Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 150)

Complex electronic motor protection relays require a separate control voltage supply that for
example can also be provided via the communication link.
Thermal simulation
Thermal simulation, i.e. the simulation of motor heating based on the measured motor current is
in simpler relays usually performed on the basis of a single-body replica similar to that of a
bimetal relay. Complex devices often also use more complex thermal replicas that more closely
simulate motor heating and for example make allowance for the differing cooling characteristics
of running and stationary motors. This increases suitability for intermittent operation. Motor
protection relays without thermal memory should be marked in accordance with IEC 60947-4-1
as such (on the device) (see also Section
4.1.2.2 ).
Top class devices also often take into account the influence of asymmetrical supply on motor
heating (see also Section
4.1.2.4.2 ).
An important advantage of electronic motor protection relays is the tripping time at high
overcurrents (motor starting). Realizing various trip classes (
Tab. 4.1-3 ) is a simple matter
electronically. The wide tolerance band of the individual classes needs not be used to the full
and the tripping times are usually close to upper class limit. This means that relays are well
suited for heavy-duty starting applications. In complex relays the trip class (tripping time at
7.2 · I
e
) can often be adjusted and can thus be adapted to the motor and the application.
The phase failure protection usually results in electronic motor protective devices in briefly
delayed (a few seconds) tripping, as the loss of a phase can immediately be recognized in the
measured signals. The short delay serves to prevent ghost tripping, for example due to short
interruptions of the power supply.
Additional functions
Section 4.1.2.4 provides an overview of functions that are often offered by electronic motor
protection relays. The range of options is wide and the documentation for the respective devices
is definitive in individual cases.
An advantage of electronic motor protective devices is the availability of various functions in a
single device and access to device-internal signals. Thus for example current-measuring motor
protective devices with inputs for temperature sensors are available.
Outputs – for example for the measured motor current – make separate measuring circuits
unnecessary and in particular access to the “temperature state” of the thermal replica allows the
devices to be integrated in the control environment. Thus protective tripping can be avoided by
the issue of early warnings or – as far as compatible with the processes – the motor loading can
be controlled in accordance with measured temperature rise. Integration in the communication
system turns the motor protective device into an integrated control component.
Memory functions may be useful for debugging after protective shutdowns or for maintenance.
For example the operating data before a protective shutdown can be captured or statistical data
on the operation of the drives collected. Such data is frequently offered by microprocessor-
based devices.
4.2.4.3 Thermistor protection relays
See also Section 4.1.2.3 .
4.2.4.3.1 Relays for PTC sensors
PTC sensors (Positive Temperature Coefficient) are most frequently used in low-voltage motors
for sensing the windings temperatures. Their resistance increases steeply (
Fig. 4.2-15 ) at the
rated operating temperature TNF which makes it possible to provide simple and economical
tripping devices. The sensors – normally 1 per phase – are imbedded by the motor manufac-
turer in the windings and are connected in series to terminals. The rated operation temperature
TNF is selected in accordance with the insulation class. If early warning before tripping is
required it is possible to install a second set of temperature sensors with lower operation
temperature that are connected to a separate tripping device.
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