My old fuji have only 5 blades on exhaust/cooling fan which means that the wind could be more or less 10% weaker than now standard 6 blade and 25-30% less than the 8 bladed special model. To compensate this I set the inverter frequency at 80hz or more. Although, it seems like more straight wind is going through the drum so less covected heat touch the beans. I realize now it is true that many masters of fuji roaster insist that the fuji roaster needs to be equipped with the eight bladed fan. Yes, It has to be and it is best to lower the rotation somehow. This means that the inverter driven high speed fan cannot be an alternative to the eight bladed fan, for its wind straghtness has to be soft enough to reach each beans to bring heat. I found that an external fan at the end of cyclon was better solution than inner inverter driven moter at increased frequency . In other words, less static pressure with just a bit more of air volume is essentical. This accelerated interior moter is only useful to speed up the cooling down process after roasting.
To avoid this shortcoming, it is best to separate cooling and exhaust functions by adopting the independent exhaust fan.
In my case, I desided to go on with double serial fans, at least for a while. See, wether I could find better external fan than the current 140W DC fan. Perhaps, it would be easier to fit in the exclusive fan when it is set separately then I go independent but I hate the costly choice for now.