Page 106 - Power Electronics Handbook

Chaoter 4
Electromagnetic compatibility

4.1 Introduction

Power electronic circuits, by switching large amounts of current at high
voltages, can generate electrical signals which affect other electronic
systems. These unwanted signals give rise to electromagnetic interference
(EMI), also known as radio frequency interference (RFI), since they occur
at higher frequencies. The signals can be transmitted by radiation through
space or by conduction along cable.
Apart from emitting EMI, the control circuit of power systems can also
be affected by EMI generated by its own power circuitry, by other circuits
or by natural phenomena. When this occurs the system is said to be
susceptible to EMI. Any system which does not emit EMI above a given
kvel, and is not affected by EMI, is stated to have achieved
electromagnetic compatibility (EMC).
This mer first describes some of the concepts behind electromagnetic
compatib~ty, followed by a description of the sources of EMI in power
electronic systems. The method by which the effects of EM1 can be
.. .
mmlrmsed, using circuit techniques and shielding, to achieve EMC are
then considered, followed by an introduction to the regulatory position on
EMI. Finally the principles involved in the measurement of EMI are
explained.

4.2 EMC concepts
There are three elements to any EMC system, the source of the EMI, the
media through which it is transmitted, and the receptor, which is any
system that suffers adversely due to the received EMI. Therefore
eiectrompcpletk compatibility can be achieved by reducing the EMI levels
from the source, blocking the propagation path of the EMI signals, or by
making the receiver less susceptible to the received EM1 signals (Tihanyi,
1995).
The source of the EMI is primarily any system where the current or
voltage changes rapidly (for example, the breaking of current by relay
contacts, arcing of motor commutators, high-frequency switching such as
the rapid turn-on and tum-off of a thyristor).
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