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AFCA Forums > Antique Fan Collectors Association > Pre-1950 (Antique) > It baffles me how a speed coil can go bad and stop working.

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It baffles me how a speed coil can go bad and stop working.  Rate Topic 
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 Posted: Mon Oct 7th, 2019 01:59 am
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Mike Crenshaw
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It is very baffling how a speed coil can actually go bad and stop working aka all speeds are the same.  And usually a symptom is when the coil's outer covering is all charred.

I would think that the speed coil would last indefinitely as it is completely enclosed in the fan housing and protected from the elements and the outside environment.  Apparently, the speed coil in the small Westy tank I bought at Fan Fair is toasted and needs a rewind, and this is the second toasted speed coil I have seen in my many years of fan collecting.

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 Posted: Mon Oct 7th, 2019 03:03 am
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Steven P Dempsey
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I there a chance the fans are running on too high a voltage (120 vs. 110, etc) Those wires are so fragile.I use a VARIAC, etc to drop down too recommended voltage on motor tag. I just opened up an R&M 3100 & the wires were not burned, just broken.

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 Posted: Mon Oct 7th, 2019 03:05 am
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Steven P Dempsey
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sorry- R&M 3500

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 Posted: Mon Oct 7th, 2019 05:51 pm
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David Allen
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As for why the speed coil fails, it's due to failure of the enamel insulation of the magnet wire. This deteriorates from heat, moisture, and vibration.

The normal operation of the coil is by reactive impedance. To simplify this effect, the coil slows down the current to the fan motor by storing some of the energy as magnetism, which then causes a voltage opposing the voltage of the power line. This happens over and over with each AC cycle of the sine wave. The important point to remember is that some of the energy is captured, and returned to the power grid instead of going to the fan motor. This energy is not lost, it's stored temporarily and released 60 times per second. The more turns of coil wire are in the circuit, the more the coil reduces the power to the motor.

When the insulation fails, there are shorts between all the windings of the coil. The coil can't build a magnetic field because the current takes a path sideways across the shorted turns of the coil, bypassing some or all of the coil. The full (or almost full) line voltage goes to the motor.

When the speed coil and the motor are working together correctly, the motor and the speed coil share the voltage drop between the two. If the motor is in a bind, or the headwire shorts out, there will be much more voltage across the speed coil than it's designed for. This will burn the winding of the coil. That will result in the problem you're seeing.

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 Posted: Tue Oct 8th, 2019 06:19 am
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Cory Baughn
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David, thank you for that response. That last paragraph made me think of speed coils in a new light.

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 Posted: Tue Oct 8th, 2019 11:28 am
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David Allen
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Cory Baughn wrote: David, thank you for that response. That last paragraph made me think of speed coils in a new light.
Cool, glad you found it useful!

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 Posted: Tue Oct 8th, 2019 07:34 pm
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Andrew Block
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The one in my Vader went bad because some idiot tried to replace the cord and rewired it completely wrong. So there's that too.

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 Posted: Tue Oct 8th, 2019 07:41 pm
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David Allen
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Andrew Block wrote: The one in my Vader went bad because some idiot tried to replace the cord and rewired it completely wrong. So there's that too.
The bastard!

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