General Motion Control

Affinity laws and other considerations – fan application

Author: Bob Parente | May 4, 2011

So, are we ready to make a motor technology choice for our fan application? I’m going to say yes, but…did you think we had defined the application well?  In every engineering “problem” there are a lot of issues that need to be understood and taken into consideration.

As “simple” as trying to find a motor to spin a fan sounds, the engineer that has to do this needs to know and use the “affinity laws” for air (and hydraulic) flow.

Simply stated they are:

  • (Q1/Q2) = (N1/N2) where Q is the volumetric air flow and N is the shaft speed. And the flow is proportional to speed
  • (H1/H2) = (N1/N2)2 where H is the head pressure and is proportional to the square of the shaft speed.
  • (P1/P2) = (N1/N2)3 where P is the power required to spin the shaft and is proportional to the cube of the shaft speed
  • (Q1/Q2) = (D1/D2) with the shaft speed held constant, flow is proportional to the diameter D of the impeller.
  • (Q1/Q2) = (D1/D2)2 with the shaft speed held constant, pressure is proportional to the square of the diameter of the impeller.
  • (Q1/Q2) = (D1/D2)3 with the shaft speed held constant, power is proportional to the cube of the diameter of the impeller.

The engineer also needs to take into consideration the shape of the fan blade and the fact that it is very similar to an airplane wing and is subject to stall conditions where increases in speed and or attack angle become ineffective. The shape of the blade (paddle, parabolic, etc,) the attack angle, the diameter, the number of blades all have an impact on the efficiency and the noise that’s produced as well as the horse power rating of the motor.

Having said all this, we’re not going to go into all that detail, but obviously somebody has to. We’re just going to select a motor technology only.
And my answer is:

A multi-tap shaded pole motor as my fist choice and, if a higher horse power is needed, then a multi-tap permanent split capacitor.

Do you agree, disagree?

Another “fan” story that I’ll share happened a number of years ago when water got into the basement of my home and I borrowed three five-foot-tall pedestal fans from the office to take home to circulate the air and dry out the basement.

The day I did this happen to be the night that I played softball. Our group arrived at the ball field on a warm summer evening and after the typical warm-up throws and some ball shagging the game was about to start. At that point I pulled the fans out of my car and set them up behind our bench. The rest of my team mates looked on a bit confused since it wasn’t that hot a night plus there was no power outlets to plug them into. They asked “what are you doing?” I replied “I brought some ‘fans’ along to watch the game.” (Drum roll, cymbal crash please.) It turned out to be our highest attendance for that season.

Coming up:

The next several blog postings will cover a “series of strikingly simple small-stepper seminars, showcasing step sequences & stall situations.” So-long

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