Fans operating backwards drawing higher amps than the correct rotation 1

[Khacker]

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?

 

[Irondoor68]

If the electric power that goes into a set of windings is not used to produce mechanical force then it will convert to heat energy. This will eventualy destroy the windings.
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.

 

[Compositepro]

Forward curved blades will move more air and draw more power than backward inclined. For the same rpm and diameter they give air more velocity. Simply geometry. I've seen squirrel cage fans run backward and the difference is dramatic.

 

[IRstuff]

Moving less air does not equate to less work. Run the fan forward, cover the outlet. Air moved is zero, but the fan is still drawing power.

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

FAQ731-376

 

[Compositepro]

IRstuff, reducing flow through a fan, blower, or centrifugal pump by restricting the inlet or outlet, does reduce the power it draws. It takes less energy to spin air around than to suck it in, accelerate it up to speed, and spit it out.

 

[tys90]

Take a look at a fan curve with the power curves on it for a backwards inclined fan. If the system is far enough down on the curve and you reduce the flow, you can slightly increase the power of the fan. The important part to remember is that peak horsepower happens at peak efficiency for these types of fans. If you are to the downside of the efficiency curve and reduce flow while keeping speed constant you will increase the power of the fan.

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.

 

[DRWeig]

My view is that you've basically turned your backward-inclined fan into a not-designed-for-it forward cureved fan. Your differential pressure at the same RPM will be much higher at all flows, thus drawing more power.

Good on ya,

Goober Dave