Question about current draw

D

dlowry

New member
My CV1800 is installed and running. As I'm running a 3-phase motor via an inverter, I can easily monitor the current. Can someone explain why the motor draws less current when the inlet is closed off? I've never understood this. Intuitively, I'd expect the opposite. With a closed system the impeller is fighting an unmoving air-mass (on the average - there is presumably huge turbulent flow in the blower housing).

So why is more power consumed when the air-mass is free to flow through the system?
 
dlowry,
A centrifugal fan imparts momentum to the air and the reaction of the air on the blades of the fan causes the motor to develop torque. The product of torque and speed is power and the motor must draw current to develop the power. Each blade literally throws air out of the outlet causing a rapid air speed increase at the output and a vacuum on the input side which causes more air to flow in. But when the input (or output) is blocked the fan can no longer throw the air. So it cannot develop much power. The air stays in the fan and the fan and air simply rotate together inside the housing. Under this condition the only power developed in the fan is that necessary to overcome the turbulence. Of course the motor itself draws a little no load current to overcome air and bearing losses within the motor.
bababrown
 
Bababrown,
Thanks for the clear explanation. I had things exactly the wrong way round in my head (not an uncommon occurence, unfortunately...)!

Drew
 
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