Page 793 - ABB_Motor _Protection_and_Control_catalogue_20221216

ABB MOTOR PROTECTION AND CONTROL 7/3

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Thermistor motor protection relays
Applications

Heavy duty starting

Direct temperature monitoring
Generally, motor damage caused by overload or Increased switch-
overheating situations can be prevented in differ- ing frequencys
ent ways. Compared to the indirect temperature
measuring which monitors the motor current, the
temperature inside the motor can be measured
by direct temperature measuring. This enables di- Single phase Operating
rect control and evaluation of different operating operation
conditions. Therefore, the consequences from conditions
overheating, such as abrasion as well as electrical
failures, can be prevented. The direct measuring Phase
principle is carried out by a combination of the unbalanced
thermistor motor protection relay and three PTC
sensors which are installed directly in the motor 07
by the manufacturer. Those 3 PTC sensors are High ambient
placed directly at the thermal hotspots, the mo- temperature
tor windings. Insufficient Breaking

cooling operation

Motor protection using current- and tempera- Operating mode
ture-dependent protective devices The thermistor motor protection relays are used
IEC 60204 stipulates that motors must be pro- to monitor the temperature of motors or genera-
tected from overheating at a rating of 0.5 kW and tors equipped with PTC sensors type A according
higher. The protection can be provided or exe- to the latest product standard IEC/EN 60947-8.
cuted by overload protection, overtemperature The sensors are built-in into the motor windings,
protection or current limiting. For motors with measuring the motor heating. In case of an in-
frequent starting and braking, and in environ- crease of the temperature in the motor, the resis-
ments where cooling may be impaired (e.g. by tance of the PTC sensors increases as well. If the
dust), it is recommended to use the overtempera- motor heats-up excessively (>2.83 kΩ), the output
ture protection option in the form of a protective relay(s) de-energize(s) and the corresponding LED
device coordinated with this mode of operation. displays the overtemperature. A short circuit and
On rotor-critical motors, overtemperature detec- an interrupted wire within the sensor circuit can
tion in the stator windings can lead to delayed and also be detected. A reset is only possible after
hence inadequate protection. In this case, the cooling down of the motor (<1.1 kΩ) or after a
standards stipulate additional protection, e.g. by wire interruption, or a short circuit within the
means of an overload relay. This combination of sensor circuit has been removed. A reset after
thermistor motor protection and an overload relay tripping can be done manually with the Test / Re-
is recommended for full motor protection in case set button, externally with a push button be-
of frequent starting and braking of motors, irreg- tween S1 and 1T2/2T2, or automatically by jump-
ular intermittent duty or excessive switching fre- ering S1-1T2/2T2.
quency.