Fans operating backwards drawing higher amps than the correct rotation
Fans operating backwards drawing higher amps than the correct rotation
(OP)
I got into a debate recently with a manufacturers rep who air balances his own kitchen equipment regarding the operation of a backward inclined centrifugal fan serving a kitchen hood. He told me that the motor was drawing higher amps when operating backwards than when it was operating in the correct rotation. The basic fan laws and laws of physics predicate this to be false. A fan operating backwards moves less air (approx. 30% to 45% of design) and thus does less work, which draws less amps. I began checking amperage on fans that I have found operating backwards and found that indeed it is some times true that the motor will draw more amps running backwards.
Why is this? Does anybody have any insight into this or have a logical reason for this?
Why is this? Does anybody have any insight into this or have a logical reason for this?





RE: Fans operating backwards drawing higher amps than the correct rotation
An easy and quick demonstration of this is to remove a coil from a solonoid valve. No more mechanical work and the coil turns into a heater.
RE: Fans operating backwards drawing higher amps than the correct rotation
RE: Fans operating backwards drawing higher amps than the correct rotation
Running the blades backwards means the blades are running substantially less efficiently, so even with less moving air, the efficiency factor can drive the consumption above the draw when running correctly.
TTFN
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RE: Fans operating backwards drawing higher amps than the correct rotation
RE: Fans operating backwards drawing higher amps than the correct rotation
Forward and radial tip fans would not increase power in this situation as their power is more or less proportional to volume (at constant speeds). Vaneaxial fans have have a funky power curve.
RE: Fans operating backwards drawing higher amps than the correct rotation
Good on ya,
Goober Dave