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# Questions about BHP on Fan Performance Curve and also How to Select a Motor for a Fan

## Questions about BHP on Fan Performance Curve and also How to Select a Motor for a Fan

(OP)
So, I recently found out that the power that a motor has and uses are two different things (from what I understand). So the input power of a motor is electrical power, using the equation P = I*V, and the output power is mechanical power, which can be calculated using the equation P = Torque*Angular Speed. Okay, so, from my current understanding, we select a motor for a fan by looking at the operating point on a fan performance curve given our system's CFM and resistance to airflow, and then use the value on the BHP Curve corresponding to our operating point to select our fan's motor size (HP Value). Okay, so if what I've said so far is correct, here are my first few questions. How exactly is the bhp curve for a fan performance curve calculated? Is it calculated or is it tested? If it's tested, what is the testing procedure? I looked at the Chapter on Fans in the 2016 ASHRAE Handbook and could not find anything regarding the calculation or testing for BHP on a Fan Performance Curve. How is it done?

After figuring out what the bhp value on a fan performance curve represents, my next question would be what is the proper process for selecting a fan's motor size? As I said before, we would use the bhp value but it's hard for me to understand how we would select a fan's motor size based on the bhp value because I'm not sure how that value comes about. So, once that is established, if anyone knows or understands, how do you use this value to properly select the fan motor size? Do you take into account any motor efficiencies (or belt efficiencies)? One thing that popped into my mind while I was writing this is that they must be using a motor when they are testing the fan, and they would perhaps, I presume, test and record the brake horsepower using that fan motor. If that's the case, would that brake horsepower value change for different motor sizes? I'm sorry if this is all too much or if it doesn't make any sense but hopefully someone here can answer any of my questions or lead me in the right direction. Thank you.
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