SIEMENS
10/158
Siemens IC 10 · 2017
Relays
SIRIUS 3RN2 thermistor motor protection
10
■
Benefits
•
Thanks to direct motor protection, overdimensioning of the
motors is not necessary
•
No settings on the device are necessary
•
Semiconductor compatible output thanks to versions with
hard gold-plated contacts
•
Rapid error diagnosis thanks to versions that indicate open
and short circuits in the sensor circuit
•
All versions with removable terminals
•
All versions with screw or spring-type terminals with push-in
functionality
■
Application
Direct motor protection through temperature monitoring of the
motor winding offers 100 % motor protection even under the
most difficult ambient conditions, without the need to make ad-
justments on the device. Versions with hard gold-plated contacts
ensure, in addition, a high switching reliability that is even higher
than an electronic control.
Direct motor protection
•
At increased ambient temperatures
•
When switching frequency is too high
•
When start up and braking procedures are too long
•
Used together with frequency converters
(at low speeds)
ATEX approval for operation in areas subject to explosion
hazard
The SIRIUS 3RN2011, 3RN2012-...30, 3RN2013, 3RN2023
thermistor motor protection relays for PTC sensors are certified
according to ATEX Ex II (2) G and D for environments with
explosive gas or dust loads.
Motor protection using current- and temperature-dependent
protective devices
IEC 60204 stipulates that motors must be protected from over-
heating at a rating of 0.5 kW and higher. The protection can take
the form of overload protection, overtemperature protection or
current limiting.
For motors with frequent starting and braking and in environ-
ments where cooling may be impaired (e.g. by dust), it is recom-
mended to use the overtemperature protection option in the form
of a protective device coordinated with this mode of operation.
A good choice in this case is the use of 3RN2 thermistor motor
protection devices.
On rotor-critical motors, overtemperature detection in the stator
windings can lead to delayed and hence inadequate protection.
In this case the standards stipulate additional protection,
e.g. by means of an overload relay.
This combination of thermistor motor protection and an overload
relay is recommended for full motor protection in case of fre-
quent starting and braking of motors, irregular intermittent duty
or excessive switching frequency. To prevent premature tripping
of the overload relay in such operating conditions, a higher set-
ting than that normally required for the operational current is cho-
sen. The overload relay then performs stall protection, and the
3RN2 thermistor motor protection relay monitors the temperature
of the motor windings.
✓
Full protection
❍
Partial protection
-- No protection
Application
Motor protection
Current-
dependent
only, e.g. with
overload relay
Temperature-
dependent only,
e.g. with thermis-
tor motor protec-
tion relay
Current-
and tem-
perature-
dependent
Motor protection in case of
Overloading in
uninterrupted duty
✓
✓
✓
Long start-up and
braking operations
❍
✓
✓
Irregular intermittent duty
❍
✓
✓
Excessively high switching
frequency
❍
✓
✓
Single-phase operation and
current unbalance
✓
✓
✓
Voltage and frequency
fluctuations
✓
✓
✓
Stalling of the rotor
✓
✓
✓
Switching on a stalled rotor
of a stator-critical motor
✓
✓
✓
Switching on a stalled rotor
of a rotor-critical motor
✓
❍
✓
Elevated ambient
temperature
--
✓
✓
Impeded cooling
--
✓
✓
© Siemens AG 2016