Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 149)

4.2.4.2.1 Principle of operation
Fig. 4.2-14
Basic functional modules of electronic motor protection relays
Current measurement
For the processing in the electronic circuits, the motor current is measured and converted into
an electronically compatible signal. Depending on the principle of operation of the electronics,
this signal may be analog (more commonly in simple devices) or digital (in complex devices).
Current transformers or magnetic field sensors (for example Hall sensors) are most commonly
used for measurement. As the signals are processed electronically, virtually no control signal
power is drawn. This results in low heat losses that facilitate control cabinet climate control,
which is a major advantage of electronic motor protection relays.
Another advantage of electronic signal processing is the wide current ranges of the devices,
which leads to a considerable reduction in the number of versions in comparison to thermal
motor protection relays and which simplifies planning and inventory. In addition they usually
have a higher precision of the ultimate tripping current thanks to the narrow tolerances of the
components used. IEC 60947-4-1 has made allowance for the technical possibilities of elec-
tronic motor protection relays by introducing additional trip classes (
Tab. 4.1-3 ).
Depending on the principle of current measurement, there are restrictions with respect to the
permissible frequency range. When current transformers are used, frequencies significantly
below the supply frequency are not acceptable because of saturation, especially applications
with direct current are not possible. Applications with variable frequency drives require separate
consultation.
The short-circuit withstand capacity of the main circuits is usually very high. Exceptions can be
versions with small rated currents, if conductors with small cross-sections are used.
Power supply / internal supply
Simple electronic motor protection relays that are intended as substitutes for bimetal thermal
relays obtain the supply for the electronic circuits directly from the measuring circuit and hence
require no supply by a separate control voltage. The simplicity of application carries with it a
restriction to the basic functions of motor protection – basically to thermal protection and the
phase failure protection. In case that other protective functions are offered it should be noted
that setting up an internal supply can take a certain time during which these functions are not
available.
I
U
Powe Operator interface r supply
Thermal image
Complementary
functions
Output-
circuits
Communication-interface
M
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LVSAM-WP001A-EN-P - April 2009 4-41