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Motor Amps

I read the article on the use of variable-frequency drives on forced-draft fans at the University of Rochester ("Variable-Frequency Drive Improves Boiler Operation," January 2011,

Why do the amps on the motor not change significantly when the demand on the boiler changes with the damper-controlled airflow?

I put a monitor on a motor, and the amps did not change significantly.
James B. Jones
Augusta, Ga.

Co-author's response:
I do not know the type of air handler you have or where the dampers are positioned. Therefore, a rough explanation of why your amps do not drop significantly when you throttle the flow via a damper on a centrifugal air handler basically is based off of this equation:

Fan hp = (cfm × psi) ÷ (229 × fan efficiency)

When you drop your cfm with an outlet damper, for example, your pressure (inside your fan) goes up, and your fan’s efficiency drops severely. Amp reduction is minimal.

In the case of inlet dampers, flow, pressure, and fan efficiency drop. Therefore, inlet dampers are more electrically efficient than outlet dampers. However, they don’t compare to reducing the speed of the blower and taking advantage of the affinity laws:

hp = speed3

Following are links to documents providing a much more thorough explanation:

Dean Williams
EMA of NY Inc.
Cortland, N.Y

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