Inductive Coupling

Inductive Coupling

Mechanism Inductive coupling - or sometimes known as transformer coupling - is a coupling via the magnetic field. This occurs between lines running parallel to one another. Current changes in a wire cause a fluctuation in the magnetic field. The resulting magnetic field lines affect parallel running wires and induce an interference voltage there. A current now flows which overrides the useful signal as an interference signal. Inductive coupling is caused in parallel running lines in cables, wire harnesses and cable ducts. Well known sources of interference are:  Conductors and electrical equipment with high and fluctuating operational and interfering currents (short circuit currents)  Lightning discharge currents  Capacity switching  Welding current generators The following circuit diagram shows the construction of inductive coupling. Current changes in circuit 1, which are caused by switching large loads or those that are caused by a short circuit, are producing a fluctuation in the magnetic field.

Size of the interference

The interference voltage caused by the inductive coupling depends on the coupling inductivity MK between the two conductors and the current change time di/dt on the power line:

Coupling inductivity MK Coupling inductivity MK is determined by the circuit arrangement. The coupling is at its largest if the circuits lie tight together as with a standard transformer.

1 Circuit 1 2 Circuit 2 h Distance between the outgoing and return lines of the circuit loop or between signal lines and the ground plate. d Distance between the circuit loops (cable spacing) l Distance that the lines run in parallel

Realistic example values for the coupling inductivity:  Tightly packed cable: h = 2 mm, d = 4 mm MK = 80 nH/m  Cable spacing 10 cm: h = 2 mm, d = 100 mm MK = 1.5 nH/m

Example: Cable spacing influence The following calculation example for inductive coupling of two electric circuits shows the influence that cable spacing has on the amount of induced interference voltage: Increasing the space between cables from 4 mm (tightly packed cable) to 10 cm reduces the induced voltage in the disturbed circuit by 98 percent!  Parallel cable length

 Switching current in power cable

 Duration of the current surge:

The induced voltage in the disturbed circuit depends on

Cable spacing d Coupling inductivity MK Induced voltage in the disturbed circuit USt

4 mm (cable tightly packed) 80 nH/m 80 V

10 cm 1.5 nH/m 1.5 V