In calculation the tripping time the measured output current in operating points below
the frequency limit is evaluated by a factor between 1 and 2. The determination of this
factor is a function of the stator frequency. The increased thermal load of self-
ventilated motors in the lower speed range is therefore considered.
The table shows in extracts factors for motor rated frequency 50Hz.
Frequency limit 572
18.5.2 Motor Protection by I2t- Monitoring
To protect the motor against overload the I2t monitoring provides a further possibility
for the user.
This kind of motor protection is mainly used in servo technology. When using servo
motors the I2t- monitoring is a proven alternative to motor protection switch.
By integrating temperature-dependent parameters, measurable or known, the heat-
ing of a mathematical model is simulated.
The kind of the I
2
t monitoring mode can be selected by Operation Mode 571.
This parameter is switchable via data set.
The I²t monitoring works by function (I
act
n
)² as shown in the figure.
The monitored value is evaluated via a PT1 element with the thermal time constant of
the stator.
If the output of PT1 element is bigger than 120%, then an error message is generat-
ed and the drive switches off. The threshold of 120% prevents, that an overshoot
leads to an immediate shutdown.
In the application should be avoided exceeding 100% capacity of the stator winding
permanently.
248 Operating Instructions ACU 06/13
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