Technical data 321
Cyclic loads
If the load cycle is shorter than 10 seconds, the thermal time constant of the heatsink
(approximately 80 seconds) can be ignored, and the following procedure can be
applied to find out whether the drive can handle the cycle.
1. Determine the rms value (I
2rms
) of the output current over the whole load cycle.
2. Determine the maximum instantaneous rms value (I
2peak
) of the output current
during the load cycle.
3. Determine the point (I
2rms
, I
2peak
) on the graph below.
If the point falls within the region bordered by a solid line, the load cycle is safe.
For I
2contxk
and I
2max
, use the ratings stated for the drive type and switching
frequency used.
If the point falls within the shaded area, a more detailed study is required.
The above procedure can also be applied to longer load cycles by dividing the cycle
into subcycles no longer than 10 seconds. If any of the subcycles fail the test, a more
detailed study is required.
I
2rms
I
2peak
I
2contxk
0.75×I
2contxk
I
2max
I
2contxk
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