Assembling and Installing Systems

Coupling Mechanisms

Interference can reach the programmable controller via four different coupling mechanisms, depending on the transmission medium (conducted or non-conducted) and distance between interference source and the equipment.

Coupling Mechanism

Cause

 

Typical Interference Sources

 

 

 

 

Direct Coupling

Direct or metallic coupling always

Switched devices (supply affected

 

occurs when two circuits have a

 

by inverters and external power

 

common conductor.

 

supply units)

 

 

Motors being started

 

 

Different potentials of component

 

 

 

cases with a common power

 

 

 

supply

 

 

Static discharges

 

 

 

 

Capacitive Coupling

Capacitive or electrical coupling

Interference pickup via parallel

 

occurs between conductors which are

 

signal cables

 

at different potentials.

Static discharge of the operator

 

The degree of coupling is proportional

Contactors

 

to the voltage variation as a function

 

 

 

of time.

 

 

 

 

 

 

Inductive Coupling

Inductive or magnetic coupling occurs

Transformers, motors, electric

 

between two conductor loops through

 

welders

 

which current is flowing. Interference

Parallel AC supply cables

 

voltages are induced by the magnetic

Cables with switched currents

 

fluxes associated with the currents.

Signal cables with a high

 

The degree of coupling is proportional

 

to the current variation as a function of

 

frequency

 

time.

Unconnected coils

 

 

 

 

Radiated Interference

There is a radiation path when a

Local transmitters (for example,

 

conductor is subjected to an

 

two-way radios)

 

electromagnetic wave. Impinging of

Spark gaps (spark plugs,

 

the wave results in induced currents

 

collectors of electric motors,

 

and voltages.

 

 

 

welders)

 

 

 

 

 

 

 

A-6

Automation System S7-400

Hardware and Installation

 

A5E00850741-01

Page 174
Image 174
Siemens S7-400 installation manual Coupling Mechanisms, Capacitive Coupling, Inductive Coupling, Radiated Interference