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[SOLVED] ac motor speed controller

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hermhart7

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I have a 120VAC AC blower... it's a Lasko type FB Model 655704. The spec says ... motor TP 1.0 amp. It has 3 speeds. But even the slowest speed is faster and noisier than I'd like. So I would like to build a simple Triac speed controller for it. For my application, I don't think I need be concerned about RF interference.
I'm looking at this design... https://electronics-diy.com/1000w-ac-motor-speed-controller.php

Here are my questions:
1. Does my blower have a universal type motor with brushes?
2. Is the circuit sited above a good circuit to use?
3. For the 500K pot, is a simple basic 1/4W one OK?
4. This circuit doesn't give spec for the diac and triac. Below is another similar circuit which does specify these, but it says it's for an induction motor. Can I use these (D3202U, and T27008) for my circuit?

https://www.google.com/search?q=ac+...=ZH5tV9LfF4XNjwS5yIOACg#imgrc=vBc3XegYxoak1M:
 

No. The vast majority of the small fans utilize a shaded-pole induction motor.
The good thing about this motor type is that you can still control its speed with a simple Triac dimmer.
 


if the fan already has an electronic controller then it must be bypassed. Even though the second circuit says for induction motors, it will work exactly the same as the first. Cheap induction motors can only be speed controlled by balancing their torque (running current) against their load. So its not speed control, but torque control and is not got very good.
Frank
 

You haven't described the noise problem and if I assume it is from winding and magnetostiction, this circuit will not fix that. If it is bearing resonance from fatigue, you might be able to tune it to an anti resonant low speed, maybe.

Can you open it to determine the speed control method is R switched Triac with C delay or switched plastic caps of two values? If the latter, the current is reduced by increased impedance of a smaller cap, which may cause a different sound.(not sure). Is it a loud tick sound less than 60 Hz or a rumble at 60Hz or a constant buzz rate that reduces in nuisance level as the speed increases with white noise from the blade and safety grill at full speed.... thereby being more noticable when the blades are quieter?

The Switched series Cap method is more common since shaded poles can saturate if the triac trigger phase-phase is not perfectly symmetrical, thereby producing a net DC offset with the phase controlled triac, which reduces L at low speeds from core partial saturation and thus creates current spikes with greater tick noises. But the series cap eliminates DC offset from mismatched trigger thresholds and phase triggers.

FWIW, I dont know for sure how it sounds or is designed. AC series motor caps tend to have higher ESR than the best plastic caps, which accelerates aging.
Did the noise always exist?

- - - Updated - - -

For example Halogen filaments on Triac control make quite a bit of noise when very dim due to the surge currents when cooler and filament vibration with lower R and higher current when dim but for a shorter interval. But fan motor windings should (!) be tight. (unless a very cheap motor)
 
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OK. Regarding the noise... this is a new fan. The noise is merely the whirring sound of the air being blown. I want to use this blower to blow cooled air into our bedroom at night for sleeping, so I want to keep any noise, even whirring, to a minimum. Since the CFM is greater than necessary for this application, I thought to reduce the speed in order to reduce the whirring. So I'll go in there and find out what type of speed reduction I already have. This seems like a reasonably rugged unit, but yet quite inexpensive. There is only a slight speed difference between low, medium and high speeds, so it might be a cheap method being used.
 

Perhaps my Lasko fan is similar to yours? Window box fan, 3 speeds. A bit loud at all speeds.

To operate it at reduced speed, I use an autotransformer. It steps down house voltage half, from 120 to 60V. All 3 speeds work okay. However they are little slow, being 1/2 of the previous speeds.

So I would prefer to operate it at 60-70 percent. It is possible to adjust a triac to that range. (This assumes a triac is compatible with your fan.) I made a triac-based fan control for a squirrel-cage fan on our wood stove. However the control broke when I tried it with my window fan. I have not yet diagnosed it.

Just to relate my experience in case it's of interest.
 
I am sure you can use a conventional triac dimmer used for filament lamps with your fan; the speed control of the fan will continue to provide some adjustments.
 
THe quietest fans are large blades moving at low RPM, not the reverse. By removing the safety screen, with a bigger plenum, one can significantly ( 30dB or more ) reduce the acoustic noise from fringe eddy current noise in axial fans. Even radial fans ( squirrel cage) have inherent intake and exit noise from the safety screen.

This is the design trade-off for safety, small size and low blade noise.

Pick any two.

I have seen 10 HP axial fans on golf courses by the edge of the green to prevent fungus growth that are whisper quiet in Georgia, USA. ( Bear's Best)
 

THe quietest fans are large blades moving at low RPM, not the reverse.

If a fan with a large blade moves at a high RPM, the tip velocity will be large and the air flow will be turbulent at the tip that will contribute to the noise. Fans with large blades (diameter) usually have low RPM so that the velocity at the tip is streamline. In addition there will be problems of resonance and the bearings and slow fans (low power) can get away with nylon bearings easily.

The problem is with small fans that need to move large volume of air under low pressure differential. They usually have blades with large areas and large angles. Often the number of blades are increased but the idea remains the same. It is often difficult to make them silent because they have their own body resonance in addition to the bearing problem...
 

BradtheRad #7... You addressed a question I was wondering about, namely reducing the AC voltage at the plug. This would prevent the need for invasive intervention. So I gather that at half voltage there is still enough magnetism for the motor to function happily. Also I gather that speed can also vary with voltage, as compared with the triac circuit where speed varies with pulse width. As to the triac-based control you made which broke, did you bypass any existing electronic speed control as Chuckey #4 says I should?

C_mitra #8... You say that I can use a conventional triac dimmer. I assume you mean I can plug my power cord straight into the dimmer. So do you disagree with Chuckey #4? Or are you saying that my internal existing speed control is not electronic?

