Methods of Induction

Figure 1. A parallel consisting of a grounded power line (fed by generator G and connected to motor M) and a two-wire, or metallic, telephone line. The reason for considering a grounded power line will be explained on page 557.

The magnetic field offers the second medium for introducing interfering currents into communication circuits. Thus, in Fig. 1, assume that a grounded alternating-current power line parallels a metallic two-wire telephone line as indicated. The power-line current will then produce a magnetic field about the power wire, and this field will link with the telephone line as shown in Fig. 2. Instantaneous induced voltages, represented by the generators e₁ and e₂, will be produced in the telephone wires. Since wire 1 is closer to the power line than wire 2, wire 1 will have a greater voltage induced in it by the alternating magnetic field, and thus a difference of potential e₁-e₂ will exist between wires 1 and 2. This difference of potential (often called metallic-circuit induction, and sometimes transverse induction) will cause a current to flow in the telephone circuit, and noise from the power line will accordingly be heard in the telephone receivers.

Figure 2. The magnetic field of the power line of Fig. 1 links with the telephone conductors.

The magnetic field causes a voltage to be induced in series with each wire. As Fig. 3 indicates, one end of each wire is positive and the other is negative. A difference of potential (often called longitudinal-circuit induction) will also exist between the wires and ground. If a high-impedance voltmeter V is connected as in Fig. 3, the voltmeter will deflect because the circuit is completed through the distributed capacitance represented by C_g1.

Figure 3. The magnetic field of Fig, 2 induces voltages in each telephone wire as indicated by e1 and e2. Since e1 is greater than e2, noise currents will be forced around through the telephone sets. These induced voltages tend to act between the telephone wires and ground as indicated by the fact that voltmeter V will indicate the presence of a voltage. This voltage to ground will force currents through capacitive and leakage paths to ground.

The third method by which noise energy is introduced into paralleling telephone circuits can be explained from a consideration of the power-line voltages and distributed capacitances between the circuits involved (Fig. 4). Considering for the moment one wire only, suppose that C₁ represents the distributed capacitance between the power wire and wire 1, and that C_g1 represents the capacitance of wire 1 to ground. The effective voltage E between the power wire and ground will divide inversely proportional to the capacitances, and thus E_g1 = EC₁/(C₁+C₂). The voltage from wire 2 to ground will be less than E_g1 since the respective distances between wires are greater, but the capacitances to ground are the same.

Figure 4. The power-line voltage E to ground acting through the distributed capacitance raises the telephone line wires to unequal voltages above ground. The voltage Eg1 exceeds Eg2. The shape of the electric field is approximately as indicated.

Since the two voltages to ground are unequal, the wires will have a difference of potential between them, and this will cause noise currents to flow through the connected telephone sets. The way in which the two telephone wires are raised to a voltage above ground is also shown in Fig. 5. The induced voltages are represented by the small generators E_g1 and E_g2.

From this section it is apparent that the power-line current and the resulting magnetic field induce voltages which act as small generators connected in series in the line wires (Fig. 1). Also, the power-line voltage and the resulting electric field raise the paralleling telephone-line wires to voltages above ground, and these voltages act as small generators connected between the telephone wires and ground (Fig. 5).

Figure 5. Showing Fig. 4 somewhat in perspective. The power-line voltage acting through the distributed capacitances raises the telephone wires to unequal voltages above ground as shown by the small generators. The voltage Eg1 exceeds Eg2. The internal impedance of each generator consists largely of the capacitance between the wire and ground.

In this discussion and the one which follows, it has been assumed that the telephone line is paralleled by a power line consisting of a single grounded wire. Such lines seldom exist for commercial power purposes, but, as will be discussed later, under certain conditions a three-phase power line has the characteristics of a grounded circuit as just considered. Also, the grounded system applies for the single trolley wire of an electric railway.