ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

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. ??

well im not really sure about the 2 but no a dimmer & a fan speed control are 2 different creatures.

What is the operational differences between a fan speed control and a dimmer, assuming they both have a similar wattage rating. Don't they both shunt the power through a resistor (old style dimmer)?

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).]

Spkjpr,

can you explain a little more. what are SCR and VFD. tring to learn as much as i can. thanks

dan

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!"

Spkjpr,

thanks for all that info. you the man!!

Just some FYI stuff:

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.

Scott SET