Operating Instructions ACU
In the calculation of the triggering time, the output current measured in working points below the
frequency limit will be valued at a factor between 1 and 2.
This factor is determined depending on the stator frequency. In this way, the higher thermal load of
self-ventilated motors in the lower speed range is considered.
The table shows an extract of the factors for a 50 Hz motor.
Frequency Limit 572
19.5.2 Motor protection by I
2
t monitoring
I2t monitoring is another way to protect the motor against thermal overload. This motor protection
method is mainly used in servo systems.
Wherever synchronous servomotors are used, I2t monitoring is a proven alternative to motor circuit
breakers. By integrating measurable or known motor parameters, heat-up is modeled based on a
mathematical model.
The I2t monitoring feature can be selected via Operation Mode 571.
The settings are data set change-over settings.
I²t monitoring is done, as shown in the illustration, via (Iact/Irated)².
The monitored variable is valued via a PT1 element with the thermal time constant of the stator. If
the PT1 element output exceeds 120%, an error message will be reported and the inverter will shut
down. The 120% threshold ensures the overshooting will not result in immediate shutdown.
Permanent exceeding of the 100% load of the stator winding should be avoided in the application.
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