CABLE TESTING 695 



form must also be independent of the particular combination of 

 transformer windings which are used to obtain the required 

 voltage and must be uninfluenced by the method of voltage regu- 

 lation. The instruments commonly used for measuring the 

 voltage, i.e., electrostatic voltmeters and electrodynamometer 

 voltmeters, give the effective or r.m.s. value of the voltage. 

 If the wave is sinusoidal, the maximum value is obtained by 

 multiplying the effective value by \/2. 



If the voltage wave is not sinusoidal, resonance effects due to 

 the capacity of the cable and the reactance of the apparatus 

 may be present, so that from this cause the wave may be distorted 

 in a manner dependent on the length of the cable under test. 



The effect resulting from taking the insulation through cycles 

 of electrostatic stress depends not only on the maximum voltage 

 but on the number of times the cycle is repeated in a second. 

 If the voltage wave be very greatly distorted, this effect and the 

 consequent weakening of the dielectric strength of the cable due 

 to a prolonged application of the test voltage, are abnormal and 

 the results will not be comparable with those obtained with a 

 very different wave form. 



The following series of oscillograms serve to show the neces- 

 sity for the proper equipment when dielectric strengths are to 

 be measured. 3 The generator used in the tests was a motor- 

 driven, 25-kva, 220-volt, 4-pole, 25-cycle, single-phase alternator, 

 having 10 slots per pole and a conductor belt five-eighths the 

 pole pitch. The transformer capacity was 50 kva., 220-50,000 

 volts. The secondary consisted of four separate 12,500-volt 

 coils. The high-tension winding had a total of 12,512 turns; the 

 low-tension 55 turns. The reactance voltage was about 6 per 

 cent. This transformer was operated at high saturation. 



Fig. 435 shows that the e.m.f . of the generator is nearly enough 

 sinusoidal and if the wave form could be maintained, satisfactory 

 tests would be possible. Fig. 436 shows the result when the 

 transformer, with the secondary circuit open, is attached to the 

 generator. The exciting current is much distorted owing to the 

 changes of permeability incident to working the core at 

 saturation, and a small third harmonic appears in the P.D. wave. 



A cable having a capacity of 0.13 microfarad was attachec 

 the secondary. Such a load will take a large leading current 



