The I²t operation is different on the MM420 to the MM440.
The purpose of I²t is to calculate or measure the motor temperature and disable the inverter if the motor is in danger of overheating.
The protection system has been tested extensively on both the MM420 and the MM440, and is approved as an "Internal Overload Motor Protection" (UL 508C Section 42 - There are no conditions of acceptance).
The motor temperature will be dependent on many factors, including the size of the motor, the ambient temperature, the previous history of the motor's loading, and of course, the load current. (The square of the current actually determines the heating of the motor and the temperature rises with time - hence I²t).
Because most motors are cooled by built in fans running at motor speed, the speed of the motor is also important. Clearly a motor running at high current (maybe due to boost) and a low speed, will overheat more quickly than one running at 50 or 60 Hz, full load. The MM420 and MM440 take account of these factors.
The drives also include inverter I²t protection (i.e. overheating protection) in order to protect the units themselves. This operates independently of the Motor I²t, and is not described here.

MM420 I²t Operation

The I²t system used in MM420 is relatively simple, because the motor model is simple, fewer parameters are involved, and there is no motor temperature feedback.
The measured motor current (r0027) is compared with the rated motor current (P0305), and other motor parameters (P0304, P0307, etc.), and the temperature of the motor calculated, a calculation which also includes the output frequency (motor speed) to account for fan cooling. If parameter P0335 is changed to indicate a forced cooled motor, the calculation is modified accordingly.
Where parameters are not entered by the user, such as P0344 (motor weight), a calculated value will be used based on a Siemens motor.
If required, the motor time constant can be adjusted using P0611, in effect overwriting the calculated value.
The resulting temperature is displayed  in % of maximum temperature in r0034.
When this value reaches the value set in P0614 (default 100%), a warning A0511 occurs. If no action is taken and the temperature reaches 110%, then the inverter trips, showing F0011.
The reaction to the warning can be changed from this default using P0610; for example, the drive can react as though a current limit has occurred, or a fault forced immediately.
The warning level of P0614 can also be adjusted to raise and lower the warning or trip level as required.
Parameter r0034 is particularly useful to monitor if the calculated motor temperature is rising excessively.

MM440 I²t Operation

Because MM440 uses different monitoring and calculation methods, the parameters relating to motor I²t are different.
The measured motor current is still displayed in r0027. The motor temperature    in degrees centigrade is now displayed in r0035. This temperature is derived either from a KTY84 temperature sensor mounted in the motor, or from a calculated value. The value from the KTY84 is used only when P0601 = 2; in all other cases (including loss of signal from the KTY84) the calculated figure is displayed.
The MM440 uses a much more sophisticated model to calculate motor temperature than the MM420. Therefore many other parameters are involved, including, for example, P0625, the ambient temperature.
Parameter P0604 can now be adjusted to set the threshold temperature in comparison with r0035 (compares with P0614 in % and r0034 in MM420). P0610 will change the reaction as before.
P0611 has no equivalent in MM440.

Practical Operation and Testing

Both I²t systems have been extensively tested and have been shown to protect many different sizes of motors under different operating conditions. This leads to a high degree of confidence in the I²t protection system, partly as a result of improved current measurement methods compared with MM3.
In order to demonstrate the system performance, the warning thresholds can be reduced (MM420 = P0614, MM440 = P0610) and the progress of the motor temperature monitored (MM420 = r0034, MM440 = r0035).

 Hi,

       I2t Means Maximum Current duration capacity of the electrical components 

       Like Fuse,SCR etc. 

      The Overloaded capacity of the components calculated as Square of the Peak current.

      And peak current times in Seconds.

 For Example :

         Rated Current = 5A

         Peak Current   = 10A

  Peak current duration = 5 Sec.

  So, I2t of the components is = (10)2-(5)2x 5 Sec

                    I2t of the component = 375A

         It means component max current withstanding capability is 375 Amps in 5 Sec...

          If load goes more than this value the component ll be fail.