Fan startup current is too large

Centrifugal fan shell duct air flow is actually also fluid mechanics (resistance) research category, close the door to quickly form and achieve the required pressure difference between the inside and outside conditions (easy to suck the wind), motor impeller in the closed channel no-load, the resistance of the air did not increase. If the damper is opened, with the increase in the amount of suction air (motor startup speed gradually accelerated) so that the impeller in the fixed flow channel resistance also increased, which is usually said to start with load, which will make the motor startup current is too large, damage to the equipment.