Every electrical or electronic device can emit electromagnetic fields that can influence and therefore interfere with other devices - electromagnetic coupling occurs. The aim is to design all electrical systems in such a way that they do not influence others and cannot be influenced themselves.

Before automation technology devices such as control units, power supply units, drives or frequency converters can be sold and used, they must pass an EMC test as part of CE certification. This is to ensure that the limit values of existing standards are not exceeded. However, anyone who concludes from this that no problems will occur if only certified devices are used is wrong: in practice, the automation landscape is heterogeneous and consists of a large number of different components and assemblies. Power supply cables run through the systems, and a fieldbus or network infrastructure ensures system-wide data exchange. Functional earthing and equipotential bonding also play a major role. From an EMC perspective, networked machines and systems are highly complex systems, each with their own individual EMC fingerprint.

Triggers for electromagnetic interference

The basic cause of electromagnetic interference is high voltages or currents that are switched with steep edges and thus cause a large electromagnetic field change. The steeper the edges or the higher the frequency, the more EMC interference occurs. This can happen when switching large inductive loads such as powerful electric motors as well as in the event of a sudden electrostatic discharge, for example when a plastic transport box is charged to several kV and then comes into contact with an earthed system component. These effects can also occur with the cable asymmetry discussed later in the article if an interference current is induced on the shield cable or on the PE (protective earth) when supplying power to a drive. Potentially, all electronic assemblies and signal lines that are in the area of influence of the generated electric and magnetic fields can be disturbed. Defects in the installation, such as inadequate shielding of signal lines or insufficient equipotential bonding, increase the risk of interference and system downtime.

Transmission paths of EMC interference

In this supply cable with a length of approx. 30 m, the PE and the shield are connected to each other at the fuse and in the switch cabinet. As a result, a current of 1.8 A flows in the circuit. The resulting total current of 0.35 A has a capacitive cause.

© Leadec

The electromagnetic interference between the interference source - the system that emits interference - and the interference sink - the interfered components - is called coupling. A distinction is made between

• Galvanic (direct) coupling: The interference source and interference sink have a conductive connection, usually through common supply or ground lines. If a current flows on these, electromagnetic interference can be coupled.
• Capacitive (electrical) coupling: The interference source and interference sink are installed in close proximity, for example in the same cable or cable duct. Interference is caused by the electric field generated by the voltage change during switching operations.
• Inductive (magnetic) coupling: The interference source influences the interference sink through a magnetic field. The alternating field generated by the current flow induces an interference voltage in the interference sink.
• External interference source: This includes, for example, a lightning strike, which is particularly important to consider when lines run outdoors in large-scale installations.
  

Cable asymmetry as one of the causes

If you take a close look at troubleshooting in electrical automation technology and industrial networks, you will notice that nowadays faults are increasingly caused by currents on PE (protective earth) or PA (equipotential bonding). One reason for this is the geometric asymmetry between the individual conductors. This is caused by the fact that the various conductors do not all have the same mechanical distance from the PE. These currents occur in particular where drives are switched or controlled by frequency converters. According to experts from Leadec, the high-frequency voltage pulses caused by switching processes or PWM (pulse width modulation) control are now one of the main causes of EMC interference.

Speed-controlled drives are increasingly being used in modern machines. At the same time, the miniaturization of electronics is progressing. The fact that these faults are increasingly occurring can be attributed to the ever-increasing level of automation. More and more automation technology and sensors are being used in systems. As a result, components and structures are becoming more compact. There is also less and less space for routing signal and power supply cables. The higher packing density requires a reduction in power loss, which is achieved with steeper edges, for example. However, steeper and sharper edges cause more interference and higher frequencies. As the speed and control accuracy of the drives increases, so too does the clock frequency of the control and therefore also the risk of high-frequency EMC interference.

Due to the cable asymmetry, high-frequency currents can be induced on the PE or PA (potential equalization), which can cause interference. For example, Leadec has measured stray interference currents in the range of 10 A and with a frequency of 0 to 5 GHz. Such shield currents are generally undesirable, as they can interfere with the device electronics on the way to ground or be transferred to the data line via the transfer impedance. This increases the risk that data communication will be impaired and sooner or later the system will come to a standstill. High currents on the PA or on the PE can lead to unexpected effects and faults in the control technology.

No patent remedy

The author: Gerhard Bäurle is a technology journalist for Leadec.

© Leadec

There is no textbook recipe for dealing with all EMC phenomena in one go. As a rule, it makes sense to carry out a basic inspection of the system in the next step, i.e. a kind of inventory in which all components and devices are recorded and analyzed. This includes various measurements of interference variables such as shielding currents, interference levels on the supply lines and the EMC load in the system. Installation or installation faults are often found during the basic inspection. These are faults such as an unconnected shield in the connectors, missing shields or long cable lengths - in other words, faults that were overlooked during acceptance after the initial installation or after conversion work. Eliminating such faults can significantly improve the overall stability of an installation. If high currents are still measured on the PE, the PA or on the shield after these measures, a design change is required, which companies such as Leadec can help with.