Furnaces Start Up - FD Fan Upstream Valve & Torque Explained

[arouna_roro]

when starting up a furnace , the instructions of vendor say that : FD fan upstream valve should be close in the beginning to increase the fan TORQUE

what does this mean , and how does this fan work while closing the upstream valve in the beginning then opening it slowely !

THx

 

[Jeff Rosenbury]

Electric motors often have problems with starting torque and inrush current. There are various methods to handle this problem. Apparently the vendor chose to decrease fan speed, thus increase motor slip. The air pressure imbalance would increase the load on the motor. (Slip is the angle between the current and the shaft in an induction motor.)

This would normally cause some heating of the motor, so perhaps they chose this method to limit thermal shock?