Why the ventilation of the ATV61D15N4 is greater than the the ventilation of the ATV61D37N4 although it is smaller rating?
Both drives have different frame size. The air flow requirement is calculated based on the maximum power dissipation in the same frame size. In this case the design of airflow is by considering 18KW drive. For the heat sink size(frame size), it is calculated as 252 m3/hour air flow is required to dissipate the heat loss.
The same difference can be observed in other ratings also, for example 75KW drive require 406 m3/hour, where as the 110KW drive require 402 m3/hour only.
It is the calculated value as per the design of VFD power components and heat dissipation capacity of heat sink size.