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44.2.3. MAXIMUM LOCKED ROTOR TIME – ENHANCED
If a more precise calculation is required, when the ratio between LRC and FLC is different from 7.2, you can refer to the
graph below, where the x axis shows the LRC/FLC ratio, and the y axis shows the multiplicative constant to be applied
to the LRT to calculate the value of parameter C267:
Figure 67: Set up of parameter C267 depending on the LRC/FLC ratio
Example 1b: When using a 7.5kW motor, the multiplicative constant corresponding to an LRC/FLC=8.2 is approx. 46 if
referring to the graph above.
As a result, the motor thermal time constant that you would select is 27.3 x 46, C267 = 1257s, which is a more accurate
value than 1080s computed in Example 1a.
Example 2: The 250kW motor in Table 119 can be approximated to have a trip class of:
690 x 79
IEC Class = ------------- = 90.85
100 x 6
Because this value is not given in Table 118, the motor thermal time constant that you would select is directly C267 =
90.85 x 36 = 3260s, or 90.85 x 33 = 2998s if the value “33” is considered, resulting from Table 119 with a ratio between
LRC/FLC=6.9.
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