Hello, i have and old fan here that has been with me for like 10 years. I plan to rework him entirely, give him a new look and some new functions, including some speed control.

I was googling something to control an AC circuit using PWM dc techniques and i found this:



At a first look it seens to work, but has anyone used it before ? The way i see i should face almost no problem with this arrangement. As it doesn't interfere in the frequency of the ac line or the voltage(Maybe a bit).

Also i have some other questions:

- By changing the frequency of the AC line to 120hz(It's 60hz) what would happen ?

- How does these variables relates to each other ? The RPM, the voltage and the frequency of the AC. Can anyone point me to a good link about single phase induction motors and it's characteristics ? Formulas, equations... that sort of things.

 

Traditionally attempts to control the rpm of 1ph induction motors have not been that successful, if this is a fairly small fan then I would expect it to be a shaded pole variety, these can are typically be controlled by Triac type phase control.
If it is a capacitor run motor, you may find it dropping out of run on the lower rpm's.
Max.

 

Why would it drop out of run?

 

The capacitor is calculated to provide optimum phase angle (90°) control between the start and run winding, a 1ph induction motor without any phase shifted winding the field just oscillates back and forth across 180°, (single phase).
So by varying the frequency of the supply the phase angle between main and run winding changes, in some cases to the detriment of the motor performance, especially under load.
Max.

 

I see.. i thought the pwm in AC changed only the average level of AC that reached load.

 

This is why you see extremely few 1ph VFD's, if at all, compared to 3ph versions available.
Max.

 

Even if i use a zero crossing detector to put the pulses applied to the mosfet in the same phase as the AC motor? And then insert a very high frequency pwm into the AC wave, dimming it ?

 

I would try running your fan motor off of a simple variable AC power source like a variac transformer and see how it reacts to having it's voltage reduced. If the motor works fine and shows a fairly proportional speed to voltage applied ratio then a simple Triac based speed control similar to what is used in a common incandescent light dimmer will work.

No need to complicate what should be a simple process.

 

I know i know that this is a rather different approach, but i would like to see if it works.

I have several optocouplers and opamps here, but no triac around, i would like to use the components i have around. That is an Arduíno or a clock signal generator IC( I have many of them), some of those that i said.

 

You haven't confirmed as to whether the motor is shaded pole or capacitor type?
Max.

 

Oh sorry, its a shaded pole 50w, 127v AC induction motor. Very weak one.

 

Then the Triac would be the optimum, the effects of any variation in frequency would be unpredictable with the shaded pole, as it would with the cap motor.
Max.

 

I see, i'll look in to that. Thanks @MaxHeadRoom

 

@MaxHeadRoom I could not find any triac on the local electronic shops. I may try the pwm approach.. i have one last question, lets say i use a zero crossing detector to put the control signal(the one that is applied to the fet) and the AC line in phase.

Then when the AC line crosses the zero i initiate a series of pulses, like something around 10000... to chop the voltage that reaches the motor.

This would look like pwm right ? My concern is, am i changing the frequency of the line? Because i know the speed of the motor is dependent on frequency.

I would be doing the same thing from that image in the first one.

 

Hi,

I know that this is an older post, but I may have some recent, relevant comments. To set up context, my 3 speed 16" floor fan finally died. The rotor bushing seized; cleaning and lubing the bushing worked for a while and then it seized again. Got 30+ years of summer service from this fan.

So, purchased a new but poorly made 3 speed fan. Thin plastic, but usable with one caveat: the low speed setting is still quite fast and noisy. It's a capacitor induction motor type.

Tried some obvious speed control options, lamp dimmers, and four motor speed controls I purchased online. Only the Staco 201 variac worked with no motor hum at very low fan speeds. From several sites, the PWM AC controller was recommended to not produce motor hum at low speeds.

Good solution, however, wanted a solidstate solution. After hours of searching online, the best solution appears to be a PWM AC controller. Unfortunately, most circuits that found used a micro: either a PIC or Arduino.

The remaining circuits were in research papers and gave no circuit design details. I will box up my small 2A Staco variac and use it to control my little fan. I still though, would like to find a reasonably simple PWM circuit fan controller that doesn't require a micro.

Would love to hear any thoughts and better yet, some AC PWM circuits that I can experiment with.

Thanks,

-Neko

 

Use a micro. VFDs I’ve worked on use a 6 step wave running at 10-100khz