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Ceiling fans starting full speed - Why?

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atferrari

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Please let me tell in advance that electric motors are my stumbling block.

There is lot of info about ceiling fans out there but I could not find an answer to this: why all (that I have run across) do start at full speed? Because the motor cannot do otherwise?
 

ronsimpson

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The most hard work a fan does is starting. AC or DC fans
When a fan gets old the bearings are not good and the fan turns hard. (dirt in the bearings)
While a old fan will run at 1/4 power it is not enough power to start the fan. If you started the fan at 1/4 power it would likely not move. Just sit and hum. If you pushed the fan with your hand the fan would then run just fine.
All, well designed, fans start at full power to break loose the bearings and get it moving. After the fan is turning you can back down the power.
 

jpanhalt

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All of my ceiling fans are operated by wall switches, rather than the integral switches in the units. They start just fine are the same speed at which they are turned off -- which is usually the slowest speed. They are 25 years old.

I suspect the reason they start full speed is because that is the next position after "off." That is not to disagree with Ron's theories.
 

atferrari

Well-Known Member
The most hard work a fan does is starting. AC or DC fans
When a fan gets old the bearings are not good and the fan turns hard. (dirt in the bearings)
While a old fan will run at 1/4 power it is not enough power to start the fan. If you started the fan at 1/4 power it would likely not move. Just sit and hum. If you pushed the fan with your hand the fan would then run just fine.
All, well designed, fans start at full power to break loose the bearings and get it moving. After the fan is turning you can back down the power.
I see. It makes sense. But then, another question: motors for big cranes, they also start like that? They do not sound like that.

BTW, risking to delay the thread I started: I recall using PWM for small DC motors. My code created very low duty cycle and they started flawlessly. Sheer luck? Very small load, maybe?

A real stumbling block.
 

dknguyen

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I see. It makes sense. But then, another question: motors for big cranes, they also start like that? They do not sound like that.

BTW, risking to delay the thread I started: I recall using PWM for small DC motors. My code created very low duty cycle and they started flawlessly. Sheer luck? Very small load, maybe?

A real stumbling block.
First, I have no idea what you mean about ceiling fans needing to start at full speed. I've never seen one that does.

Second, it all comes down to money. PWM (and electronic speed controls in general) are almost always a luxury. They lets you gradually turn up the motor speed during startup (so-called soft-start) but the vast majority of motors aren't equipped with these electronics because they aren't needed and increase cost. The performance might not be as elegant or versatile, but the people paying (factor owners) don't care because it costs more money, or the users (aka homeowners) known so little about things that they wouldn't know the difference even if it was present. For these things, manufacturers and installers will try and get away with the BARE minimum.

Even applications like ceiling fans that demand variable speed don't necessarily demand electronic speed controls and almost always try to get away without it because it's cheaper. Make no mistake: they are a high-end feature.

This also holds true for applications that cannot or should not start at full speed. There are many schemes for providing variable speed or starting a motor (two different things) and they are almost always used before PWM or electronic speed controls.
 
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MaxHeadRoom78

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Ceiling fans are typically shaded pole motors and they have very poor starting torque, so it does not take alot to stall them, having said that the simple Triac phase controllers are usually capable of starting a shaded pole motor at low speed if the motor is in fairly good condition, bearings etc.
Large crane motor are a different animal completely, 3ph for one, and also older versions are wound rotor motors for restive control. Now many are VFD control.
Max.
 

dr pepper

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I maintain industrial machinery, pretty much all have motors.
Synchronous, or rather semi synchronous motors start & run at the same setting unless they have a star/delta starter, this is so they dont pull so much current at start up, this is the reverse of starting at full speed, technically a motor that is being started in star mode is undervoltaged.
Machines with a speed controlled motor usually have a Pid or other form of control loop, depending on the design the motor can be given full power untill the target speed is reached.
For small ceiling fans the designers probably want to save copper and make the low speed windings as small as possible meaning the fan needs a kick to get it going from the high speed winding.
 

MaxHeadRoom78

Well-Known Member
All induction motors are Asynchronous.
There is no low speed winding on a shaded pole motor, the phase shift occurs due to a change in flux in that part of the iron where the shaded pole (shorting ring) is located.
It doesn't take much to stall them for this reason they are used mainly in (low load) fan applications.
Max.
 

ronsimpson

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Where I live it is common to vent the entire house with a big fin. The switch for fan speed is "off, fast, medium, slow". It does not circle. You can not get from off to slow or from slow to off.

If you are watching the motor, with your hand on the switch or PWM control, and the motor does not start, you just increase the power. With 3 volts on a 12V motor and some start, 4volts and most start, 5V and all start. It does not matter much because you have you hand on the control.

