A customer ask about there ceiling fans not working correctly, only one speed sometimes. It sounded like someone installed a standard dimmer, not a fan speed control. They also have a remote control not working right either. I'm sure this wouldn't be good for a fan or the dimmer itself. Light dimmers vary the voltage, not sure about fan speed controls, maybe they vary the frequency or the cycles. ??
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Ron, the newer fan controls use an SCR to chop the AC line I believe. Much like a VFD drive in a commercial application. Several of the ceiling fans I have seen warn about using a rheostat for speed control, as in don't. Some of the speed controls also appear to be transistorized. (Man these 20 hr days are killing me. I don't type well in the first place)
[This message has been edited by spkjpr (edited 03-28-2002).]
Daniel, an SCR is a Silicon Controlled Rectifier, a diode that has a Gate used to turn it on so it will conduct. It is used in many industrial and other circuits to control how long the sine wave is on. Were I work we use them to control spot welders so we can control how much heat we apply to the metal. A VFD is a Variable Frequency Drive. Used an motors to control the speed, once again we use them at work to control the speed of a straightner, used to flatten the metal as it comes off a giant coil,so it will match the speed of the stamping press. Any time you have a question, jump in and ask. All of us here are more than willing to help, I even found a man here who answered my question about a crane control when I couldn't find any info about it. Good luck and remember as a n instructor once told me" a thirst for knowledge is a good thing, it means you are still alive!"
Many Dimmers and speed controls use Triacs [like 2 SCRs facing both directions] as the AC Amplifying / Controlling device.
Some fire a Diac for Gate conduction, some drive a SCS to amp the Triac's Gate.
The Speed control which is intregal with the typical Ceiling Fan Permanent Split Capacitor Motor ["PSC"] is comprised of a Tapped Autotransformer section that is wound with the Stator Winding [Primary], and is connected ahead of the Run Winding. This is a simple method for speed control of an AC Induction Motor which limits the available KVA to the Rotor [Secondary] and the overall True Power [Wattage]. By reducing the Voltage, the Motor will reduce it's Rotor speed until it can draw enough True Power to drive the Rotor [simply reduced the cubic feet per minute of air moved].
I should have a schematic of the PSC Motor posted in the Reference section if anyone needs a visual reference. Let me know if one is needed.
Normally, speed of AC Motors is determined by the Frequency applied to the Rotor [higher Frequency = higher speed]. A 2 Pole AC Induction Motor driven at 60 Hz will have an unloaded speed of 3600 RPMs, and slip down to around 3450 at full load. A 4 Pole AC Induction Motor driven at 60 Hz will be 1800 unloaded / 1725 full load. Double the Frequency and the speed doubles. The 4 Pole motor effectivly cuts the Frequency Induced to the Secondary [Rotor] in 1/2, or Induces AC at 30 Hz to the Secondary.
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