Power issues with fixed-frequency fans

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This post was last edited by zzyhncs on 2017-1-19 at 10:30. We have encountered a very strange problem: We would like experts to explain and discuss it. We consulted with and used a highly renowned blower company in Germany; they claim, and the technical parameters they provided also indicate, that when the inlet valve of the blower is closed tightly, the air flow rate is very low and the pressure rise is also low. At this point, the motor still operates at its rated speed, but its power decreases significantly as the air flow rate and pressure rise drop, meaning the current becomes very low. May I ask why? How can such a low power level persist over an extended period of time when the frequency of the induction motor remains constant? Can this be explained from the working principle of motors and other aspects related to how motors function?