SunnySkyGuy #9, and C_mitra #10... I have a 3-speed axial in addition to the 3-speed radial I have been asking about. You're definitely motivating me to try removing the safety screens on both of them. One of the features I prefer about the radial is the compact size... It takes less space in the hall-way so we can more easily walk around it. It also seems to me that I can more specifically direct the flow of the radial. (Rifle vs shotgun approach). But right now I need to get to the bottom of the noise issue. I think for both fans, the bearing noise is a smaller percentage of the total, with resonance at the screen and blade being more significant.
 

That's exciting. Can I plug my fan directly into this circuit even though the fan unit has its own internal speed control? At this point I do not know what the nature of it's internal speed control is.
 

BradtheRad #7... You addressed a question I was wondering about, namely reducing the AC voltage at the plug. This would prevent the need for invasive intervention. So I gather that at half voltage there is still enough magnetism for the motor to function happily. Also I gather that speed can also vary with voltage, as compared with the triac circuit where speed varies with pulse width. As to the triac-based control you made which broke, did you bypass any existing electronic speed control as Chuckey #4 says I should?

My experience tells us the fan speed does not hold to mains frequency. It can be changed by voltage, and therefore also triac control. Both methods alter the power going through the fan.

This doesn't mean I understand the speed control inside the fan. I didn't open it up. I merely experimented with parts I had around.

My triac control has no heatsink. I may have overloaded it by trying to use it on the window fan. I think I was lucky it was adequate to control the wood stove fan.
 

C_mitra #8... You say that I can use a conventional triac dimmer. I assume you mean I can plug my power cord straight into the dimmer.
Right, most common light dimmers would not need any changes; they use phase control and effectively reduce the overall power being supplied to the load.
So do you disagree with Chuckey #4? Or are you saying that my internal existing speed control is not electronic?

I strongly suspect that the built-in speed control is either resistive or capacitive - you can easily make out if you run the fan at the lowest speed setting and feel the heat produced in the regulator.
 

OK, BradtheRad #7, I have tried what you did, namely reducing the voltage using a transformer. It works great. I ran it for an hour or so and everything seems OK. Today I picked up a light dimmer at a thrift store and I'd like to try that, too. I admit to being a little edgy about doing this though when I read something like the following website ...

**broken link removed**

... but I have demonstrated that my motor seems to respond happily to voltage reduction, and per BradtheRad #14 it would follow that therefore I can also use my new light dimmer. So where is the fallicy in the reasoning of the website I just sited? Is it answered by Chuckey #4
Cheap induction motors can only be speed controlled by balancing their torque (running current) against their load. So its not speed control, but torque control and is not got very good.
Frank

The author in icmag points out that whatever method is used to reduce the speed, causes the magnetism to decrease, and hence also the inductive reactance, thus causing the current to increase and risk overheating. Why do I not need to be afraid of that happening to my motor?
 

The author in icmag points out that whatever method is used to reduce the speed, causes the magnetism to decrease, and hence also the inductive reactance, thus causing the current to increase and risk overheating. Why do I not need to be afraid of that happening to my motor?

That is correct, in essence.

But you see that you are decreasing the voltage applied; this causes the current to decrease, this in turn causes the magnetism to decrease and that is the cause of the decrease speed. Once the speed is reduced, the back emf produced is going to get less (what the author is calling inductive reactance) and that causes the current to increase.

Now you can see the net result: the voltage is reduced, the speed is reduced but the current is reduced only slightly. The reduction depends on the load and what we are actually trying to do is to reduce the speed by controlling the torque and load. They are balanced always at different speeds.
 

The author in icmag points out that whatever method is used to reduce the speed, causes the magnetism to decrease, and hence also the inductive reactance, thus causing the current to increase and risk overheating. Why do I not need to be afraid of that happening to my motor?

ICmag could have a point about slower frequencies causing higher current draw. This is similar to the caution about taking a transformer built for 60Hz, and running it at 50Hz. For safety we need to de-rate the transformer, that is, draw less power from it.

It might be the same with a fan motor. However notice the ICmag article does not mention the case of a step-down transformer. My experience is that it appears to be safe for my fan. To be thorough, I need to measure Ampere draw, of course.

The ICmag article causes questions to multiply:

* The author does not recommend an inexpensive light dimmer. These typically contain a triac. So, is there a reason we cannot we make a triac control which reduces power to a level which is safe for the fan?

* The fan has a 3-speed control knob, and it is safe for the motor. Is there any reason we can't get creative with the slowest speed, change a component value, and slow it down further, safely?

* The author recommends: "Using a variable frequency drive. They do not change current, voltage, magnetic field, or any other factor other than frequency. " This slows down the motor. Won't this cause increased current draw, risking the motor? If not, why not?
 

* The author recommends: "Using a variable frequency drive. They do not change current, voltage, magnetic field, or any other factor other than frequency. " This slows down the motor. Won't this cause increased current draw, risking the motor? If not, why not?

I cannot access the original article, but using the VFD is not the panacea for all. If you change the frequency, you need to revise the applied voltage.

For any given motor, VFD will be usable only over a range of limited frequency. And I believe this to be true even for motors designed specifically for VFD.

Basically motors are current controlled devices and the motor does not know about the temperature rise. More current is more torque.
 

To return to the original question, a 1000 W "high velocity" fan in a small enclosure is most likely using an universal motor, similar to a vacuum cleaner. If so, the VFD discussions are off-topic.

You need little technical knowledge to determine if a motor is using brushes, just listen how it comes to a stop after being switched off. Question is if the three-step speed control uses electronics or simply field winding taps. I guess the latter. In this case it can be easily slowed down by a triac controller.
 

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