19
Electrical Connection
Motor protection
Motor protection with PTC resistor sensors
The AC motors contain between two and six PTC resistor sensors connected in series,
depending on the motor size and type. The PTC resistor sensors are embedded in the
windings of the motors in a way that optimises recording of the winding temperature. PTC
resistor sensors are temperature-dependent semiconductor resistors.
When the maximum permissible winding temperature of 135 °C is reached, the resistance
value increases rapidly. This response, combined with full motor protection electronics,
causes the motors to switch off.
NOTE
Unlike thermal contacts, PTC resistor sensors are not subject to wear. They provide a high
degree of operating safety as they are shock-resistant, vacuumresistant and not sensitive
to pressure.
DAMAGE TO THE UNIT!
The separate sensor with external leads must not be connected to voltage as this would
damage it beyond repair. Measuring voltages greater than 5 V are not permitted.
Connect the sensor only to full motor protection tripping devices.
INDUCTION EFFECTS
We recommend the use of shielded cables (such as JY (st) Y, 2 x 2 x 0.5) to prevent the
effects of induction on the PTC resistor measuring circuit. Connect the shielding on one
side with the potential equalisation (PE).
Full motor protection electronics
For motor protection we recommend our EVO-M 912.MVS 1.0 full motor protection trip-
ping devices. They have been specially harmonised with the PTC resistor sensors used
in the motors. As operation of the full motor protection electronics is based on the clo-
sed-circuit current principle they are intrinsically safe. A locking shutdown after a fault and
restart following power failure generally occur in conjunction with EVO-M 912.MVS 1.0 full
motor protection tripping devices.
The electronic full motor protection tripping devices switch the motors off where
the following occur:
– Overloading
– Ambient temperatures exceed permissible limits
– The rotor is locked
– Insufficient cooling
– Short circuit in winding
– Wire breakage and loose contacts in the measuring leads
+135
C
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ta
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Temperature T
Operating point of
tripping device
