12 



SYNCHRONOUS MOTORS 



will become zero, and will then change sign if the difference in phase 

 increases. By reason of the symmetrical construction of the motor 

 the torque will have points of maximum and minimum value at equal 

 distances between the points of zero-value, i.e., in the positions where 

 the poles of the rotor are midway between the poles of the stator. 



To sum up, taking as abscissae the difference of phase / of the poles 

 of the rotor, expressed in terms of the interpolar space L, and taking 

 as ordinates the torque C, the representative curve will take the form 

 shown herewith (Fig. 9), the magnetic strength at the armature-poles 

 being supposed constant, i.e., assuming the currents that produce this 

 magnetic flux to be constant. 



The machine will have stable operation for the difference of phase 

 comprised between the two maximum points B and C (the positive 



.Operating as Motor 



'Opemt/ng as ^Generator 



FIG. 9. 



maximum being due to a lag, and the negative maximum being due 

 to a lead), because every accidental advance (or lag) is corrected 

 of itself by a contrary variation of the torque. If the rotor lags, for 

 example, in consequence of a passive mechanical resistance, the increase 

 in torque compensates for this resistance. 



When the motor is running without load, its condition corresponds 

 to the position O, at which there is no phase-difference. When the 

 motor is loaded, i.e., whenever mechanical resistance is applied to the 

 shaft, the position of the poles of the rotor changes in phase and comes 

 to a point O', such that the couple O'm shall balance the resisting couple. 

 If the resisting couple is greater than the maximum torque, the machine 

 can no longer run; and even for positions of m which are a little 

 below A/, the machine will fall out of step, in consequence of unavoid- 

 able oscillations. 



