Motor temperature monitoring, residual hazards, motor blocking

Question:
Temperature sensor KTY, PT1000, PTC, thermal contact TKO: 
Why can a motor overheat in special cases despite activated temperature monitoring?

Answer:
In the operating instructions of the Lenze motors, the following note on the use of the temperature sensors can be found under the chapter 'Residual dangers':
  • Built-in temperature sensors are not full protection for the machine. If necessary, limit the maximum current, switch off the function block after a few seconds of operation with I > IN (of the motor), especially if there is a risk of stalling. Built-in overload protection does not prevent overloading (of the motor) under all conditions.
Especially when the motor current increases abruptly to high values, in the direction of several times the rated motor current, the temperature of the stator winding can increase so much at certain points that the winding is damaged even before the excess temperature can be registered via the temperature sensor. Such a case can occur, for example, if the motor is blocked, the current increases abruptly and then remains on for a longer period of time. 
Due to the following aspects, the actual temperature of the winding cannot be detected quickly enough, although the sensor is located directly in the stator winding:
  • Thermal capacity of the sensor,
  • thermal resistance of the insulation of the temperature sensor,
  • thermal conductivity of the sensor connections,
  • not all winding parts are heated evenly during standstill.
  • Use of an oversized drive controller and thus particularly high overcurrent factor.
The motor can only be protected from overheating/overload by further monitoring if such special cases occur. 

Solution approaches:
  • Current device series: Activation of motor load monitoring (I²xt).
  • Predecessor products: By means of a function block circuit or user program, motor current values that are untypically high for the application can be detected and the duration of their effect can then be limited. For this purpose, a fault can be triggered, for example, so that the drive controller switches off. To detect an unintentional blocking of the drive, the actual speed value can be monitored in addition to the signal for the torque in order to switch off the drive controller before the motor overheats.

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URL for linking this AKB article: https://www.lenze.com/en-de/go/akb/200401077/1/
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Taxonomy: 13.125 three-phase AC motors , 13.710 three-phase AC motors , 13.711 three-phase AC motors , Motors-AC , 9300 vector EVF93xx-EH hoist frequency inverters , 9300 vector EVF93xx-EV frequency inverters , 9300 vector frequency inverters , 9300 Cam Profiler (EK) servo inverters , 9300 Position Controller (EP) servo inverters , 9300 Register Controller (ER) servo inverters , 9300 Servo PLC Logic (EI) servo inverters , 9300 Servo PLC Technology (ET) servo inverters , 9300 Standard Servo (ES) servo inverters , 9300 servo inverters , 8400 BaseLine frequency inverters , 8400 HighLine frequency inverters , 8400 StateLine frequency inverters , 8400 TopLine frequency inverters , 8400 frequency inverters , 9400 HighLine servo inverters , i700 axis module E70ACM , i950 servo inverter i95A
AKB-ID: 200401077
Last update: 2024/01/11
Category: FAQ, Tipps und Tricks

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