proportional to the 

 product of the flux <i> and that component of the armature M.M.F. 

 in quadrature with $, and that for a given value of < the necessary 

 M.M.F. R will depend upon the reluctance of the magnetic circuit; 

 e.g., if a synchronous motor has its pole faces bored back so as to 

 increase the reluctance of the magnetic circuit, a larger resultant 



