Hello..... I was just at Home Depot trying looking for a quick fix to a problem. I need to slow down an AC fan motor (measured at 2.3 A full speed/ 120V.) I was finding all ceiling fan controls are rated at
1.5 A. max. But then I found in the back some other lighting controllers that were unusual. One was for low-voltage magnetic and the other electronic transformer types. It connects to the AC mains, and the magnetic load is downstream. It seems to me that this controller rated at 5 A. might fit the bill. Since the inductance/reactance is designed into this device, why couldn't it work....? The motor type...don't know. However, it doesn't use a starting cap.The windings are on the motor casing. Comments please......Thank you.
Ceiling fans usually don't work very well with the standard triac- based lamp dimmers, Les. But if you want to try, there are many dimmers on the Home Depot website that can handle 600W (about 5A) at
120VAC. I'm sure some of them are in the store. You might want to try one of those if you've got some spare time and don't mind taking it back if it doesn't work well (probably).
If you want to get something that will work better, try the NuTone Ventilation control switch Model 72W (Home Depot Internet/Catalog #
100490891). It's a little pricey at $53.18 ea., but has a different type of circuit that will work better with the inductive load of a fan motor. Call ahead to see if the store has it in stock.
Thanks Chris, I'll look into your suggestion. And Phil, the device is a whole house fan, 1600 cu. ft/min displacement, hence the large current draw. I want to slow it down to 33% if possible to cut down on noise. Domestic made, though I don't know if the motor itself is from US. I know Triacs are intended for resistive loads, so my eyes lite up when I noticed the claim that this "special dimmer" was intended for inductive loads. So the next question is what components are in this unusual dimmer, and would it suffice in my intended experiment........ I don't want to damage the windings in the motor by overheating.
I've been doing some reading about motor speed control lately.
I've found that it's important to find out exactly what type of motor you have before choosing a speed control.
Most common small devices such as fans, aquarium pumps, etc, are powered by single phase shaded-pole induction motors. At 2.3 amps, you might have one of these, but at that power level, you're in the area where a more complex motor might do the job better. You need to open the fan and take a look at the motor before you proceed.
If it's a shaded-pole motor, you need some sort of Volts/Hz speed control, with a variable output frequency and the voltage reduced as the frequency is reduced.
If the Nutone controller Chris mentioned will do the job, the off the shelf solution is cheaper in terms of money and time than you'd invest if you tried to build one. I couldn't find any specs on the device with a quick Google search, so I don't know if it's suitable for your app or not. You might email Nutone with the details of the fan you're trying to control. (Their site is
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I roughed out a Volts/Hz controller circuit as a discussion point on this topic in another forum. I'm not sure it will work as designed, but it will give you an idea of the sort of thing you'd have to build if you decided to do the job yourself. You can find the schematic here:
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The microcontroller would set the switching frequency of the inverter, from the motor's design frequency down to some lower level, as the speed control pot is adjusted. As the frequency is reduced, the voltage is also reduced linearly (V = freq * design V / design freq) using the micro's PWM output and monitored by the micro's ADC input.
Note that this schematic is simply a talking point. I haven't breadboarded it or, for that matter, even given it much thought. It probably needs debugging.
If your fan doesn't use a single phased shaded-pole motor, your speed control device might actually be simpler. When you know for sure what sort of motor you're controlling, you might do some Googling.
Good luck with your project. When you've solved your problem, please report your solution so the rest of us can learn from your experience.
Tom.... I really appreciate your thorough analytical treatment. I was suspecting this might involve controlling voltage and frequency parameters, but hoped it wouldn't require my own design or assembly. I will definitely look over your suggested materials, and of course, I need to determine the type of motor I have before proceeding further. I'll check back with results when I dig thru the details.
Ya know, it's funny you should ask this. I just installed a GAF WHFS24M (4500 CFM) attic fan that I got from Home Depot. It was way too loud even at its low setting. I'll bet this is the same unit you have, eh?
I tried an X10 type lamp dimmer. It worked, but the dimmer got real hot at certain settings and I wasn't sure whether it would harm the motor because it doesn't feed the motor with a sine wave unless at full output. I think my motor is a permanent split capacitor (PSC) type. There is a capacitor on my motor. I'll post a picture of the motor label on alt.binaries.schematics.electronic since it describes the capacitor.
