General Motion Control

Choosing the best motion control solution – Fan application

Author: Bob Parente | January 25, 2011

To recap the last posting, we were discussing a fan application.

Our fan requirements are that it’s going to be used on a pedestal fan in a home or office environment:

  • The motor only needs to spin its shaft in the clockwise direction. It’s blowing air not exhausting it. Plus we could always turn the fan around and have it blow in the opposite direction.
  • We don’t need and infinite number of speeds, so we don’t need a speed control. Unless the speed control saves us money. Three speeds (low, medium, high) in this application will work well.
  • We don’t care if the motor accels or decels in a particular time frame or stops in an exact position.
  • It should hold its set speed reasonably well once it reaches it. (What is reasonably well, +/- 5%, 10 %?)
  • It’s going to operate indoors in a home or office type environment. Fans don’t work well in a vacuum, even though some people think that their office operates in one.
  • It or its control is going to operate from a 120 VAC, 60 Hz line.

If we have a good understanding of the advantages and disadvantages of each of the motor technologies and armed with the application’s constraints, then we can begin to zero in on the motor that best suits our needs.

Our motor technology list is:

  • AC Motors with no control. (Line driven.)
  • DC Motors and controls
  • AC Motors and controls
  • Brushless motors and controls
  • Stepper motors and controls
  • Servo motors and controls
  • Direct drives and controls

We’ll review the bold listing in this blog posting

For AC Motors with no control (line driven.) we have:

  • Shaded pole
    • Advantages
      • Simple design
      • Brushless, more reliable
      • No capacitors
      • No centrifugal switch
      • High volume-low cost motor
      • Single direction
      • Low starting current
      • Speed can be varied with simple triac speed controls
      • Can have multi-speed connections
    • Disadvantages
      • Low starting and running torque
      • Single direction
      • Low efficiency
      • Largest size is about ¼ HP.

Shaded pole motors are easily identified by the copper ring surrounding each pole.

My neighbor has a very nice three season room that was used more for storage than anything else for a long time. They decided a few years ago to clean it out and use it as a room. When they did, they found a fan installed in the floor area that was designed to blow air from the main house into the three season room. The problem was that it hadn’t been used in a very long time and it didn’t work. They invited me, Mr. Fix It, to take a look. The fact that it didn’t trip a breaker or smoke when they turned it on made me think it was a shaded pole motor (low starting current,) The best access to it was from the basement and I could see the copper rings on the motor body, so I was able to verify that it was a shaded pole motor and only a couple of things could go wrong with it. One was that it could have an open winding (not likely since it was hardly used.) or two, the shaft had seized (more likely since it was hardly used.)

I used a spray lubricant on the shaft-bronze-bushing area and, knowing that this motor technology has low starting torque it would have a difficult time trying to overcome the long-time-not-used bearing friction, I used a long shafted screw driver to spin the fan blade. That freed up the motor’s shaft.

A click of the switch and the fan was humming away and my neighbor and I were celebrating with a nice cold beer.

Permanent split capacitor

  • Advantages
    • Reversible
    • Capable of lots of starts and stops
    • Good starting and running torque
    • Brushless, more reliable
    • No centrifugal switch
    • Economical
    • Reasonable speed regulation
    • Multi-speed connections
    • Low starting current
    • Efficient
  • Disadvantages
    • Largest size is about 1 HP.
    • Not suitable for variable speed operation other than with the multi speed connections

Are there other advantages or disadvantages that you would like to add?

We’ll continue with more motor technologies next time.

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