If the given acceleration time is too short, the change of the output frequency of the inverter far exceeds the change of the rotation speed (electric angle frequency), the inverter will trip due to the overcurrent flowing, and the operation will stop, which is called stall. In order to prevent the motor from stalling and continue to run, it is necessary to detect the magnitude of the current for frequency control. When the acceleration current is too large, the acceleration rate is appropriately slowed down. The same is true when slowing down. The combination of the two is the stall function.