Most people do not like fan noise. In computers and test equipment it is common for the fan to start very slow and increase with temperature of the components inside. (people do not complain about fan noise if it increases slowly)
One company I do designs for said their highest failure rate part is the fan. It just makes it through the warranty time then stops. I noticed the fan trouble got much worse when they changed the fan speed circuit. (years ago) Their circuit starts the fan voltage at 0 and increases it to 12V with temperature. A two year old fan often will not start with a slowly increasing voltage.

My "machine" starts the fan (at power-up) with 12V for 0.5 seconds. Then the voltage drops back to 6 volts. (5 volts when new, 6V at 1 year and 7 volts at 2 years) I got a large box of old "will not start" fans. I used these fans for the prototypes. They all started of you hit them hard with 12V. (no old fan started with 6V) All old fans, after being started, will run down to a very low voltage.
 

atferrari

Well-Known Member
First, I have no idea what you mean about ceiling fans needing to start at full speed. I've never seen one that does.s)
I am not saying they need to. Those (all) I can recall, do start at full speed. In other words: as soon as the switch is ON, the fan is spinning at maximum speed.

By now, it seems that Ron hit the nail.
 

jpanhalt

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Ceiling fans are typically shaded pole motors and they have very poor starting torque, so it does not take alot to stall them, having said that the simple Triac phase controllers are usually capable of starting a shaded pole motor at low speed if the motor is in fairly good condition, bearings etc.
Large crane motor are a different animal completely, 3ph for one, and also older versions are wound rotor motors for restive control. Now many are VFD control.
Max.
Not according to this history: http://www.dtvintagefans.org/forum/archive/index.php/t-15378.html

Also, every ceiling fan in my house is reversible with a switch.
 

dknguyen

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I am not saying they need to. Those (all) I can recall, do start at full speed. In other words: as soon as the switch is ON, the fan is spinning at maximum speed.

By now, it seems that Ron hit the nail.
I know what you meant. None of my ceiling fans start this way and I've never seen one that does.
 

ronsimpson

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I suspect the reason they start full speed is because that is the next position after "off."
I think it's as simple as that. Every other answer on here is probably just either confusing the issue or completely irrelevant!
I like the reasoning. The switch is labeled "fast" so it must be fast. So I am going to relabel my switch. :)
OFF comes next to FAST for a reason. Switches are labeled that way for a reason. Engineer(s), that understands motors, chose to make it that way from the beginning.
If people that complain, had much control over switches, then OFF would be next to SLOW.
I have a wife that thinks that with enough complaining the FAST/SLOW will work differently. Forty years of complaining has made no difference. Maybe 41 will work.
 

jpanhalt

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I like the reasoning. The switch is labeled "fast" so it must be fast. So I am going to relabel my switch. :)
OFF comes next to FAST for a reason. Switches are labeled that way for a reason. Engineer(s), that understands motors, chose to make it that way from the beginning.
If people that complain, had much control over switches, then OFF would be next to SLOW.
I have a wife that thinks that with enough complaining the FAST/SLOW will work differently. Forty years of complaining has made no difference. Maybe 41 will work.
Apparently you need a real brain teaser: Why are toggle switches in aircraft and in the US almost always installed with the toggle-up position being on? Now, what about in the horizonal position?;)

John

Edit (underlined): My toaster and many rotary switches are labeled like that.


 
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hyedenny

Active Member
While a old fan will run at 1/4 power it is not enough power to start the fan. ... After the fan is turning you can back down the power.
Every ceiling fan I've ever seen has a wall switch to turn on the fan (regardless of what position the speed or direction switches are in), so this makes absolutely no sense.
Additionally, if the motor is in such poor shape that it won't start at low speed, it shouldn't be run at any speed!

The switch for fan speed is "off, fast, medium, slow". It does not circle. You can not get from off to slow or from slow to off.
Not true. Many fans have switches that cycle through speeds in a loop with repeated pulls of a chain.
 

dknguyen

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Wait, is the OP asking about the wall on/off switch causing the fan to start at full speed. Or is he asking about the order of the chain you pull to control the speed and why it always seems to go from OFF->Fast->Medium->slow->Repeat and never Off->Slow-Medium->Fast->Repeat?
 

atferrari

Well-Known Member
Wait, is the OP asking about the wall on/off switch causing the fan to start at full speed.
Yes, switch combined with the pot controlling the speed. No chain-controlled here. They are more and more hard to find locally.

In other words: as soon as the switch is ON, the fan is spinning at maximum speed.By now, it seems that Ron hit the nail.
This is the one:

fan control.jpg
 
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