164 SYNCHRONOUS MOTORS 



excitation R must be supplied in order to produce the same flux < 

 made necessary by the same impressed E.M.F. ; but with this same 

 flux, the same load will require the same torque component OBo of 

 the armature M.M.F. A. Thus although R and F have increased, 

 OBo is unchanged, and the angle 6 reduced, for the same ratio of F 

 to R. This decrease of the angle 6 for a given torque corresponds 

 to a stiffening of the elastic band in the mechanical coupling, and 

 increases the break-down torque (when 6 = 90). In fact, the above- 

 described change in the synchronous motor is commonly called a 

 stiffening of the field, in that the latter is then less distorted by a 

 given armature M.M.F. 



H will be observed in this connection that every synchronous 

 motor has a natural period of oscillation about its mean running 

 position, which depends upon the moment of inertia of the revolving 

 part and the stiffness of the electromagnetic coupling. These oscil- 

 lations correspond to a variation of the coupling angle 6 about its 

 mean value, and a pulsation of power-flow. If the angular velocity 

 of the supply alternator should oscillate about a mean value, the 

 resulting oscillation transmitted to the synchronous motor through 

 the electromagnetic coupling would depend upon the amplitude and 

 frequency of the impressed oscillation, upon the moment of inertia of 

 the revolving part of the motor, and upon the stiffness of the coupling. 

 If the impressed oscillation should have the same frequency as that 

 of the freely oscillating motor, the latter would tend to increase its 

 amplitude of oscillation until it would swing past the point of maxi- 

 mum torque (6 = 90) and break down. If, on the other hand, the 

 frequencies were widely different, the amplitude of the transmitted 

 oscillation would be greater, the stiffer the coupling and the less, the 

 moment of inertia of the revolving part. 



These phenomena are usually covered by the term " hunting " 

 or " phase swinging," a quantitative analysis of which is given in 

 Chapter IV. 



Length of Air-Gap 



It will be interesting here to review the relative merits of high 

 and low synchronous impedance in a synchronous motor; i.e., of a 

 soft and a stiff electromagnetic coupling, or what is equivalent, the 

 relative merits of a short and a long air-gap. 



Take first a system in which there is a considerable pulsation of 



