SYNCHRONIZATION OF TELEVISION 60S 



it is necessary to hold the synchronization within approximately }/2 of 

 the width of one element. This gives 0.144 degree divided by 2 or 

 0.07 degree as the requirement within which synchronization should 

 be held. By way of comparison it might be mentioned that the 

 angular twist in a length of 6 ft. of 1-in. steel shafting operated at 

 rated load is of about the same order of magnitude. 



An ordinary four-pole synchronous motor when operating at full 

 load, unity power factor, has an angular phase displacement of about 

 20 electrical degrees between the impressed and back e.m.f. This 

 corresponds to 10 mechanical degrees since the motor has two pairs 

 of poles. If this motor is operated at constant load and the line 

 voltage is varied, the phase angle will decrease with increasing voltage, 

 or when the voltage is held constant and the load is varied the phase 

 angle will increase with increasing load. It is at once apparent 

 therefore that the ordinary type of synchronous motor will not even 

 approach the degree of precision required for the reason that any 

 minute change in line voltage or load will cause variations in its 

 phase angle of lag with respect to the impressed frequency of a far 

 greater amount than 0.07 degree. Consider, however, a motor 

 having 120 pairs of poles. Allowing 20 electrical degrees as the 

 normal full load phase displacement, this would be equivalent to 20 

 divided by 120 or 1/6 degree mechanical phase displacement. Even 

 this amount is over twice the required permissible displacement of 

 0.07 degree. Since the variation of the phase displacement is the 

 important factor and not the absolute amount of displacement, it is 

 evident that if the line voltage and load are held reasonably constant 

 a synchronous motor with 120 pairs of poles should be sufficiently 

 precise. 



Another requirement in addition to close phase synchronization is 

 regulation of the acceleration or deceleration of the generator at the 

 transmitting end. Such regulation is required due to the fact that 

 an appreciable time is taken for the transmission of the synchronizing 

 current a distance of 220 miles (circuit length) between New York and 

 Washington. The velocity of propagation over the cable was approxi- 

 mately 19,000 miles per second while that of the picture on the open 

 wire of 285 miles circuit length was about 175,000 miles per second, 

 the corresponding times of transmission being .0116 second and 

 .0016 second, leaving a difference of .01 second approximately. Since 

 the total permissible error in synchronization is .07 degree, it is reason- 

 able to allow .02 degree as error due to acceleration regulation. Let 

 a be the acceleration in degrees per second per second. Substituting 

 in the formula 5 = 3^a/^ gives .02 = 3^a(. 01)^ or a = 400 degrees 



