SOiME ASPECTS OF NOISE INDUCTION 487 



characteristic sound is changed, indicating the effects of increasing the 

 harmonic content of the voltage wave. 



■1P^^»"^^ 



A A A ./:Ct\ A A. .A A 



^^.y^ W v/ V v/ \ 



Fig. 11 — Oscillograms of normal and distorted balanced voltages. 



11-^ — Impressed voltage, 



U-B — Balanced distorted voltage to neutral. 



In practice, harmonic voltages and currents may arise not only from 

 generating and transforming equipment but also may occasionally 

 arise from some particular load equipment such, for example, as certain 

 types of rectifiers or rotating machinery. 



Residual Voltages and Currents 



The inductive influence of a voltage or current of a given magnitude 

 and wave shape depends to a considerable extent on the dimensions of 

 the circuit in which it acts. For balanced currents or voltages (or 

 balanced components of the actual currents or voltages on line wires), 

 which, as discussed before, are confined to the wires of the power cir- 

 cuit, the dimensions of the circuit are much smaller than for the residual 

 currents or voltages which involve the earth as part of their circuit. 



In order to illustrate the relative inductive influences of a given 

 magnitude and wave shape of voltage, when acting in a balanced 

 manner and as a residual, the miniature power line is energized in two 

 different manners. First (the normal manner) the voltage is impressed 

 on it through a bank of transformers connected "delta" on the supply 

 side and " Y-grounded" on the line side. With these connections, the 

 voltages impressed on the three line wires are approximately equal and 

 120 electrical degrees apart and thus are closely balanced. Next, 

 using the same transformer connections, the line wires are energized in 

 parallel to earth and consequently, the vector sum (residual) is equal to 

 three times the normal phase-to-neutral voltage. The power circuit 

 connections used are shown in Fig. 12 and the telephone circuit con- 

 nections used are the same as shown in Fig. 4-A . The increase in the 

 noise from the loud speaker when residual voltage is used shows that 

 the influence of the power line is greater under these conditions. 