Anyway, I ended up getting a variac at Fry's. It's the Philmore 48-1205 and the Fry's p/n is 4638758. It's rated at 500VA and it costs $70. I feel better about it since it alway drives the motor with a sinusoid.
The variac works extremely well. I can turn it down to 72 (out of 120) on the dial and the fan sucks (which is a good thing, in this case) and with minimal noise. I measured the current to be 1.3A @ knob = 72 and 2.4A when it's plugged directly into the wall.
Bob.... Great timing on your part. I have the smaller (1600 cu.ft/min model) but the current draw is almost the same. As I said, I haven't inspected the motor type yet (as it's in the ceiling already) and the included pamphlet could care less about technical information. The X10 idea: I'm surprised it just didn't crisp itself. I assume they use Triacs and could buzz loudly. The Variac idea has been bounced alot for this purpose, but the only ones I've seen before are desk models with big footprints. So, this Fry's device sounds VERY interesting. And I assume it's a wall-mount type switch ( will fit typical electrical wall box).... Right?
I have a 2 and 4 year-old, and breadboarding is a thing of the past. That's why I'm looking for turnkey devices, and go back to being a Dad....... (although it's hard not to want to tinker around)
Unfortunately, this variac is 5" high and its round section is 5" diameter. I don't think that an autotransformer type of variac, with 500VA capacity, could be any smaller.
Someone probably makes a smaller sinusoidal output switching type of motor/voltage controller but I couldn't find one. I'm not saying that sine waves are the only way to go, it's just what makes me feel "comfortable".
When your kids are older, remind them how much trouble they were when they were this age, and how perfect you and I were. ;-D
Variacs and fixed transformers are large and expensive. An inexpensive alternative (if you don't need continuously variable speed) is to use dropping resistors to lower the voltage. It could be just one (for fixed speed operation) or several selectable ones.
Based on Bob's observation that 72 volts at 1.3 amps was about right in his case, a 50 ohms 100 watt (allowing a safety factor on the power rating)resistor might do the trick . I see that
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50 ohm 220 watt resistors for $3.75. They also have 10 ohm 25 watt resistors for $1 each. With five of these, you could try a wide range of series/parallel combinations to find the speed you like best.
If you try this approach, keep in mind that the resistors will dissipate about 70 watts of heat. You might mount them on a metal plate or heat sink by the fan, where the air flow will give good convection cooling. I assume that this is an attic fan, so appearance shouldn't be an issue.
Yes, I had that hunch the boys didn't come up with some miracle toroid to cheat the huge dissipation factor. But it never ceases to amaze me what new materials come forth.
I agree here.
ha-ha. but it's fun to see things thru "new eyes".
And in regards to Ninja, resistance is certainly fast and easy. I just hate the inefficiency. But it could work, and why bother with a resistor...how about a
100 watt bulb in series for starters. Sure, in 3000 hours we need another limiter, but hey, you get some light as well !!
I've used light bulbs as voltage droppers. It's not as stupid as it sounds. In fact, you could keep a supply of different wattage bulbs and screw in the one you want for the desired fan speed.
Yes, the light bulb technique was my first thought. In general, I like the positive temperature coefficient as a safety device; It limits current on short circuit to a lower level than a resistor would do.
But I wonder whether the PTC could cause start-up problems with a motor. Don't they need a surge of current to get going?
Another concern is that a 100 watt bulb would drop the voltage far too much. Don't you need to drop about 50 volts at 1.3 amps? A 300 watt bulb maybe (or three 100w in parallel), but now you're talking about a specialty bulb (or multiple bulbs in parallel). $ $ $
I suspect that resistors will turn out to be far less expensive than bulbs and sockets. For local availability, though, light bulbs win hands down!
That's the nice part of speed controlling a fan. They don't have a startup torque that must be overcome. They provide zero load at zero rpm, then build as the speed builds.
Personally, I think the light bulb idea is brilliant, and should work well.
i use a 25 watt tungsten paralleled with my screw in "high efficiency flourescents" in my rooms overheads with automatic IR switches never had any trouble with starting the florescents after that.
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