Seite(n) 2
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Ausgabe 0810
EN
Over temperature protection
Winding protection
Winding protection
PTC thermistors
The most comprehensive protection against thermal overloading of the motor is offered by positive temperature
coefficient thermistors which are integrated in the motor winding. Owing to their low thermal capacity and internal
thermal contact with the winding, they provide precise monitoring of the winding temperature. When a limit
temperature is reached (rated tripping temperature), the resistance of the thermistors changes rapidly. This
resistance is evaluated by tripping devices and can be used to open auxiliary circuits. PTC thermistors cannot be
subjected to high currents and voltages. This would result in destruction of the semi-conductor component. The
temperature tolerance for switching is low with ± 5 K. This makes it also possible to quickly switch on the drive
again. Motors provided with this type of protection can be used for operating conditions with high frequency
starting, starting against heavy loads and voltage and frequency fluctuations in the mains supply. PTC thermistors
to DIN 44081 are suitable for tripping devices with 2,5 V DC output voltage and 4 kΩ tripping resistance. The
resistance for each PTC thermistor is between 10 and 250 Ω at temperatures of –20 °C to θNat –20 °C (Nat =
rated tripping temperature). The resistance of each PTC thermistor changes in the kΩ range when the rated
tripping temperature is reached.
Number of PTC thermistors:
• Motors with one winding:
3 PTC thermistors (1 per phase); protection against overload, excessively high starting frequencies, two-phase
starting, inadequate cooling, blocked rotor.
• Motors with two windings:
3 PTC thermistors; 1 PTC thermistor in the low-speed winding, 2 PTC thermistors in the high-speed
winding.
Important: Thermistors only provide protection against overload, exces sively high starting frequency, inade
quate cooling. No protection against blocked rotor and two-phase starting.
• Special design (must be specified in the order):
3 PTC thermistors in the low-speed winding, 3 PTC thermistors in the high-speed winding, protection against
overload, exces sively high starting frequency, twophase starting, inadequate cooling, blocked rotor. If, in addition
to PTC thermistor switchoff, a warning is required when the winding temperature is too high, an additional tripping
device and double the quantity of PTC thermistors are required. PTC thermistors used for warning purposes are
supplied with a rated tripping temperature 10 K lower than that of PTC thermistors used for switching off. The
required tripping devices must be ordered separately.
Temperature detectors
Temperature detectors integrated in the motor winding provide adequate protection for slowly increasing motor
temperatures and, therefore, offer protection against thermal overloading. These bimetallic switches (normally
closed) can switch off an auxiliary circuit on reaching a limit temperature. The circuit can only be closed again
when the motor has significantly cooled down. Owing to the large thermal time constants, these switches are not
suitable for high motor current increase rates (e.g. short-circuit or blocked rotor). The control voltage of the
temperature detectors must not exceed 250V according to EN 60204. The maximum switching current is 6,3A at
250V.
# = Änderungen gegenüber
vorheriger Ausgabe
1321/PM Standard cranes
Seite(n) 2
Seite 2
Ausgabe 0810
EN
Over temperature protection
Winding protection
Number of temperature detectors:
• Motors with one winding:
1 temperature detector
• Motors with two windings:
2 temperature detectors; 1 temperature detector in the low-speed winding, 1 temperature detector in the
highspeed winding.
• Special design:
3 temperature detectors in the lowspeed winding, 3 temperature detectors in the highspeed winding.
Not possible for frame sizes Z63, 71 and 90 A for design reasons.
# = Änderungen gegenüber
vorheriger Ausgabe
1321/PM Standard cranes