Temperature monitoring of electric motors Thermistor overload relays EMT6 comply with
the characteristics for the combination of protective devices and PTC sensors to VDE 0660 Part 303. They are
therefore suitable for monitoring the temperature of series motors. When designing
motor protection, it is necessary to differentiate between stator-critical and rotor-critical motors:
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Stator-critical
Motors whose stator winding reaches the permissible
temperature limit quicker than the rotor. The PTC sensor fitted in the
stator winding ensures that the stator winding and rotor are adequately
protected even with stalled rotor.
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Rotor-critical
Squirrel-cage motors whose rotor in the event of
stalling reaches the permissible temperature limit earlier than the
stator winding. The delayed temperature rise in the stator can lead to a
delayed tripping of the thermistor overload relay. It is therefore advisable to
supplement the protection of rotor-critical motors by a conventional overload relay.
Three-phase motors above 15 kW are usually rotor-critical.
Overload protection for motors in accordance with IEC/EN 60204. These standards specify that
motors above 2 kW used for frequent starting and stopping should be adequately protected
for this type of duty. This can be achieved by fitting temperature sensors. If the
temperature sensor is not able to ensure adequate protection with stalled rotors, an
overcurrent relay must also be provided.
Generally, where there is frequent starting and stopping of motors, intermittent operation and
excessive frequency of operation, the use of overload relays in conjunction with thermistor overload relays is to
be recommended. In order to avoid premature tripping out of the overload relay in these
operating conditions, it is set higher than the predefined operational current. The overload relay then assumes
stalling protection; the thermistor protection monitors the motor winding.
Thermistor overload relays can be used in conjunction with up to six PTC sensors to DIN 44081 for direct
monitoring of temperatures in EEx e motors compliant to the ATEX directive (94/9 EC). Copies
of PTB certification are available on request.
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