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Choker coils, resistors, etc.  Rate Topic 
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 Posted: Thu Sep 21st, 2017 07:03 pm
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Richard Daugird
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I am very limited in my knowledge about electrics, but want to learn. From what I have read, multiple speed motors use either a choker coil or a resistor to slow them down(maybe other meathods as well?). Could something like this be added to a single speed fan to make it variable speed?

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 Posted: Thu Sep 21st, 2017 11:43 pm
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David Allen
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Hi Richard.  For a shaded-pole or capacitor-run AC motor, you can reduce the voltage to slow the motor.

A resistor can be used to reduce the voltage. This passes the current through a resistance element made of some nickel-based alloy or carbon composition. The energy which does not reach the motor is turned into heat, and the resistor gets very hot.

A choke coil (also called a reactor or a 'speed coil' in the fan world) does this by producing a magnetic field which oscillates through an iron core. It reduces the current "reactively" and does not generate as much heat as a resistor. The energy which does not reach the motor is reflected back into the power supply lines as reactive current; instead of being lost.

An autotransformer or a buck transformer are two ways to accomplish the same effect. These reduce the voltage to the motor by recycling some of the voltage back to the incoming power supply. Little to none is lost.

The three items above are "fixed" in their effect. They have a specific amount of reduction in speed. Some have 2 or more possible speed taps; however each tap is a different, fixed speed. There is no in between.

If you are looking for infinitely variable speeds, you will need a more complex device.  Two common ones in the fan world are the variac and the thyristor control.

A variac is a transformer with "thousands" of possible voltage taps, and a movable arm which slides across the taps and allows selection of a nearly infinite range of speeds.

The thyristor control is an electronic device such as the Lutron we all are familiar with as a light dimmer. These are offered in a version which is suited for motor speed control. This works by changing the shape of the AC waveform and blocking out parts of the alternating signal to reduce the power delivered to the motor. This is the smallest and lightest full variable device, however it can introduce a buzzing sound to the motor which some people find objectionable.

Hope this is helpful to you!



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 Posted: Thu Sep 21st, 2017 11:45 pm
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David Allen
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Oh I forgot to add, some motors have extra windings in the stator which act like choke coils and reduce the motor's current and speed. These have extra wires coming from the motor to the switch to choose the speeds.

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 Posted: Fri Sep 22nd, 2017 03:07 am
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Richard Daugird
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Very helpful, thanks!

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 Posted: Fri Sep 22nd, 2017 07:37 pm
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Richard Daugird
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Would it be safe to control a Mobilaire, a later model with the hex motor, with one of these? Says it will handle 6 amps.
http://www.galco.com/buy/KB-Electronics/KBWC-16K?source=googleshopping&gclid=CjwKCAjw6ZLOBRALEiwAxzyCW5FsLBSbihj2DoiDSmAhm_yiU1zCe_NRTD2YRaeUs1HUMuK_hp2avxoCFgIQAvD_BwE

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 Posted: Fri Sep 22nd, 2017 08:16 pm
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Craig Baxter
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Very helpful to me also!

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 Posted: Fri Sep 22nd, 2017 11:37 pm
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Steve Sherwood
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Good information, but be careful trying to slow down a fan motor. If the voltage is low it will cause problems, as in burning the motor out. Lets say the motor is rated at 110vac and you drop the voltage to 90vac. The motor might over heat and burn out. Also the current will drop if the voltage is dropped. In my opinion you would need a choke coil or a buck boost to make it work. If you use a basic resister, you will create a current limiting device which will have a voltage drop also. It will effect the magnetic field.

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 Posted: Fri Sep 22nd, 2017 11:41 pm
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Richard Daugird
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Thanks Steve. These things move so much air, it would be nice to have variable speed. Wish in one hand...

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 Posted: Fri Sep 22nd, 2017 11:47 pm
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Jeff Jones
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I've (We've) been using variac's for a long time to slow fans...I've never onced noticed a motor getting hotter from it...if anything they run cooler. Burnout could be an issue if it doesn't start, or stalls from a power interruption then fails to re-start...but how would just running it at a reduced voltage cause a burnout if its not getting hot, or hotter than at full?

I know that logic applies for DC series wound motors, they will try to pull MORE current to compensate if the voltage is low. Have seen that with engine starter motors getting burned up from being used with weak batteries over extended periods of time.

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 Posted: Sat Sep 23rd, 2017 12:18 am
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Henry Carrera
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Jeff Jones wrote: I've (We've) been using variac's for a long time to slow fans...I've never onced noticed a motor getting hotter from it...if anything they run cooler. Burnout could be an issue if it doesn't start, or stalls from a power interruption then fails to re-start...but how would just running it at a reduced voltage cause a burnout if its not getting hot, or hotter than at full?

I know that logic applies for DC series wound motors, they will try to pull MORE current to compensate if the voltage is low. Have seen that with engine starter motors getting burned up from being used with weak batteries over extended periods of time.

That's accurate information. Our fans, or a fan type load, will run cooler and draw less current with lower voltage. A series wound DC motor actually draws less current as well. The problem is, instead of a quick higher RPM burst to start a motor, people stay on the switch much too long and the duty cycle of the motor is not designed for that kind of abuse.


Last edited on Sat Sep 23rd, 2017 12:26 am by Henry Carrera

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 Posted: Sat Sep 23rd, 2017 01:38 am
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Jeff Jones
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They actually will draw more current if loaded  and spinning slower than designed because there is less back emf. Slower the motor spins the less back emf to "fight" the current draw. 

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 Posted: Sat Sep 23rd, 2017 02:22 am
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David Allen
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Jeff Jones wrote: They actually will draw more current if loaded  and spinning slower than designed because there is less back emf. Slower the motor spins the less back emf to "fight" the current draw. 
This is true, however we have to remember that fan blades have an exponential power curve with speed, so a very small reduction of speed will dramatically reduce the power needed to turn the blade. It seems that most of these fans will respond well to reduced voltages as a means to reduce the speed.

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 Posted: Sat Sep 23rd, 2017 03:01 am
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Richard Daugird
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Can someone recommend a dummy friendly speed control method?

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 Posted: Sat Sep 23rd, 2017 01:16 pm
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David Allen
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Richard Daugird wrote: Can someone recommend a dummy friendly speed control method?
Yep!  The one you found earlier would be very simple. http://www.galco.com/buy/KB-Electronics/KBWC-16K

Just wire the motor for "high" speed, then use this control in the same manner as an off-on switch. :D

That's how I wired my Ilg 12" box fan. It works great.


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 Posted: Thu Sep 28th, 2017 03:51 am
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John McComas
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KB Electronics makes very good controls.
They make up to a 15 amp model for 120 VAC.

http://www.kbelectronics.com/Fan_Speed_Controls_Triac/Wall_Mounted_Fan_Controls.html

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