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William K.
User Rank
Platinum
over riding the 3 speed switch.
William K.   3/28/2012 9:47:42 PM
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Almost all fans offered to both consumers and many commercial applications do indeed start with the highest speed setting because some of these fans are such junk that they wont start at the low speed setting, although they will run there once spinning. That is because the motor design is marginal and the phase shifting capacitor needed to make the motor run is a cheaply made device, with no capacitance to spare. So to avoid thye stall condition the fastest speed is what comes on first. That is useful because the inferior lubricant on many of these devices does tend to turn sticky and to restrict rotation. So the high speed start is definitely a "CYA" type of design choice.

OldGeek
User Rank
Iron
Re: 3-speed fan motor control
OldGeek   3/28/2012 5:32:20 PM
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Yes Rob, I think you'll agree that regardless of the sequence of operation, the string would have been sucked into the fan as soon as it was put on "High".

I think OLD_CRUMUDGEON has a good point too. If mounted in a narrow hallway I could see the air velocity being more likely to cause the string to be sucked into the inlet than if mounted in a large room.

Still, for it to happen the string must not have had much if any weight on it!

OldGeek
User Rank
Iron
Re: 3-speed fan motor control
OldGeek   3/28/2012 3:32:13 PM
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Kenish, you are right! I didn't think about blowers in automobiles!

Yes, DC motors have a steeper torque vs. speed curve than the multi-speed motors used in household fans and are much less prone to sticking. At reduced speed settings, the AC units are essentially constant torque devices where the torque is determined by the current in the windings. The current is controlled by tapped windings in the motor or by externally increasing the impedance. At lower speeds, these motors present an almost pure inductive load that shows little variation with speed. I once replaced the blower motor in a window a/c unit with a higher horsepower "universal" replacement. On turning it on, I found that I had no change between highest and lowest speed settings because the blower didn't load it enough to slow it down!

Rob Spiegel
User Rank
Blogger
Re: 3-speed fan motor control
Rob Spiegel   3/28/2012 3:27:01 PM
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OldGeek, your last comment may be the most germane when it comes to uncovering the monkey business in this story. The fact that the pull cord could get sucked up into the airstream may be the real design flaw.

kenish
User Rank
Platinum
Re: 3-speed fan motor control
kenish   3/28/2012 2:50:52 PM
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OldGeek- Same in my house, even an over-the-range microwave with a software-controlled vent hood starts at the highest setting.  OTOH, the blower motor in cars starts with the low setting.  Perhaps because they are DC motors?  I'm an EE but not a motor expert.

uniquity@uniquitypsych.com
User Rank
Gold
Re: Fan switches on the fritz
uniquity@uniquitypsych.com   3/28/2012 2:09:59 PM
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I have a number of fans in my home.  Four of them are ceiling fans with pull chains.  All of the came with the first pull starting at the highest speed.  One of them had to have the switch replaced, and it ended up with the slowest speed as the first speed.  I have never had a problem with that.  I also have a whole house fan with ceiling louvers.  It is controlled by a wall switch that is temperature sensitive.  You can adjust the switch so the fan will shut off when it cools down the room. 

I have four other ceiling fans that are controlled by rotating wall switches.  They are variable speed, and they all start at the high speed settings.

If the original story had a fan strong enough to pull the chain up, and into the fan, it seems to me the easiest way to solve that problem whould had been to add a heavier weight to the bottom of the pull cord.

williamlweaver
User Rank
Platinum
Re: Fan switches on the fritz
williamlweaver   3/28/2012 1:35:24 PM
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Yep. The first official Aircraft Carrier. http://en.wikipedia.org/wiki/File:Samuel_Pierpont_Langley_-_Potomac_experiment_1903.jpeg

The Navy's 1st "operational" Aircraft carrier was named in his honor. http://en.wikipedia.org/wiki/USS_Langley_(CV-1)

thrashercharged
User Rank
Iron
Re: Fan switches on the fritz
thrashercharged   3/28/2012 12:57:04 PM
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Langley launched his machines from an aircraft carrier in the Potomac River... and continuously fished them out from the bottom.

I seriously doubt Langley used an aircraft carrier!  A large boat perhaps!

OldGeek
User Rank
Iron
Re: 3-speed fan motor control
OldGeek   3/28/2012 12:25:22 PM
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Interesting since EVERY fan in my home (ceiling fans, stand mounted and range exhaust) uses the "Off/High/Medium/Low" sequence.  The range hood is a variable speed unit that the pot is wired to go slower the farther you turn it. All three window air conditioners in my home and garage use that paradigm as well.

In fact, I can't think of a single instance where a fan I've delt with starts out on the lowest setting.

Tecsonics got it right, small motors are notorious for being difficult to start. Starting out at high speed insures that there is enough initial "kick" to get it moving instead of sitting "locked rotor" and burning up. It also gives immediate feedback to the user that the fan is actually "on".

To the original article, that does sound like a "never tested" assembly. The lack of a factory attached weight on the pull cord is a SERIOUS error!

OLD_CURMUDGEON
User Rank
Platinum
"WHOLE HOUSE" fans, etc.
OLD_CURMUDGEON   3/28/2012 12:24:00 PM
I agree with one of the other bloggers.  It seems that some of these comments are confusing ceiling fan operation with whole house central fan operation.  One comment was that the ONLY thing the two devices have in common is the word "fan".

I have installed whole house fans, gable-end attic space fans AND consumer ceiling fans for my own homes, for nontwchnical relatives, and for friends over the years.

The whole house fans that I've installed are designed w/ a dedicated fan-duty squirrel cage motor, in the 1/2 to 1 hp power range.  All of these fans were powered using a separate wall-mounted speed control switch w/ a horsepower rating.

Regarding "systems engineering"...... Having spent several decades doing systems engineering, both from a hardware point of view and a software point of view, I would offer that the manufacturers of these whole house fans really can't do a satisfactory, all-inclusive systems engineering test, since each attic space will have its own unique airflow equation.  Some houses may have a peak vent, in which case the attic air will flow more easily; some houses will be designed w/ a truss system for the roof members, providing considerable flow resistande, others may have corner breaks, etc., while others may have soffet vents.  ALL of these conditions will alter the flow rate of the air through an additionally provided (self-closing) louver device.

In fact, in our current house in FLA, we have a gable end attic fan w/ a gravity/flow operated louver.  It has been in place for close to 20 years, and now it is evident that the rivets holding the louver blades to the frame are wearing, resulting in erratic operation of the louver when the fan is energized.  However, this fan was installed to assist in "cooling" the attic space to aid the living space HVAC unit.  Since the house has recently been re-roofed, and a full-length high-flow (passive) ridge vent installed, we hardly use the fan any more.

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