Centrifugal Fan

November 2, 2011

Centrifugal Fan

The ninja almost, but not completely, understood what he was trying to do here.

The exhaust fan has a three phase motor. Three phase motors run in one direction or another depending upon the phase rotation of the power. The phase rotation is easily changed by reversing two of the three wires connected to it, which the ninja did.

Unfortunately, he assumed that reversing the direction of the motor would reverse the direction of airflow when the fan was on. For propeller or turbine fans, this assumption is true. But centrifugal fans blow air in the same direction no matter which direction they turn.

Instead of the gas being sucked into the building and killing the occupants, it was blown back at the ninja and he dies instead. Victory to our heroes at the Green Building Council!

Side note: what if the fan had a single phase motor? In that case, reversing the wires would have made no difference. The motor always turns in the same directions. But for three phase motors, the connection sequence of wires is critical to the performance of the fan. I can hardly count the number of times I have “fixed” a design problem by reversing the direction of the fan. It is one of my favorite fixes.

Mark Robison, PE

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